Ee ron Ret" 4 seer are teit isa rn Ft + te + 2 cats ne eget tf oe Bsaspececatete oes see siaesefeta tit ; eta hake ot he rede) elo hae + ks sitieiele : , + “prbrets 4% Tete wahes i tats efergieanss Se haes *. 4 were Sree: aot Ferg ff } PAPI : IAD A i” 7 wae itt AT URBANA-CHAMPAIGN GEOLOGY re. ¥. th) oe ae Nhs Wg : ‘ p A 7 Lk e om i * 7} " » “ pa . . + ; ; ny +2 ‘ ‘ ; P any eh oh if ‘ Ye 4 o . %. ; -, GEOLOGY. By Tuomas C. CHAMBERLIN and ROLLIN D. SALISBURY, Professors in the University of Chicago. (American Science Series) 3 vols. 8vo. Vol. I. Geological Processes and Their Results. Vols. II and III. Earth History. (Not sold separately.) A COLLEGE TEXT-BOOK OF GEOLOGY By Tuomas C, CHAMBERLIN and ROLLIN D. SALISBURY, (American Science Series.) 8vo. INTRODUCTORY GEOLOGY. A TEXT-BOOK FOR COLLEGES. By Tuomas C. CHAMBERLIN and ROLLIN D. SALISBURY. (American Science Series.) 12mo. PHYSIOGRAPHY By RoLuin D. SALISBURY. (American Science Series.) 8vo. The same. Briefer Course. 12mo. The same. Elementary Course. 12mo. ELEMENTS OF GEOGRAPHY By Rotiin D. SALISBURY, HARLAN H. BARROwsS and WALTER S. Tower, of the Department of Geography, The University of Chicago. (American Science Series.) 12mo. MODERN GEOGRAPHY By Rotimn D. SALISBURY, HARLAN H. BARROws and WALTER S. Tower, of the Department of Geography, The University of Chicago. (American Science Series.) I2mo. HENRY HOLT AND COMPANY, PUBLISHERS New YORK AND CHICAGO AMERICAN SCIENCE SERIES. INTRODUCTORY GEOLOGY pe eeXT-BOOK FOR COLLEGES BY THOMAS C. CHAMBERLIN AND ROO IN sD: SALISBURY Heads of the Departments of Colbey and Saas The University of Chicago Ds Ks oN) tay iy) BS 4 NEW YORK HENRY HOLT AND COMPANY 1924 Coryricnt, 1914 BY HENRY HOLT AND COMPANY The Lakeside Press R. R. DONNELLEY & SONS COMPANY CHICAGO a AA 545O e teps ae J oe ©, zi ¥ 7 - J ox A f . sone A ey a | “3 j ’ / Ss —' T- U ‘ PREFACE This volume is an abbreviation and simplification of COLLEGE GEOLOGY, published five years ago. Many technical details have been omitted, but the general purpose and scope of the volume is not altered fundamentally. It is intended to present an outline of the essential features of geology with as few technicalities as the nature of the subject permits. Part I deals with geological processes, and with the materials on which they operate, while the y> theme of Part II is historical geology. The effort has been to treat “ these topics in such a way as to give the student not merely an - understanding of the subject, but also an understanding of the means by which the present status of the science has been reached. N The theoretical and interpretative elements which enter into “the general conceptions of geology have been used freely, because | they are regarded as an essential part of the evolution of the science, 2 since they often help to clear and complete conceptions and to stim- “ ulate thought. The aim has been, however, to characterize hypo- "y thetical elements as such, and to avoid confusing the interpreta- \ tions based on hypothesis with the statements of fact and estab- .~ lished doctrines. ~~) >< In many cases the topics discussed will be found to be pre- . sented in ways differing widely from those which have become S familiar. In some cases, fundamentally new conceptions of familiar [subjects are involved; in others, topics not usually discussed in 2y text-books are stated with some fullness; and in still others, the emphasis is laid on points which have not commonly been brought YMnto prominence. Whether the authors have been wise in depart- _ ding to this extent from beaten paths, the users of the volume must } decide. Note University of Chicago, February, 1914. HEDRICK ' Jans CONTENTS PAR ie! THE MATERIALS. OF THE EARTH AND PROCESSES CHAPTER A II. III. IV. WHICH AFFECT THEM PRELIMINARY OUTLINE THE EARTH IN THE SOLAR SYSTEM THE GRAND DIVISIONS OF THE EARTH GEOLOGIC WORK OF THE ATMOSPHERE MECHANICAL WoRK CHEMICAL WorK ; THE ATMOSPHERE AS A CONDITIONING “AGENCY ; SUMMARY WORK OF GROUND (UNDERGROUND) WATER GENERAL FACTS ‘ WorK OF GROUND- WATER ORE DEPOsITs . ‘ SUMMARY . SPRINGS AND ARTESIAN W ELLS : WORK OF RUNNING WATER EROSIVE WorRK ANALYSIS OF EROSION CONDITIONS AFFECTING RATE OF EROSION RATE OF DEGRADATION FEATURES RESULTING FROM SPECIAL, ConDITIONS OF ERo- SION . oe SAC OY etre 1 ee: EFFECTS OF UNEQUAL “HARDNESS THE EROSION OF FOLDs . ADJUSTMENT OF STREAMS TO Rock STRUCTURES - INFLUENCE OF JOINTS ON EROSION EFFECT OF CHANGES OF LEVEL . AGGRADATIONAL WoRK OF RUNNING WATER . ALLUVIAL TERRACES epee. ae V. WORK OF SNOW AND ICE IcE IN GENERAL GLACIERS THE STRUCTURE OF GLACIER IcE MotTION OF GLACIER ICE THE WorkK OF GLACIERS GLACIO-FLUVIAL WoRK ICEBERGS Vil PAGE i) Vili CHAPTER VI. XIII. XIV. CONTENTS WORK OF THE OCEAN GENERAL FACTS PROCESSES IN OPERATION IN “THE SEA MOVEMENTS OF SEA-WATER . DEPOSITS ON THE OCEAN-BED LAKES OUTLINE OF THEIR WorRK AND HISTORY MOVEMENTS AND DEFORMATIONS OF THE EARTH’S BODY (DIASTROPHISM ) MINUTE AND Raprp (SErsMIc) MOVEMENTS, EARTHQUAKES SECULAR MOVEMENTS sl an. ype arn VULCANISM INTRUSIONS . EXTRUSIONS THE CAUSE OF VuLc ANISM MATERIALS OF THE EARTH AND THEIR ARRANGE- MENT IGNEOUS Rocks SEDIMENTATION AND SEDIME NT. ARY Rocks. INTERNAL CHANGES IN IGNEOUS AND SEDIMENTARY Rocks; METAMORPHISM : , VARIOUS CLASSIFIC. \TIONS AND NoME NCLATURES ~ PAROLE HISTORICAL GEOLOGY THE ORIGIN OF THE EARTH HYPOTHESES STAGES OF THE EARTH’S HISTORY PRIOR TO) THe KNOWN ERAS STAGES UNDER LAPLACIAN HYPOTHESIS STAGES UNDER PLANETESIMAL HYPOTHESIS THE ARCHEOZOIC ERA GENERAL CONCEPTIONS GENERAL CHARACTERISTICS OF ARCHEAN Rocks — DISTRIBUTION OF ARCHEAN ROCKS . THEORETICAL CONSIDERATIONS : : GENERAL TABLE OF GEOLOGIC TIME DIvIsIONsS ‘ THE PROTEROZOIC ERA FORMATIONS AND PHySICAL HISTORY THE PROTEROZOIC OF THE LAKE SUPERIOR REGION ; GENERAL CONSIDERATIONS RELATING TO LAKE SUPERIOR PROTEROZOIC : , PROTEROZOIC OUTSIDE “LAKE ‘SUPERIOR REGION , LirE DurRING PROTEROZOIC ERA CLIMATE OF PROTEROZOIC ERA . PAGE 167 170 173 189 201 206 427 228 229 241 246 205 285 297 299 397 310 314 317 320 323 323 325 331 337 340 342 342 CONTENTS 1X THE PALEOZOIC ERA CHAPTER PAGE XV. THE CAMBRIAN PERIOD PORMATIONG ANO WL HYSICAL) HISTORY: Ur Se eaerho wk ow. (34g Bee TH APASEUROAN To) ig Wa eo ce ce yO NE ey te ny 38S XVI. THE ORDOVICIAN (LOWER SILURIAN) PERIOD Pewee TIONS AND AE HYSICAL, HISTORY 2. Gl. pes a 4 1967 LIFE . a XVII. THE SILURIAN (UPPER SILURIAN) PERIOD FORMATIONS AND PHysIcAL HisToRyY . . . .:. . . 388 Pre ee Seca RTM a eg a aad XVIII. THE DEVONIAN PERIOD FORMATIONS AND PHysICAL HISTORY . .. . . . . 402 ema Pl eee UR i gl aR XIX. THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PE- RIOD FORMATIONS AND PHysICAL HISTORY Uaets AN 2) Pea ataG See te SS Un ron fey tele ehh. ke 1432 XX. THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PE- RIOD FORMATIONS AND PHysicAL History . .. . . . . 44I SERIE WAY ost at ae ERA rm ts ye fers erage XXI. THE PERMIAN PERIOD FORMATIONS. AND PHYSICAL. History a CG eee my RT Ge OE i a a a PROBLEMS OF THE PERMIAN . 4 THE MESOZOIC ERA XXII. THE TRIASSIC PERIOD PORMATIONSFAND) LHYSICALMELISTORY. (07) So. we, 48d ce Re ROM ae eh Les eh eee Ta MM, ee aK lee! AOR XXIII. THE JURASSIC PERIOD FORMATIONS AND PHysICAL History .., .. . . 502 Mei ts ee we Pence ey Nia is) in. 5) gt VSOF XXIV. THE COMANCHEAN (LOWER CRETACEOUS) PERIOD RURMATIONS AND PHYSICAL HISTORW My) ge eh. 1 88 Pera ene, ars Ty ee hee I Me WM yet ge a S28 XXV. THI CRETACEOUS PERIOD FORMATIONS AND PuysicAL History . . . . . . « 532 Pra nT Sa | ae Re og! che seis flee (SAO XXVII. XXVIII. XXIX. XXX. CONTENTS THE CENOZOIC ERA THE EOCENE AND OLIGOCENE PERIODS FORMATIONS AND PuHysicAL HISTOR Dire Se ee a a a eS OLIGOCENE FORMATIONS . OLIGOCENE LIFE THE MIOCENE PERIOD FORMATIONS AND PHysICAL HISTORY LIFE . re eyes : THE PLIOCENE“ PERIOU FORMATIONS AND PuHysicAL HISTORY LIFE . : ee ee A THE PLEISTOCENE OR GLACIAL PERIOD FORMATIONS AND PHYSICAL HISTORY CAUSE OF GLACIAL CLIMATE Lire. ee oa THE HUMAN OR PRESENT PERIOD FORMATIONS. LIFE . APPENDIX REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS . REFERENCE TABLE OF THE PRINCIPA?, GROUPS OF ANIMALS VII. VIII. XVI. Winn Obe PIA TES FACES PAGE DUNES IN CONTOUR STREAMS DISAPPEARING IN SAND, GRAVEL, ETC., IN AN ARID REGION YOUTHFUL VALLEYS, SHORE OF LAKE MICHIGAN THE WIDENING OF A RIVER VALLEY BY MEANDERING OF THE STREAM YOUTHFUL VALLEYS IN A REGION OF SLIGHT RELIEF AND OF GREAT RELIEF Fic 1. ToPpoGRAPHIC MATURITY Fic 2. IRREGULARITIES OF COAST DEVELOPED BY EROSION AND DEPOSITION TopoGRAPHIC OLD AGE CUSHETUNK AND RounD MownrtaIns, N. J. . Fic. 1. ENTRENCHED MEANDERS. CONODOGUINET CREEK, PA. Fic. 2. SECTION OF THE CALIFORNIA COAST AT OCEANSIDE, SHOWING CHANGES OF LEVEL OF THE LAND ALLUVIAL FAN AT THE BASE OF MOUNTAINS. CUCAMONGA, CAL. THE ALLUVIAL PLAIN OF THE MIsSsouRI AND BiG Sioux; S. DAK. GLACIERS OF GLACIER PEAK, WASH. GLACIERS AND CIRQUES OF THE BIGHORN MOUNTAINS AN ILt-DRAINED PLAIN OF GLACIAL DRIFT; SOUTHERN WISCONSIN Fic. 1. SHORE-LINE OF MARTHAS VINEYARD, Mass. See ILAND-TIED ISLAND 2 2.50 soos 6 #8 oe PmeeUPPeR END OF SENECA LAKE, N.Y. . . s . « «6 + 20 21 GEOLOGY lore Wael boa | THE MATERIALS OF THE EARTH AND PROCESSES WHICH AFFECT THEM CHAPTER I PRELIMINARY OUTLINE Geology 1s the history of the earth and its inhabitants. It treats of the rocks and of the agencies and processes which have made them, and from the rocks, their structures, and their fossils, it attempts to make out the stages through which the earth and the life which has dwelt upon it, have passed. Subdivisions. So broad a science has many subdivisions. Cosmic or Astronomic Geology treats of the outer relations of the earth; Geognosy treats of the materials of the earth, and its most important branch is Petrology, the science of rocks; Structural Geology deals with the arrangement of the rocks; Dynamic Geology deals with the forces involved in geologic processes; Physiographic Geology treats of the face of the earth, or topographic form; while Paleontologic Geology, or Paleontology, concerns itself with the fossils that have been preserved in the rocks, and with the faunas and floras that have lived in the past. The succession of events in the earth’s history constitutes Historical Geology, which is worked out chiefly from the succession of beds of rock formed through the ages, and from the fossils they contain. Besides these general subdivisions, there are special applications of geologic knowledge which give rise to other terms. Thus Economic Geology is concerned with the industrial applications of geologic knowledge, and Mining Geology, a sub-section of economic geology, deals with the application of I 2 , PRELIMINARY OUTLINE geologic facts and principles to mining. Other similar subdivisions might be mentioned. Dominant processes. ‘Three sets of processes, still in operation on the surface of the earth, have made much of the record on which the science is based. ‘These processes have been designated dzas- trophism, vulcanism (volcanism), and gradation. Diastrophism includes all movements of the outer parts of the lithosphere, whether slow or rapid, gentle or violent, slight or extensive. Many parts of the land, especially along coasts, are known to be sinking slowly relative to the sea-level, while other parts are known to be rising. The fact that sediments originally deposited beneath the sea now exist in some places at great elevations, together with the fact that certain areas which were once land are now beneath the sea, proves that similar changes have taken place in the past. Earthquakes are another illustration of diastrophism. Vulcanism includes all processes concerned with the movements of lava and other volcanic products, whether they issue at the surface or not. Vulcanism and diastrophism may be closely associated, for many local movements are associated with volcanic eruptions. Gradation includes all those processes which tend to bring the surface of the lithosphere’ to a common level. Gradational processes belong to two categories — those which level down, degradation, and those which level up, aggradation. ‘The transportation of material from the land, whether by rain, rivers, glaciers, waves, or winds, is degradation, and the deposition of the sediment, whether on the land or in the sea, is aggradation. Degradation affects primarily the higher parts of the lithosphere, and aggradation the lower. | THE EARTH IN THE SOLAR SYSTEM Though supremely important to us, the earth is but one of the minor planets which revolve about the sun. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mercury, Venus, and Mars, are smaller. There are hundreds of asteroids, but all together they do not equal the mass of the smallest planet. Jupiter, the largest planet, has more than three hundred times the mass of the earth. The earth’s position is in no sense distinguished, for it is neither the outermost nor the innermost, nor even the middle planet. In the inner group of four to which it belongs, it is the largest. Its average distance - THE EARTH IN THE SOLAR SYSTEM 4 from the sun is about 92.9 million miles, and its period of revolu- tion, 365% days, is longer than that of any other one of the inner planets, and shorter than that of any one of the outer group. The orbit of the earth, like the orbits of the other planets, is an ellipse. The inclination of the earth’s axis, nearly 231%°, is less than that of the axis of some planets, and more than that of others. The earth is peculiar in having one unusually large satellite, which has a mass !/8: of its own. The larger planets have several satellites whose combined mass exceeds that of the moon, and a few individual satellites may be larger than the moon; but no other is 1/81 of the size of the planet about which it revolves. The moon has played an important part in the history of the earth, for it is the chief cause of tides, and tides are efficient in the wear of the shores of the oceans and in the distribution of marine sediments. Tides probably have been important ever since the ocean came into existence. The most important external relation of the earth is its depend- ence on the sun. Its mass is less than 1/300000 that of the sun, upon which it depends for nearly all its heat and light, and, through these, for nearly all of the activities that have determined its history. A little heat and light are received from other bodies, and an im- portant source of energy is found in the interior of the earth itself; yet all of these are so far subordinate to the great flood of energy which comes from the sun, that they are quite insignificant. The dependence of the earth on the sun has been intimate throughout its past history, and its future is locked up with the destiny of that great luminary. Meteorites. There are multitudes of small bodies, called mete- orites, passing through space in varying directions and with varying velocities. Great numbers of these reach the earth daily as “shoot- ing stars.” Some meteorites revolve about the sun like planets, but some of them do not belong to the sun’s family. Some consist almost wholly of metal, chiefly iron alloyed with a little nickel; some consist of metal and rock intimately mixed; and some consist wholly of rock. Since meteorites are thought to throw some light on the early history of the earth, they are of interest to the geologist. The amount of material added to the earth by the infall of meteorites is now slight compared with the whole body of the earth; but their contributions in the past may have been greater. 4 PRELIMINARY OUTLINE THE GRAND DIVISIONS OF THE EARTH The constitution of the earth. The materials of the earth fali into three grand divisions: (1) The atmosphere, (2) the hydros phere (water sphere), and (3) the lithosphere (rock sphere). The atmosphere. Since the atmosphere is a part of the earth, its history falls within the province of geology. It is an intimate mixture of (1) all those substances that do not become liquid or solid under the temperatures and pressures which exist at the earth’s surface, together with (2) such transient vapors as the various liquid and solid substances of the earth throw off. The first are the principal gases of the atmosphere, and consist of nitrogen about 78 parts, oxygen about 21 parts, carbon dioxide about .o3 part, together with small quantities of argon, and several other sub- stances. Chief among the second group is water vapor, which varies greatly in amount from time to time and from place to place. Here, too, belong the gases which issue from volcanoes, and many volatile organic substances. Dust and other matter suspended in the air are regarded as impurities rather than constituents of the atmosphere; but they are important because they affect the tem- perature and light of the air, and the condensation of its moisture. The mass of the atmosphere is estimated to be 1/1200000 of the total mass of the earth. It exerts a pressure of about fifteen pounds per square inch at sea-level. Its density decreases upward, but its actual height is not known. There is no direct evidence of its existence above a few hundred miles, but there are theoretical grounds for believing that it reaches much greater heights. Geologic activity. The atmosphere is the most mobile and active of the three great subdivisions of the earth. Its direct and indirect effects on water and rocks are so great that it must be regarded as one of the great agents of change in the earth’s history. The func- tion of the atmosphere in sustaining life and promoting all that depends on life is obvious. The hydrosphere. The water which lies upon the surface of the solid earth is about 1/4950 part of the earth’s mass. Were the solid part of the earth perfectly even, this amount of water would make a universal ocean a little less than two miles deep; but owing to the unevenness of the lithosphere, most of the water is gathered in the great basins which affect its surface. These basins are all connected, so that anything which changes the level of the water in one, changes it in all. GRAND DIVISIONS OF THE EARTH 5 The area of the oceans is estimated at 143,259,300 square miles, or about 72% of the earth’s surface. The area of the true oceanic basins is only about 133,000,000 square miles, but the basins are somewhat more than full, and the ocean water overflows them, lapping up on the continental shelves to the extent of more than I0,000,000 square miles. If the uppermost 600 feet of the ocean water were removed, the true ocean basins would be just full. About 4/s of the ocean has a depth of more than a mile, and more than half of it a depth exceeding two miles. Its greatest depth is nearly six miles, and its average about two and one-half miles. The shallow waters which lie upon the continental shelves, or extend into the interiors of the continents, such as the Baltic Sea and Hudson Bay, are epicontinental seas, for they lie upon the low borders of the continental platforms. Those detached bodies of water which occupy deep depressions in the surface are to be re- garded as true abysmal seas. Such, for example are the Mediter- ranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as low as many parts of the true ocean basin itself. Besides the oceans, the hydrosphere includes all the water of streams and lakes, together with that which is in the pores and fissures of the litho- sphere. The waters of the earth become a true hydrosphere only when the ground water is considered. All other waters of the earth are small in amount, compared with the ocean. Of all geological agents operating on the surface, water is the most obvious and apparently the greatest. Through rainfall, surface streams, underground waters, and waves, water is constantly modifying the surface of the lithosphere, most obviously by carry- ing sediment from the higher land and depositing it in the various basins. The hydrosphere is the great agency for the degradation of the land and the building up of the basin bottoms. The beds of sediment which it lays down follow one another in orderly succes- sion, each later one lying on an earlier. In this way, they form a time record. Relics (shells, bones, etc.) of the life of each age are embedded in the sediments, and record the history of life from age to age. The historical record of geology is dependent largely on the fact that the waters have buried, in systematic order, relics of the life of successive ages. The lithosphere. The atmosphere and hydrosphere are outer shells, rather than true spheres, though both penetrate the litho- sphere to some extent. The lithosphere, on the other hand, is an 6 PRELIMINARY OUTLINE oblate spheroid with a polar diameter of 7,899.7 miles, and an equatorial diameter of about 26.8 miles more. Its equatorial cir- cumference is 24,902 miles, its meridional circumference 24,860 miles, and its surface area about 196,940,700 square miles. Its average specific gravity is about 5.57. ‘The oblateness of the spheroid is the result of the rotation of the earth. The earth is not a perfect spheroid. Its equatorial diameters are not exactly equal, and the continental protuberances are, on the average, some three miles above the bottoms of the oceans. The forces or agencies which produced the continental platforms and abysmal basins, and the great undulations, foldings, and volcanic extrusions of both, are yet subjects of debate. It is customary to look upon the continents as the great features of the earth’s surface, but in reality the oceanic depressions are the master feature. They exceed the continental protrusions in breadth, and they are much farther below sea-level than the continents are above it. If the earth be regarded as a shrunken body, the settling of the ocean bottoms has doubtless been the greatest diastrophic movement. The following table shows the relative areas of the lithosphere above, below, and between certain levels. Per cent More than 6,000 feet above sea-level. ...........0.0+0+eevusees 203 Between sea-level and 6,000 feet above.............+.0+s00e uae 2555 Between sea-level and 6,000 feet below... ............-.0+acaee 14.8 Between 6,000 and 12,000 feet below sea-level.................. 14.8 Between 12,000 and 18,000 feet below sea-level...............-. 39.4 Between 18,000 feet and 24,000 feet... ....6..1+) os cee ge From these estimates it appears that if the surface of the litho- sphere were graded to a common level by cutting away the conti- nental platforms and dumping the material in the ocean basins, bringing all to a common level, this level would be about 9,000 feet below sea-level. The continental platforms may be conceived as rising from this common plane rather than from the sea-level. The bottoms of the ocean basins have broad undulations ranging through many thousands of feet; but they have not those irregu- larities of form that give variety to land surfaces. The ocean bot- toms are also diversified by volcanic peaks, many of which consti- tute islands. From many of them, the solid surface slopes down rapidly to abysmal depths. Many of the volcanic islands are GRAND DIVISIONS OF THE EARTH 7 isolated mountains whose heights and slopes would seem extraordi- nary, if the ocean were removed. The surface of the land is diversified similarly by broad undula- tions and volcanic peaks, as well as by narrower wrinklings and foldings of the crust, and ali of these irregularities have been carved into varied and picturesque forms by erosion. In this respect, the land differs radically from the bed of the sea. The outer part of the lithosphere is often called the crust of the earth. The old notion that it was the solid portion overlying a liquid part beneath is now generally abandoned. The crust is merely the outer, cooler portion of the lithosphere. Its thickness is undefined, but a shell several miles thick, and perhaps a few score miles, is penerally meant when this term is used. Materia's of the lithosphere. Mantle rock. The great ei of the lithosphere probably is composed of solid rock, but the solid rock is very generally covered by a layer of loose material such as soil, clay, sand, gravel, and broken rock, known collectively as mantle rock. The mantle rock of many places consists of the decayed products of underlying rocks. The upper part of mantle rock constantly is being blown away by wind and washed away by water, while the lower part is being renewed constzntly by the Fig. 1. Soil and subsoil arising from the decay of limestone resting on the sneven surface of the rock beneath. Southeastern Missouri. (Buckley.) 8 PRELIMINARY OUTLINE decay of the rock below. The mantle rock of some other areas, as the northern part of North America and the northwestern part of Europe, consists chiefly of an irregular sheet of commingled clay, sand, gravel, and bowlders (drift) deposited by great glaciers, com- parable to that which now covers Greenland. In still other places, especially along the flood plains of streams, the mantle rock consists of deposits made by rivers. Along the shores of lakes and seas, there are beach gravels and sands. The thickness of the mantle rock varies from almost nothing to hundreds of feet (Fig. 1). Solid rock. Mantle rock is absent in some places, and there the surface of solid rock appears. It is common on the slopes of steep-sided valleys and mountains, on the slopes of cliffs which face seas or lakes, and in the channels of swift streams, especially where there are falls or rapids. In all lands inhabited by civilized peoples there are numerous wells and other excavations ranging from a few to several hundred feet in depth, and occasional wells and mine- shafts go much deeper. In these, and even in many of the shallower excavations, solid rock is encountered, and in most regions excava- tions as much as a few hundred feet deep reach it. We infer, there- fore, that solid rock is nowhere far below the surface. Varieties of solid rock. If the mantle rock were stripped from the land, the solid part beneath would be found to be made up of many kinds of rocks, all of which may be grouped into three classes. Fig. 2. Stratified rock. Trenton Limestone, Fort Snelling, Minn. (Calvin.) CLASSES OF ROCKS 9 By far the larger part of the land surface would be of stratified rock, and the remainder of rocks without distinct stratification. The latter are divided into two great groups, igneous rocks, and meta- mor phic rocks. The essential feature of stratified rock (Fig. 2) is its arrangement in layers. The layers may be distinct or indistinct, and thick or thin. In many cases thick layers are made up of many thinner ones. In composition, most stratified rock corresponds somewhat closely with sediments now being carried from land and deposited in the sea; that is, these rocks are made up of gravel, sand, or mud, the particles of which are cemented together. The bedded arrange- ment of stratified rocks and of recent sediments is the same, and the ee ay ae > ee ae i Fig. 3. Diagrammatic representation of the relations of igneous rock to stratified rock. The igneous rocks, represented in black, have been forced up from beneath. markings on the surfaces of the layers, such as ripple-marks, rill- marks, wave-marks, etc., are identical. Furthermore, many of the stratified rocks of the land, like the recent sediments of the sea, contain the shells and skeletons of animals, and some of them the impressions of plants. Many of the relics of life found in the strat- ified rocks belonged to animals or plants which lived in salt water. Because of their structure, their composition, their distinctive markings, and the remains of life which they contain, it is confi- dently inferred that many, if not most, of the stratified rocks which lie beneath the mantle rock of the land originally were laid down in beds beneath the sea, and that the familiar processes of the pres- ent time furnish the key to their origin. Igneous rocks may be defined as hardened lavas. They sustain various relations to stratified rocks, as illustrated by Fig. 3, in which some of the igneous rock is represented as lying beneath the stratified rock, some above it, and some interbedded with it, while some cuts across its layers. From these relations it is possible to tell some- 10 PRELIMINARY OUTLINE thing of the order in which the rocks were formed. Where stratified rocks are broken through by lavas, it is clear that the stratified rocks were formed first, and the lavas intruded later. Lava sheets intruded between beds of stratified rock can be told from those which flowed out on the surface and were subsequently buried, for Fig. 4. Metamorphic rock. (Ells. Can. Geol. Surv.) in the former case the sedimentary rocks, both above and below the igneous rock, were affected by the heat of the lava, while in the latter case only those below were so affected. Most metamorphic rock has cleavage; that is, a tendency to break in one direction rather than in another. The cleavage of metamorphic rock may look much like stratification, but it is really very different. The tendency to break along certain planes is not due to the fact that the rock was deposited in layers originally, as in the case of stratified rock, but is the result of the changes which the rock has undergone since it was formed. ‘The structure shown in Fig. 4 is known as schistosity —a structure characteristic of much metamorphic rock. Metamorphic rock may be derived from both igneous and sedimentary rocks. More commonly than otherwise, metamorphic rocks lie beneath THE EARTH’S INTERIOR Te sedimentary beds, or come to the surface from beneath them, Many of them are broken through by igneous rocks. Concerning the great interior of the earth, little is known except by inference. From the weight of the earth,' it is inferred that its interior is much more dense than its surface. From its . behavior under the attraction of other bodies, it is believed to be at least as rigid as steel. Its interior cannot, therefore, be liquid, in the usual sense of that term. From volcanoes, and from the tem- peratures in deep borings, it is inferred that the interior is very hot. 1 The specific gravity of its outer portion is about 2.7, less than half that of the earth as a whole (5.57). CHAPTER II THE GEOLOGIC WORK OF THE ATMOSPHERE Since the atmosphere is a part of the earth, its activities and its history are proper subjects of geologic study. As a part of geology, the study of the atmosphere is restricted, commonly, to its effects on the other parts of the earth. The origin and history of the atmosphere must, however, be considered, in any thorough- going history of the earth. In the history of the earth, the atmosphere has played a part comparable to that of water, though its record is less clear. Its direct work is partly (1) mechanical and partly (2) chemical. Its indirect effects are even more important, for it furnishes the condi- tions under which (3) the sun produces its temperature effects, and (4) evaporation and precipitation take place. The atmosphere, ' too, furnishes the necessary conditions for plants and animals, and the important influences that spring from them. MECHANICAL WORK The mechanical work of the atmosphere is accomplished chiefly through its movements. A feeble breeze moves particles of dust, a wind of moderate velocity blows dry sand, and exceptionally strong winds move small pebbles. The principal movement of the wind is horizontal; but every obstacle against which it blows deflects some of the air, and some of it is deflected upward. Furthermore, there are exceptional winds, in which the vertical element predominates. Particles of dust are caught by these upward currents, and carried to great heights. This facilitates their transportation great distances. Dust.! Transportation of dust by the wind is nearly universal. No house, no room, and scarcely a drawer is so tightly closed but that dust enters, and the movements of dust in the open are much 1 Udden, Jour. of Geol., Vol. II, pp. 318-331; also Pop. Sci. Mo., Sept., 796. 12 MECHANICAL WORK ~ 13 more considerable. The dustiness of the atmosphere in dry regions during wind-storms is familiar proof of the efficiency of the wind as a carrier of dust. Under special circumstances, it is possible to determine roughly the distance and height to which dust is carried. In the great eruption of Krakatoa in 1883, large quantities of volcanic dust (pulverized lava) were shot up to great heights into the atmosphere. The coarser particles soon settled; but many of the finer ones, caught by the currents of the upper air, were carried around the earth in 15 days, and some of it traveled round the earth repeatedly. Its presence in the air was known by the historic red sunsets which it caused.! Dust from volcanoes is shot into the atmosphere rather than picked up by it. Dust picked up by the wind is perhaps transported as widely, but, after settling, its point of origin is less readily deter- mined. It would perhaps be an exaggeration to say that every Fig. 5. Vertical face of loess near Huang-tu-Chai in northern Shan-si. The vertical faces are the result of erosion. (Willis, Carnegie Institution.) square mile of land has particles of dust blown from every other square mile of dry land; but such a statement probably would in- volve much less exaggeration than might at first be supposed. 1A brief account of the influence of the dust on sunsets is found in Davis’s Elementary Meteorology, pp. 85 and 119. 14 GEOLOGIC WORK OF ATMOSPHERE Extensive deposits of wind-blown dust are known. Consider- able beds of volcanic dust, locally as much as 30 feet thick, are known in various parts of Kansas and Nebraska, hundreds of miles from the nearest volcanic vents. In some parts of China there is an extensive earthy formation, the loess (Fig. 5), in some places reaching a thickness of hundreds of feet, much of which is believed to have been deposited by the wind. ‘The loess of other regions has been referred to the same origin, and much of it is quite certainly eolian. From the flood plains of such rivers as the Missouri, clouds of dust are swept up and out over the adjacent high lands at the present time, especially when the surface of the flood plain has become dry after floods. This dust is very like loess, if, indeed, it is not loess. The transportation of dust is important wherever strong winds blow over dry surfaces free or nearly free of vegetation, and ccm- posed of earthy or sandy matter. Its effects may be seen in such regions as the sage-brush plains of western North America. The roots of the sage-brush hold the soil immediately about them, but between the clumps of brush where there is little other vegetation, the wind has in many places blown away the soil to such an extent that the base of each shrub stands up several inches, or even a foot or two, above its surroundings. Some of the mounds in this posi- tion are due partly to the lodgment of dust about the bushes (Fig. 6). Fig. 6. Shows the effect of sage-brush or other similar vegetation in holding sand or earth, or in causing its lodgment, in dry regions. EOLIAN SAND 5 Since dust is carried to a considerable extent in the upper air, its movements and its deposition are affected but little by obstacles on the surface of the land, and when it falls it is spread more or less uniformly over the surface. | Much of the dust transported by the wind is carried out over seas or lakes and falls into them, causing sedimentation over their bottoms. No determinations of the amount of dust blown into the sea have been made, but it is safe to say that, if such determination were possible, the result would be surprising. Sand. Winds do not commonly lift sand far above the surface of the land, and its movement is therefore interfered with seriously by surface obstacles. A shrub, a fence, a ETE SJL TRL DR co a ea or even a stone may occa- G 4) ( ) sion the lodgment of sand in quantity, though it has little effect on dust. If the obstacle which causes the lodgment of sand presents a surface which the wind can- not penetrate, such as a wall, sand is dropped abundantly both on its windward and leeward sides (Fig. 7); but if it be penetrable, like an open fence, the lodgment takes place chiefly to lee- ward. Fig. 7. Diagram to illustrate the effect of an obstacle on the transportation and deposition of sand. The direction of the wind is indicated by the upper arrow. The lower arrows represent the direction of eddies in the air, caused by the obstruction. If the surface in which the obstacle was set was originally flat (dotted line), the sand would tend to be piled up on either side at a little distance from it, but more to leeward. At the same time, a depression would be hol- lowed out near the obstacle itself on either side. (After Cornish.) In cultivated regions, cases are known where, in a. few weeks of dry weather, sand has drifted into lanes in the lee of hedges to the depth of two or three feet, making it difficult for vehicles to pass. Dunes. In contrast with eolian dust, much eolian sand is aggre- gated into mounds and ridges called dunes. Some dunes are 200 or 300 feet high, but many more are no more than ro or 20 feet in height. The shape of dunes depends, among other things, on the extent and form of the area furnishing the sand, the strength and direction of the wind, and the shape of the obstacles which occasion the lodgment. The shapes of the cross-sections of dunes are influ- enced by the strength and constancy of the winds. With constant 1 Geog. Jour., Apr., 1910, p. 379. 16 GEOLOGIC WORK OF ATMOSPHERE winds and abundant drifting sand, dunes are steep on the lee side (bc, Fig. 8), where the angle of slope rarely exceeds 25°. Under the same conditions, the windward slope is relatively gentle (ad). d o Fig. 8. Section of a dune showing, by the dotted line, the steep leeward (bc) and gentler windward (ab) slope. By reversal of the wind, the cross-section may be altered to the form shown by the line adc. (Cornish.) If the winds are variable, so that the windward slope of one time becomes the leeward slope of another, and vice versa, this form is not preserved. By reversal of the wind, the section abc may be changed to adc. Where the winds erode (scour) more than they deposit, other profiles are developed. The erosion profiles may be Fig. 9. Dunes at Longport, coast of New Jersey, showing the irregular forms developed by winds which erode. very irregular if the dunes are partially covered with vegetation (Fig. 9). Topography of dune areas. From what has been said, it is clear that the topography of dune regions varies widely, but it is always distinctive. Where the dunes take the form of ridges (Fig. 1, Pl. I), the ridges may be of essentially uniform height and width for con- THE CONTOUR MAP 17 siderable distances. If there are parallel ridges, they may be sep- arated by trough-like depressions. Where dunes assume the form of hillocks (Figs. 2 and 3, Pl. I) rather than ridges, the topography is even more distinctive. In some regions depressions (basins) are associated with the dune hillocks. In some places they are hardly less notable than the dunes themselves. THE TOPOGRAPHIC MAP Since dunes as well as other topographic features are conveniently represented on contour maps, and since such maps will be used frequently in the following pages, a general explanation of them is here introduced. “The features represented on the topographic map are of three distinct kinds: (1) inequalities of surface, called relief, as plains, plateaus, valleys, hills, and aN gin Ws eh \ U MASS 3 hs RHNUNS mR ni NN ) Sat Fig. ro. Sketch and map of the same area, to illustrate the representation of topography by means of contour lines. (U.S. Geol. Surv.) 18 GEOLOGIC WORK OF ATMOSPHERE mountains; (2) distribution of water, called drainage, as streams, lakes, and swamps; (3) the works of man, called culture, as roads, railroads, boundaries, villages, and cities. “Relief. All elevations are measured from mean sea-level. The heights of many points are accurately determined, and those which are most important are given on the map in figures. It is desirable, however, to give the elevation of all parts of the area mapped, to delineate the horizontal outline, or contour, of all slopes, and to indicate their grade or degree of steepness. This is done by lines connecting points of equal elevation above mean sea-level, the lines being drawn at regular vertical intervals. These lines are called contours, and the uniform vertical space between two contours is called the contour interval. On the maps of the United States Geological Survey the contours and elevations are printed in brown (see PI. I). ‘“The manner in which contours express elevation, form, and grade is shown in the preceding sketch and corresponding contour map, Fig. to. “The sketch represents a river valley between two hills. In the foreground is the sea, with a bay which is partly closed by a hooked sand-bar. On each side of the valley is a terrace. From the terrace on the right a hill rises gradually, while from that on the left the ground ascends steeply in a precipice. Contrasted with this precipice is the gentle descent of the slope at the left. In the map each of these features is indicated, directly beneath its position in the sketch, by contours. The following explanation may make clearer the manner in which contours delineate elevation, form, and grade: “y, A contour indicates approximately a certain height above sea-level. In this illustration the contour interval is 50 feet; therefore the contours are drawn at 50, 100, 150, 200 feet, and so on, above sea-level. Along the contour at 250 feet lie all points of the surface 250 feet above sea; along the contour at 200 feet, all points that are 200 feet above sea; and so on. In the space between any two contours are found elevations above the lower and below the higher con- tour. Thus the contour at 150 feet falls just below the edge of the terrace, while that at 200 feet lies above the terrace; therefore all points on the terrace are shown to be more than 150 but less than 200 feet above sea. The summit of the higher hill is stated to be 670 feet above sea; accordingly the contour at 650 feet sur- rounds it. In this illustration nearly all the contours are numbered. ‘Where this is not possible, certain contours — say every fifth one — are accentuated and numbered; the heights of others may then be ascertained by counting up or down from a numbered contour. “9. Contours define the forms of slopes. Since contours are continuous horizontal lines conforming to the surface of the ground, they wind smoothly about smooth surfaces, recede into all re-entrant angles of ravines, and project in passing about prominences. The relations of contour curves and angles to forms of the landscape can be traced in the map and sketch. “3. Contours show the approximate grade of any slope. The vertical space between two contours is the same, whether they lie along a cliff or on a gentle slope; but to rise a given height on a gentle slope one must go farther along the surface than on a steep slope, and therefore contours are far apart on gentle slopes and near together on steep ones. “For a flat or gently undulating country a small contour interval is used; for a steep or mountainous country a large interval is necessary The smallest’ DUNES 19 interval used on the atlas sheets of the Geological Survey is 5 feet. This is used for regions like the Mississippi delta and the Dismal Swamp. In mapping great mountain masses, like those in Colorado, the interval may be 250 feet. For intermediate relief contour intervals of 10, 20, 25, 50, and roo feet are used. “Drainage. Watercourses are indicated by blue lines. If the streams flow the year round the line is drawn unbroken, but if the channel is dry a part of the year the line is broken or dotted. Where a stream sinks and reappears at the surface, the supposed underground course is shown by a broken blue line. Lakes, marshes, and other bodies of water are also shown in blue, by appropriate con- ventional signs. “Culture. The works of man, such as roads, railroads, and towns, together with boundaries of townships, counties, and states, and artificial details, are printed i in.black.” From folio preface, U. S. Geol. Surv. Explanation of Plate I. In Fig. 1, Plate I (Five Mile Beach, 8 miles north- east of Cape May, N. J.), the contour eieeval is 10 feet. There is here but one contour line (the 1ro-foot contour), though this appears in several places. Since this line connects places 10 feet above sea-level, all places between it and the sea (or marsh) are less than 10 feet above the water, while all places within the lines have an elevation of more than 10 feet. None of them reaches an elevation of 20 feet, since a 20-foot contour does not appear. It will be seen that some of the elevations in Fig. 1 are elongate, while others have the forms of mounds. (From Cape May, N. J., Sheet, U. S. Geol. Surv.) Fig. 2 shows dune topography along the Arkansas River in Kansas (Larned Sheet), and Fig. 3, dune topography in Nebraska (Camp Clarke Sheet), not in immediate association with a valley or shore. In Fig. 2 the contour interval is 20 feet. All the small hillocks southeast of the river are dunes. Some of them are represented by one contour, and some by two. In Fig. 3, where the contour interval is also 20 feet, there are, besides the numerous hillocks, several depressions (basins). These are represented by hachures inside the contour lines. In some cases there are intermittent lakes (blue) in the depressions. There are two de- pression contours (4280 and 4260) within the contour of 4300, near Spring Lake. The bottom of the depression is therefore lower than 4260, but not so low as 4240. Migration of dunes.‘ By the transfer of sand from its windward to its leeward side, a dune is moved from one place to another, though continuing to be made up, in large part, of the same sand. In their migration, dunes may invade fertile lands, causing so great loss that means are devised for stopping them. ‘The simplest meth- od is to help vegetation to get a foothold in the sand. The effect of the vegetation is to pin the sand down. Where dunes migrate into a timbered region, they bury and kill the trees (Fig. 11). On the coast of Prussia a tall pine forest, cover- ing hundreds of acres, was destroyed between 1804 and 1827. At some points in New Jersey orchards have been so far buried within the lifetime of their owners that only the tops of the highest trees } Beadnell, Sand Dunes of the Libyan Desert, Geog. Jour., XX XV, 379, 1910. 20 GEOLOGIC WORK OF ATMOSPHERE Fig. 11. Lee side of a sand dune, Cape Henry, Va. The dune is advancing on a forest and burying the trees. (Hitchcock.) ; are exposed. Trees and other objects once buried may be discov- ered again by farther migration of the sand ! (Fig. 12). Fig. 12. A resurrected forest. After burying and killing the forest, the sand was blown away, exposing the dead trees. (Myers.) . 1 Cowles. The Ecological Relations of the Vegetation of the Sand Dunes of Lake Michigan. Botanical Gazette, Vol. XXVII, 1899. An excellent study of the relations of sand dunes and vegetation. = ; _ Angle soa PLATE | a4 Fig. <1.—Dunes on coast of New Jer- Q 8 ; = ‘s © : ° ties & > WS: Ry. > sey. Scale, about 1 mile per inch. Contour interval, Het a 10. feet. (Cape May Sheet, U. = ' ag Ses S. Geol. Surv.) @ x + Fie. 2—Dunes a- long Arkansas River in Kansas. Scale, about 2 Garfie fighd | 2A miles per inch. {ia ae? | Contour interval, fpr : 20 feet. (Larned p* Sheet, U.S.Geol. |. f Surv.) ; #! \\ -_ Fic. 3.—Dunes in plains of Nebraska. Scale, about 2 miles per inch. Contour interval, 20 feet. (Camp Clarke Sheet, U. 8S. Geol. Surv.) PLATE Il Streams disappearing in the sand, gravel, ete., at the base of mountains in an arid region. Scale, about 4 miles per inch. Contour interval, 200 feet. (Paradise, Nev., Sheet, U. S. Geol. Surv.) DUNES eH Distribution of dunes. Dunes are likely to be formed wherever dry sand is exposed to the wind. They are especially characteristic of the dry sandy shores of lakes and seas, of sandy valleys, and of arid, sandy plains. Along coasts, dunes are developed extensively only where the prevailing winds are on shore. Thus about Lake Michigan, where the prevailing winds are from the west, dunes are abundant and large on the east shore, and few and small on the west. Along valleys, dunes are most numerous on the far side as the prevailing winds blow. The dunes may be in the valleys, but in quite as many cases the sand is blown up out of the valley, and the dunes are on the bluffs above. Dunes probably reach their greatest development in the Sahara, but they are conspicuous in other arid sandy tracts, as in some parts of western Kansas and Nebraska, and in parts of Wyoming. Eolian sand is not all piled up into dunes. It may be spread somewhat evenly over the surface where it lodges. Eolian sand is much more widespread than dunes are. | Wind-ripples. The surface of the dry sand over which the wind has blown for a few hours is likely to be marked with ripples (Fig. 13). While the ripples are, as a rule, but a fraction of an inch high, they throw light on the origin of the great dune ridges. If Fig. 13. A ripple-marked sand dune in a western valley. (U.S. Geol. Surv.) 22 GEOLOGIC WORK OF ATMOSPHERE the ripples be watched closely as the wind blows, they are found to shift their position gradually. Sand is blown up the gentler wind- ward slope to the crest of the ridge, and falls down on the other side. Wear on the windward side may be about equal to deposition on the leeward, and the result is the orderly progression of the ripples in the direction in which the wind is blowing, just as in the case of dune ridges. Abrasion. While the effect of the wind on sandy and dusty surfaces is considerable, its effect on solid rock is slight, except ier pk MN tae Fig. 14. Wind erosion. Cave rocks near Sierra La Sal, in Dry Valley, Utah. (Cross, U. S. Geol. Surv.) where sand and dust are driven against it. Rock worn by wind- blown sand acquires a surface peculiar to the agent accomplishing the work. If the rock is made up of lamine of unequal hardness, the blown sand digs out the softer ones, leaving the harder ones to project as ridges. The sculpturing thus effected on projecting masses of rock is picturesque and striking in some cases (Figs. 14 and 15), and is most common in arid regions. Effect of wind on plants. Another effect of strong winds is seen in the uprooting of trees. The uprooting disturbs the surface, making the loose earth more readily accessible to wind and water. Organisms of various sorts (certain types of seeds, germs, etc.), as well as dust and sand, are transported far by wind. ~ CHEMICAL WORK 23 Fig. 15. Wind erosion. Casa Colorado, Dry Valley, Utah, between Abajo and La Sal Mountains. La Plata (Jurassic) sandstone. (Cross, U.S. Geol. Surv.) Indirect effects. Other dynamic processes are called into being by the atmosphere. Winds generate both waves and currents, which are effective agents in geologic work. The results of their activities are discussed elsewhere. CHEMICAL WORK The chemical work of the atmosphere is accomplished prin- cipally in connection with water. Dry air has little chemical effect on rocks or soils. ‘The important chemical changes wrought by the atmosphere are oxidation, carbonation, and hydration. Oxidation, as used in this connection, is the union of oxygen with some con- stituent of the rock, forming an oxide. Carbonation is a union of carbon dioxide of the air with constituents of the rock, forming carbonates. Hydration, similarly, is the union of water with con- stituents of the rock. Oxidation and hydration may go on at the same time. Thus when iron rusts, oxygen and water both enter into combination with the iron. In most cases these chemical changes result in breaking up the rock, much as steel or iron is 24 GEOLOGIC WORK OF ATMOSPHERE broken up when it rusts. A few other effects of the atmosphere may be noted. Precipitation from solution. The water in the soil is constantly evaporating. Such substances as it contains in solution are depos- ited where the water evaporates, and where evaporation is long Fig. 16. Erosional forms characteristic of dry regions where erosion by the wind is effective. Fissure Canyon, north slope of the La Sal Mountains, Utah. The rock is Permian. (Cross, U. S. Geol. Surv.) continued, without re-solution of the substances deposited, the surface becomes coated with an efflorescence of mineral matter. An illustration is found in the alkali plains of certain areas in the western part of the United States. Certain substances, deposited when the water which held them in solution is evaporated, coat the pebbles and stones of some arid plains. In some places gravel is thus cemented into conglomerate. . Conditions favorable. Conditions are not everywhere equally favorable for the chemical work of the atmosphere. Since high EFFECT OF CHANGING TEMPERATURES 25 temperatures facilitate chemical action, rocks are more readily decomposed by the chemical action of the atmosphere in warm than in cold regions. Changes of temperature tend to disrupt rock, and thus increase the amount of rock-surface exposed to chemical change. ‘The elements of the atmosphere are much more active chemically in moist than in dry regions. Though the chemical changes effected by the air are slow, their importance in the course of the earth’s long history has been very great. The amount of rock which has been thus disintegrated probably far exceeds all that is now above the sea. THE ATMOSPHERE AS A CONDITIONING AGENCY Temperature effects. Changes of temperature tend to break up rocks. The heating of rock by day and its cooling by night produce some such change in it as is produced by the quick heat- ing and cooling of glass. When the surface of the rock is heated, it expands, and a strain is set up between the hotter and more expand- ed part at the surface, and the cooler and less expanded part below.! This strain is enough to make the surface of the rock shell off in many cases. Daily variations in temperature are much more important than yearly variations, because they are much more common and take place more suddenly. Variations which do not involve the freezing of water are more important in long periods of time than those which do, because they are so very much more common. The daily range of temperature is influenced especially by (1) latitude, (2) altitude, and (3) humidity. (2) If other things were equal, the greatest daily ranges of temperature would be in low latitudes. (2) High altitudes favor great daily ranges of tem- perature, so far as the rock surface 1s concerned, for though the rock becomes heated during the sunny day, the thinness and dryness of the atmosphere allow the heat to radiate rapidly at night. Here, too, the daily range of temperature is likely to bring the wedge- work of ice into play. . Since the south side of a mountain (in the northern hemisphere) is heated more than the north, it is subject to the greater daily range of temperature, and the rock on this side suffers the greater disruption. Similarly, rock surfaces on which the sun shines daily are subject to greater disruption than those 1Tt is the change of temperature of the rock surface, not the change of temper- ature of the air above it, which is considered here. 26 GEOLOGIC WORK OF ATMOSPHERE ‘ much shielded by clouds. (3) The daily range of temperature is also influenced by humidity, a rock surface bécoming hotter by day and cooler by night beneath a dry’ atmosphere than beneath a moist one. Aridity, therefore, favors the disruption of rock by changing temperatures. The color of rock; its texture, and its composition also influence its range of daily temperature by in- fluencing absorption and conduction. ‘The disrupting ‘effects of changes of temperature are slight or nil where solid rock is pro- tected by soil, clay, sand, gravel, snow, or other incoherent material. In view of these considerations, the breaking of rock by changes of temperature should be greatest on the bare slopes of isolated elevations of rock, where the atmosphere is dry. All these conditions are not often found in one. place, but the disrupting Fig. 17. A mountain top, illustrating a common condition of the rock in mountain peaks. Med . 128 ns Sg EFFECT OF CHANGING TEMPERATURES ry effects of changing temperatures are best seen where several of them are associated. . The importance of this method of rock-breaking is rarely appre- ciated except by those familiar. with high and dry regions. Moun- tain climbers know that most high peaks are covered with broken rock to such an extent as to make their ascent dangerous to the uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule, literally crumbling to pieces (Fig. 17). The piles of talus which lie on the slopes and at the bases of steep mountains are in some cases hundreds of feet in height, and their materials are in Fig. 18. Serrate mountain peaks with abundant talus. Cascade Mts., Wash. large part the result of the process here under discussion. Masses of rock, scores and even hundreds of pounds in weight, are some- times detached in this way, and started downward, and small pieces are much more common. ‘The sharp peaks which mark the summits of most high mountain ranges (Fig. 18) are largely developed by the process here outlined. Even in low latitudes and moist climates the effects of temperature changes may be seen. For example, thin beds of limestone at the bottoms of quarries have been known to expand under the heat of the sun, so as to arch up and break. 28 GEOLOGIC WORK OF ATMOSPHERE The disruption of rock by changes of temperature is one phase of weathering. It tends to the formation of a mantle of rock waste, which, were it not removed, would soon completely cover the solid rock beneath and protect it from further disruption by heating and cooling; but the loose material thus produced becomes an easy prey to running water, so that the work of the atmosphere prepares the way for that of other eroding agencies. A thermal blanket. The atmosphere is a thermal blanket to the rest of the earth. Without it the heat of the sun would reach the earth with far greater intensity than now, and it would be radi- ated back from the surface almost as rapidly as received. During the night the earth would be far colder than any part of the earth is now. In passing through the atmosphere, parts of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is radiated back into the air by which it is absorbed and retained. ‘The air thus distributes and equalizes the temperature. The constituents of the atmosphere which are most efficient in this work are water vapor and carbon dioxide, and the climate of the earth is believed to have been greatly affected by the varying amounts of these constituents, as well as by variation in the total mass of the atmosphere. Evaporation and precipitation. Perhaps the most important work of the atmosphere as a geologic agent lies in its relation to the evaporation, circulation, and distribution of water. Atmos- pheric temperature is the primary factor governing evaporation, an important factor in the circulation of the vapor after it is formed, and controls its condensation and precipitation. Mechanical effects of rain. In falling, the rain washes the atmosphere, taking from it much of the dust which the winds have lifted from the surface of the dry land. Not only this, but in passing through the atmosphere, the water dissolves some of its gases, so that when the rain reaches the land, the water is no longer pure. The dissolved gases enable it to dissolve various mineral matters on which pure water has little effect. As it falls on the surface of the land, the rain produces various effects of a mechanical nature. (1) It leaves on the surface the solid matter taken from the air. (2) Clayey soils, baked under the influ- ence of the sun, are softened by the rain, and more easily eroded by running water. (3) Under the influence of the expansion and con- ‘traction caused by wetting and drying, the soils and earths on EFFECTS OF ELECTRICITY 29 slopes creep slowly downward. (4) When rain falls on dry sand or dust the cohesion is at once increased, and shifting by the wind is temporarily stopped. Effects of electricity. Another dynamic effect conditioned by the atmosphere is that produced by lightning. In the aggregate, this result is unimportant; yet instances are known where large bodies of rock have been fractured by a stroke of lightning, and masses many tons in weight have sometimes been moved appreciable distances. Incipient fusion in very limited spots is also known to have been induced by lightning. Thus where it strikes sand it may fuse the sand for a short distance, and, on cooling, the partially fused material is consolidated, forming a little tube or irregular rod (a fulgurite) of partially glassy matter. Fulgurites are usually but a few inches long, and more commonly than otherwise a fraction of an inch in diameter. SUMMARY On the whole, the tendency of the work of the atmosphere, and of the work which is controlled by it, is to degrade the land, and to loosen materials of the surface so that they may be moved readily to lower levels by other agencies. The most important phase of the degradational work of the atmosphere is weathering, or the prep- aration of material for removal by other and more powerful agents of degradation. As we shall see, however, the atmosphere is not the only agent concerned in weathering. The wind has doubtless been an important agent in the trans- portation of dust and sand, wherever and whenever there was dry land, ever since an atmosphere has existed. If it has been as effective as now through all the untold millions of years since there have been land and atmosphere, the total amount of work which it must have done is past calculation. Wind-deposited sand, now cemented into solid rock, has been identified, even in very ancient formations. Laboratory work. The study of topographic and geologic maps, photographs, etc., illustrating wind work should be taken up in connection with this chapter. Plates XVI to XXII of Professional Paper 60 of the U. S. Geological Survey afford good illustrations of wind work. See also Interpretation of Topographic Maps, Exercise III, a laboratory manual (Henry Holt & Co.) which may be used with this text. CHAPTER III THE WORK OF GROUND (UNDERGROUND) WATER The average amount of precipitation on the land is estimated at about 40 inches per year. A part of this water sinks beneath the surface, a part forms pools or lakes, a part runs off at once, and a part of it is evaporated. The proportion of the rainfall which follows each of these courses depends on several conditions, among which are (1) the topography of the surface, (2) the rate of rainfall (or the rate at which snow melts), (3) the porosity of the soil or rock, (4) the amount of water which the soil contains when the rain falls or the snow melts, (5) the amount of vegetation on the surface, and (6) the dryness of the atmosphere. The steeper the slopes, the more rapid the rainfall, the less porous the soil, the wetter it is, and the less the vegetation, the more water will run off without sinking beneath the surface. The water which sinks into the ground becomes ground-water. The thousands of wells in lands peopled by civilized man, and the many springs which issue from the slopes of mountains and valleys prove that it is abundant and widely distributed. That ground-water is connected intimately with rainfall is shown by the following facts: (1) The level of water in wells com- monly sinks during droughts, and rises after rains; and the sinking is greater when the drought is long, and the rise greater when the rainfall is heavy. (2) Many springs discharge less water in times of drought, and others cease to flow altogether. (3) Rain-water is seen to sink beneath the surface, wherever the soil is porous. Sink- ing through the soil to the solid rock, it finds cracks and pores, and through them it descends to greater depths. Nowhere are the rocks which we see so compact and so free from cracks, when any con- siderable area is considered, as to prevent the sinking of water through them. The amount of ground-water in a given region does not depend entirely on the local rainfall. Ground-water is constantly moving, and some of it flows far from the place where it entered. Thus 30 GROUND-WATER SURFACE 31 beneath the Great Plains of the West there is much water which fell on the eastern slopes of the Rocky Mountains. It has flowed beneath the surface to the plains, where some of it is drawn out for purposes of irrigation in regions where rainfall is deficient. Ground-water surface. Water-table. Ifa well 60 feet deep fills with water up to a point 20 feet below the surface, it is because the material in which it is sunk is full of water up to that level. When the well is made, the water leaks into it, filling it up to the level to which the rock (or subsoil) is itself full. This level, below which the rock and subsoil (down to unknown depths) are full of water, is known as the ground-water surface, or water-table. In a flat region of uniform structure and composition, the ground-water surface is essentially level, though it rises during wet weather, and sinks in times of drought. Its rise is due simply to the descent of rain water; but its sinking is due to several things: (1) Where there is growing vegetation, its roots draw up water from beneath; (2) evaporation goes on independently of vegetation; (3) the water is drawn out through wells, mines, etc., and runs out as springs; and (4) it flows underground from places where the water surface is higher to those where it is lower. In these and other minor ways the ground-water surface is depressed. A well sunk to such a level as to be supplied with abundant water in a wet season may dry up during a period of drought, be- cause the ground-water level is depressed below its bottom. Thus either well shown in Fig. 19 will have water during a wet season when the water- level is at a; but well 1 will go dry when the water surface Fig. 19. Diagram illustrating the sinks to 0. fluctuation of the ground-water surfa-e; "Where the topography isnot Net weather ground-water level; b= flat, the ground-water surface is not level. As arule it is higher (though farther below the surface) under an elevation than under surrounding lowlands, as illustrated by Fig. 21. The reason is as follows: If a hill of sand is rained upon, most of the water falling on it sinksin. If the rain continues long enough the hill of sand will be filled with water, the water filling the spaces between the grains. The water in the hill tends to spread, but since the movement involves friction, the spreading 32 GROUND-WATER is slow. With the spreading, the surface of the water in the sand sinks, and sinks fastest at the center where it is highest. If no water were added, the surface of the water in the hill would, in Fig. 20. Diagram showing how rain-water, falling in one place, may flow under- ground to another and there be brought to the surface. The layer a is porous, and water entering it in the mountains follows it to the plain. . time, sink nearly to the level of the water in the surrounding land; but at every stage preceding the last, the surface of the water would be higher beneath the summit of the hill than elsewhere, though Fig. 21. Diagram illustrating the position of the ground-water surface (the dotted line) in a region of undulatory topography. farther from the surface. In regions of even moderate precipitation the water-surface beneath the hills rarely sinks to the level of that in the lowlands about them, before it is raised by further rains. The water-surface beneath lowlands also sinks. Some of the water finds its way into valleys, some of it sinks to greater depths, and some of it evaporates; but since the water-surface beneath the elevations sinks more rapidly than that beneath the lowlands, the two approach a common level. ‘Their difference will be least at the end of a long drought, and greatest just after heavy rains. Depth to which ground-water sinks. The depth to which ground-water sinks has not been determined by observation. The deepest excavations are but little more than a mile deep, and at this depth the limit of water is not reached. There is a popular belief that water sinks until it reaches a temperature sufficient to convert it into steam; but except in places where hot lava lies near the surface, this belief does not appear to be well founded. Its descent probably is stopped in quite another way. Water descends through the pores and cracks of soil and rock, MOVEMENT OF GROUND-WATER 33 and it doubtless goes down as far as they do. But it is probable that cracks do not go down more than a few miles, and that pores are limited to sirnilar depths. The reason for this is that rock, solid and unyielding as it seems, is yet mobile under sufficiently great pressure. If cracks or openings were formed in it at great depths, it is calculated that they could not persist, for the rock, under the pressure which exists there, would “flow” in and close them. The flow is, in effect, much like the flow of a stiff liquid. The outer zone of the earth, where cracks and cavities may persist, is the zone of fracture, and it is probable that the descent of water under ordinary conditions, is limited to this zone, variously estimated to have a depth of six to eleven miles.! Movement of ground-water.? Ground-water is in more or less continual movement. If all the water is pumped out of a well, it soon fills up again by inflow from the sides. Springs and flowing wells also demonstrate the movement of ground-water. Near the surface the movement is primarily downward if the rock through which it passes is equally permeable in all directions; but so soon as the descending water reaches the water-surface, its downward flow is checked, and its movement is partly lateral. Ground-water moves chiefly by slow percolation, for most of it is not organized into definite streams. Small streams are seen in some caves, and subterranean streams issue as springs in some places; but most streams which issue as springs probably have definite channels for short distances only, before they appear at the surface. The “reservoirs” from which artesian wells draw their supply are porous beds of rock, containing abundant water. As the supply is drawn off at one point, it is renewed by water entering elsewhere. Since the freedom of movement of ground-water is influenced greatly by the porosity of the rock, and since the rock is, on the average, most porous near the surface, the movement of ground-water is greatest near the surface, and less and less with increasing depth. Movement in the lower part of the subterranean hydrosphere doubtless is extremely slow. Amount of ground-water. The porosity of surface rocks varies 1 Some recent experiments suggest that, at high temperatures and under great pressures, water may enter into combination with rock material, with contraction of volume. If so, water im combination (not free) may perhaps go below the zone of fracture. Barus. Bull. 92, U. S. Geol. Surv. 2 For a full discussion of this subject see King, 19th Ann. Rept., U. S. Geol. Surv., Pt. II, and Slichter, Water Supply and Irrigation Paper 67, U. S. Geol. Surv. 34 GROUND-WATER widely, and the porosity of but few has been determined.!. From such determinations as have been made, it is estimated that the average porosity of the outer part of the lithosphere is somewhere between five and ten percent. If the porosity diminishes at a con- stant rate to a depth of six miles (where it becomes zero), the average porosity to this depth would be half the surface porosity. An average porosity of 214% would mean that the rock might contain enough water to form a layer nearly 800 feet deep, if brought out to the surface.’ It is probable that the porosity decreases in more than an arithmetic ratio, both because the deeper rocks are not so generally of porous kinds as those at the surface, and because of the pressure which tends to close openings. For this reason it may be that the figure given above is too large, even for the land. ‘The porosity beneath the sea is probably less than that beneath the land, so that for the earth, 800 feet is perhaps too high a figure, and is not to be regarded as a measurement. Fate of ground-water. Most ot the water which sinks into the earth reaches the surface again after a longer or shorter journey. Some of it is evaporated from the surface direcily, some is taken up by plants and passed by them into the atmosphere, some issues in the form of springs, some seeps out, some is drawn out through wells, and much of the remainder finds its way underground to the sea or to lakes, seeping out beneath them. A small portion of the descending water enters into combination with mineral matter. It does not necessarily follow, however, that the total supply of water is for this reason decreasing. Minerals once hydrated may be dehydrated, the water being set free. Furthermore, con- siderable quantities of water in the form of vapor issue from volca- noes, and some volcanic vents continue to steam long after volcanic action proper has ceased. It is probable that some, and perhaps much of the water issuing from these vents has never been at the surface before. The amount of water reaching the surface of the earth for the first time from volcanoes, may, so far as now known, 1 Buckley, Building and Ornamental Stones, Bull. IV, Wis. Surv.; Merrill, Stones for Building and Decoration. 2 Slichter estimates that the ground-water is sufficient in amount to cover the earth’s surface to a depth of 3,000 to 3,500 feet: Water Supply and Irrigation Paper No. 67, U.S. Geol. Surv. Earlier estimates gave still higher figures. Fuller, in a recent estimate, places the amount at about 1oo feet: Water Supply and Irrigation Paper 160, U. S. Geol. Surv. WORK OF GROUND-WATER 35 equal or even exceed the amount consumed in the hydration of minerals. | WORK OF GROUND-WATER Ground-water works chemically and mechanically, the chemical work being the more important. Chemical work. The chemical and chemico-physical action of ground-water may be grouped in several more or less distinct categories. 1. The simplest result is the solution of mineral matter. Pure water dissolves little mineral matter; but the carbon dioxide ex- tracted from the atmosphere, and the products of organic decay extracted from the soil, give the water added power to dissolve. The solvent work of ground-water is shown by the fact that all Fig. 22. Sections of petrified logs, near Holbrook, Ariz. Age probably Jurassic. water from springs and wells contains mineral matter, while rain water is essentially free from it. The subtraction of soluble matter from rock tends to make it porous, and helps it to decay. 2. One mineral substance in solution may be substituted for 36 GROUND-WATER another extracted from the rock. Thus the lime carbonate of e shell imbedded in rock may be removed, molecule by molecule, and some other substance, such as silica, left in its place. Wher the process is complete, the substance of the shell has been com- pletely removed, though its form and structure are preserved in the new material. Buried logs may be converted into stone by the substitution of mineral matter for the vegetable tissue (Fig. 22). 3. Materials dissolved from rock at one point may be de- posited in other rock elsewhere. ‘Thus a third type of change, addition, is effected. Rock may at one time and place be rendered porous by the subtraction of some of its substance, and the open- ings thus formed may later become the receptacles of deposits from solution. This is exemplified in the stalactitic deposits of many caves. Not uncommonly cracks and fissures are filled with mineral matter deposited by the waters which pass through them, making veins. 4. A further series of changes is effected by ground-water when the mineral matter it contains enters into combination with the mineral matter through which it passes. In the long course of time, changes of this sort may be so great as to change rock com- pletely. Importance of solution. Calculations have been made which illustrate in a measure the quantitative importance of solution by ground-water. Most of the mineral matter dissolved in streams was contributed by ground-water (springs, etc.) flowing to them, and the amount in stream water is determined readily. The Thames River drains an area only about one-tenth as large as the State of New York, but it is estimated to carry about 1,500 tons of mineral matter in solution to the sea daily. From the uppermost 20,000 square miles of its drainage basin, the Elbe is estimated to carry yearly about 1,370,000 tons of mineral matter in solution. Such figures make it clear that ground-water is an effective agent in the lowering of land surfaces. It is estimated that something like one-third as much matter is carried to the sea in solution as in sediment. The importance of the solution effected by ground-water is shown in another way. It is probable that most of the salt of the sea has been taken to it in solution by waters flowing from the land. The amount of salt is stupendous (Chapter VI). Furthermore, most of the limestone of the earth has been extracted from sea- PLATE Ill re 3 - f soe At Fe 3 TER & \ s 1 mS : es = “T : . \ \ ° i ; to HEX — fe NQ 8 ) ii Poe ae Shore of Lake Michigan just north of Chicago. Scale, about 1 mile per inch. Contour interval, 10 feet. (Highwood, II1., Sheet, U. 8..Geol. Surv.) PLATE IV A stream widening its valley by lateral planation. Scale, about 1 mile per inch. Contour interval, 20 feet. (Missouri, U. 8. Geol. Surv.) WORK OF GROUND-WATER 37 water, whither the larger part of it was carried by streams, and the aggregate amount of limestone is far greater than the amount of salt in the sea. Some other sorts of rock, such as gypsum, of less importance quantitatively, have had a similar history. In general, solution is probably most effective at a relatively slight distance below the surface. In the mantle rock, the materials are as a rule less soluble than below, for in many places they represent the residuum after the soluble parts of the formation from which they originated were dissolved out. Below this zone, the rock contains more soluble matter, and the water, charged with organic matter in its descent through the soil, is in condition to dissolve it. At still greater depths the water has become saturated to some extent, and, so far forth, less active. At great depths, too, the movement is less free. Increased pressure on the other hand facilitates solution at great depths. Deposition of mineral matter from solution. Mineral matter is deposited from solution under various conditions. (1) Some of it is deposited by evaporation. ‘This is shown where water seeps out on arid lands. (2) Reduction of temperature may occasion deposition. In general, hot water is a better solvent of mineral matter than cold,! and if hot water issues with abundant mineral matter in solution, some of it is likely to be precipitated on cooling. (3) Certain plants cause the precipitation of mineral matter from solution, as about some hot springs in which alge grow in profusion. These little plants are a chief factor in the deposits about the hot springs of Yellowstone Park.? (4) A fourth factor involved in the deposi- tion of mineral matter is relief of pressure. Pressure increases the solvent power of water directly; it also increases the amount of gas which may be dissolved, and this in turn increases the solvent power of the water for some minerals. As water charged with gas comes to the surface, pressure is lessened, and some of the gas escapes. In numerous cases, mineral matter is then precipitated. (5) Pre- cipitation is sometimes effected by the mingling of waters contain- ing different mineral substances in solution. Such mingling of solutions is most common along lines of ready subterranean flow, 1 This is not true in the case of minerals, such as the carbonates, dissolved and held in solution under the influence of gases dissolved in the water. 2Weed. The Formation of Hot Springs Deposits; Excursion to the Rocky Mountains, and Ninth Ann. Rept. U.S. Geol. Surv., pp. 613-76; and B. M. Davis, Science, Vol. VI, pp. 145-57, 1897. : 38 GROUND-WATER and while each portion of the water entering a crevice or porous bed might have been able to keep its own mineral matter in solu- tion, their mingling may involve chemical changes resulting in the formation of insoluble compounds, and therefore in deposition. This principle probably has been involved in the making of many veins of ore. The deposition of material held in solution is most notable at two zones, one below that of most active solution, and the other at the surface, where evaporation is greatest. Under proper con- ditions, however, deposition may take place at any level reached by water. Mechanical work. The mechanical work of ground-water is relatively unimportant. Where it flows in definite streams, the channels through which it flows are likely to be increased by me- chanical erosion as well as by solution. Either beneath the surface or after the streams issue, the mechanical sediment carried will be deposited. RESULTS OF THE WORK OF GROUND-WATER Weathering. Where the solvent work of ground-water is slight and equally distributed, its effect is to make the rock porous. If, for example, some of the cement of sandstone is dissolved, the rock becomes more porous; but if all the cement is removed, the Fig. 23. Diagram to illustrate the form and relations of caverns developed by solution. The black spaces represent caverns. Small limestone sinks are repre- sented at the surface where the roofs of caves have fallen in. rock is changed to sand. If a complex crystalline rock contains among its minerals some one which is more soluble than the others, that one may be dissolved. ‘This has the effect of breaking up the WORK OF GROUND-WATER 39 rock, since each mineral acts as a binder for the rest. It may happen that no one of the minerals is dissolved completely, but that one or more of its constitutents is removed. Such change may cause the mineral to crumble, and so destroy the integrity of the rock. These are phases of weathering. Caverns.' In formations like limestone, which are relatively soluble, considerable quantities of material may be dissolved from a given place. Instead of making the rock porous, in the usual sense of the term, caverns are developed (Fig. 23). In their pro- duction, solution may be abetted by the mechanical action of the water passing through the openings which solution has developed. Caves are numerous in central Kentucky and southern Indiana, and the size of some of them, such as Mam- moth and Wyandotte, meevery “great. A ground-plan of Wyan- dotte (Ind.) Cave is shown in Fig. 24. The aeexceate length of its Fig. 24. Ground-plan of Wyandotte Cave. passageways 1S a numM- The unshaded areas represent the passageways. ber of miles. (21st Ann. Rept., Ind. Geol. Surv.) Deposition may take place in caves after they are formed (Fig. 25), or it may even go on at the same time that the cave is being excavated. Stalactites and stalagmites are common forms of cave deposits. A stalactite may start from a drop of water leaking through the roof of the cave. Evaporation, or the escape of gases in solution, results in the deposi- tion of some of the lime carbonate about the margin of the drop, in the form of a ring. Successive drops make successive deposits on the lower edge of the ring, which grows downward into a hollow tube through which descending water passes, making its chief de- posits at the end. Deposition in the tube ultimately may close it, while deposition on the outside, due to the water trickling down in that position enlarges it. Limestone sinks. Underground caves give rise to topographic features of local importance. If the roof of a cavern collapses, it causes a sink or depression in the surface. Some regions of lime- stone caves are affected by numerous sinks formed in this way. 1 For a racy account of caverns see Shaler’s Aspects of the Earth. 40 GROUND-WATER These limestone sinks (Figs. 26 and 23) as they are called, are con- spicuous in the cave region of Kentucky, and are well known in many other limestone districts. Some limestone sinks are made in other ways. Creep, slumps, and landslides. When the soil and subsoil on a slope become charged with water, they tend to move downward. When the movement is too slow to be sensible it is called creep; when rapid enough to be sensible, the material is said to slump or slide. This may happen when the slope on which water- charged mantle- rock lies is steep (Fig. 27). Some landslides have _ done great damage. Where a_ stream’s bank are high, and of unindurated ma- terial, such as clay, — considerable masses sometimes slump Fig. 25. Stalactites and stalagmites in Marengo from the. bank into Cave, southern Indiana. (Hains.) the river, or settle away slowly from their former positions. The same thing takes place on a larger scale on the slopes of steep mountains.! In creep and in landslides gravity is the force involved, and the ground-water only a condition which makes gravity effective. ORE-DEPOSITS Many ore-deposits are but a special result of the chemical work of ground-water, and are of interest because of their industrial — value. An ore is a rock that contains a metal that can be extracted. profitably, though the term is often extended to include unwork- » 1 Russell has emphasized this point in 20th Ann. U. S. Geol. Surv., Pt. Li: pp. 193-202, and Cross, 21st Ann. U. S. Geol. Surv., Pt. Il, pp. 129-150. Fig. 25. A sinkhole of recent development near Meade, Kan. (Johnson, U.S. - Geol. Surv.) _ Fig. 27. South face of Landslip Mountain, Colo. The protruding mass on the right has slumped down. (U.S. Geol. Surv.) 42 GROUND-WATER able, lean bodies of ore material. The metal need not preponderate, or form any fixed percentage of the whole. Little gold ore contains more than a very small fraction of one per cent of the precious metal, while high-grade iron ore yields sixty-odd per cent of the metal. In iron ore, the metallic oxide or carbonate makes up nearly the whole rock; in gold ore, the metal is one of the least abundant constituents. Metals are disseminated widely through the rock substance of the earth, and even through the hydrosphere; but in their dis- seminated condition they are not ores. The concentration of the metals into workable richness, in accessible places, is the essential thing in the formation of ores. The degree of concentration re- quired is measured by the value of the metal. The chief points about ores to be considered in connection with ground-water are (1) the original distribution of the metallic materials, (2) their solution by circulating waters (or, rarely, by other means), (3) their transporta- tion in solution to the place of deposit, (4) their precipitation in concentrated form, and (5) perhaps their further concentration and purification by subsequent processes. Ores which originated in volcanic intrusions or from waters derived from lavas (magmatic waters) are mentioned but briefly here. Original distribution of ore material. For present purposes it is sufficient to regard all rocks concerned in ore-deposition as either igneous or sedimentary, and to inquire, first, how far ordinary igneous and sedimentary processes contribute to the segregation of ore material; and second, what the subsequent processes of local concentration are. Magmatic segregation. The segregation of metals in lava is known as magmatic segregation. In some instances masses of iron ore seem to have originated in this way. It is not improbable that the segregation of metallic iron and nickel, and perhaps other metals, may be common in the deeper parts of the earth, but it is not clear that many known ores originated in this way. It is probable, however, that there may be some segregation of metallic substances in lavas. While this segregation may not be rich enough to make ore, it may determine the places where subsequent concentration takes place, by the help of ground-water. Marine segregation and dispersion. In the formation of the sedimentary rocks there was notable metallic enrichment in some places. The ground-waters of the land, after their subterranean ORE-DEPOSITS 43 circuits, carried to the seas various metallic substances in solution. In the main these substances appear to have been widely diffused, and to have been distributed very sparsely through the sediments, for sediments seem to contain less ore material than igneous rocks. There are however important exceptions to this general rule of sedimentary leanness. The iron-ore beds of Clinton age ranging from New York to Alabama, and appearing also in Wisconsin and Nova Scotia, form a stratum in the midst of ordinary sediments, and contain marine fossils. ‘The great iron-ore beds of Lake Superior also were sedi- mentary in origin, and so, probably, were most other important iron deposits. Not all sedimentary iron-ore deposits are of marine origin, and most of them are not clastic. Many of the sedimentary iron ores have been changed greatly from the condition in which the ferruginous matter was first deposited. In this change, ground- _ water has been the chief agent. Beds of clastic iron ore are known in Europe. The ore matter was in older rocks, and was segregated, mechanically, during sedimentation, because it was much heavier than other contemporaneous sediments. Its superior weight had much the same effect as greater coarseness. Some limestones appear to have been enriched locally, in a lean way, in lead and zinc, and rarely in copper, in the course of their formation. This lean enrichment at the time of deposition probably determined the development of ore regions later. The lead and zinc ore regions of the Mississippi basin have been regarded as areas of this sort, the subsequent concentration of the metal into ores being the work of ground-water. The lean enrichment accompanying sedimentation has been attributed to solutions of the metals brought to the sea from neighboring lands, the metals being then precipi- tated by organic action in the sea-water.'| This organic action may have been more effective in some areas than in others, because of the unequal distribution of life and the concentration of its decaying products.. Since it is reasonable to suppose that land-waters, on reaching the margins of the water-basins, must here and there find con- ditions favorable for the precipitation of their metallic contents, it is inferred that while the processes of sedimentation tended on the whole to leanness, they gave rise to (1) some very important ore-deposits, notably many iron ores, the greatest of all ores in 1 Chamberlin. Geol. of Wis., Vol. IV, p. 5909, et seq., 1882. 44 GROUND-WATER quantity and in industrial value, and (2) a lean enrichment of the sediments of certain other areas which, after subsequent processes of concentration of the metals by ground-water, became productive. | Origin of ore regions. From these considerations it appears that the fundamental explanation of many ‘‘mining regions” is to be found in (1) magmatic segregation, so far as the country rock is igneous, and (2) enrichment during sedimentation, so far as the rock is secondary. Either of these processes may, in rare cases, give rise to ores directly; but in most cases, further concentration of the metallic substances is necessary. This concentration is effected in various ways by the help of ground-water. 1. Surface concentration. The simplest of all modes of con- centration takes place in the formation of mantle-rock. An in- soluble or slightly soluble metallic substance sparsely distributed through rock may be concentrated to working value by the decay and removal of the principal rock material, leaving the metallic matter in the residuary mantle. The tin ores of the Malay penin- sula! are examples. Crystals of tin oxide were originally scattered sparsely through granite and limestone. By the decay and partial removal of the rock, the crystals have accumulated in workable quantities. Certain gold fields and certain iron ores have acquired higher value in the same way; also certain ores of manganese, as those of Arkansas. Such residuary ores may be further concen- trated by running water, because the greater weight of the metals causes them to be left behind when the lighter substances are washed away, or because their greater weight causes them to be partially separated from the other sediments, in deposition. Gold placers are the best example. 2. Purification. A different mode of concentration and puri- fication has affected some of the great iron deposits. As already stated, the iron compounds were originally parts of a sedimentary formation, and in beds. In some cases they were sufficiently pure, as first deposited, to be worked profitably; but in most cases they were affected by impurities. From such deposits the impurities have been dissolved by the percolation of waters, and at the same time, more of the valuable metal has been added. The great Bessemer iron-ore deposits of Lake Superior are examples. Originally impure silicates or carbonates, they have been converted into rich and 1Penrose. Jour. of Geol., Vol. XI, pp. 135-155, 1903. ORE-DEPOSITS 4s phenomenally pure ferric oxides by ground-water. There are vast quantities of lean ores in the same region not thus purified and enriched.! 3. Solution and re-precipitation. Ore material may be leached out of the surface-rock by water circulating slowly through it, and carried on until it reaches some substance which causes a reaction that precipitates the metallic matter. This substance may be a constituent of some rock which the circulating water encounters; but more commonly, the precipitation seems to be due to the mingling of waters charged with different mineral substances, the mingling inducing reactions which result in the precipitation of the ore. Precipitation does not necessarily follow such commin- gling; it takes place only when the mingling waters reduce the solubility of the ore material sufficiently. Changes of pressure and temperature also may enter into the process. Otherwise stated, the general process of underground ore forma- tion appears to be this: The permeating waters dissolve the ore material disseminated through the rock, and carry it thence into the main channels of circulation, usually the fissures, porous parts, or cavernous spaces. If precipitating conditions are found there, deposition takes place. The precipitating conditions may be merely changes of physical state, such as cooling or relief of pres- sure; but probably much more generally they are found in the commingling and mutual reaction of waters that have pursued different courses, and are differently mineralized. Location of greatest solution. Water circulation is probably very slight below the depth of a mile or two, and above that depth there is little reason for supposing that the rocks of one horizon are more metalliferous than others of their kind. Thus there is no assignable reason why the igneous or sedimentary rocks at the sur- face are not as rich in ore material as the igneous rocks two or three miles below. For a given amount of water, solvent action is prob- ably greatest where the temperature and pressure are highest, that is, in the deeper reaches of water circulation; but the amount of water passing in and out of the deeper zone is small compared with that of higher levels, and the total solvent action is quite certainly much greater in the upper zone than in the lower. At thesame time, the solutions in the upper zone are quite certainly more dilute than those below. The horizon of greatest solution doubtless lies be- 1 Van Hise & Leith, various monographs of the U. S. Geol. Surv. 46 GROUND-WATER tween the surface and a level slightly below the ground-water sur- face (p. 31); in other words, in the zone where atmosphere and hydrosphere co-operate. Surface-waters are charged with atmos- pheric and organic acids and other solvents, and their general effect upon the rocks is markedly solvent down to and somewhat below the permanent water-level. Concentration by residual accumulation may take place in this zone, as already noted, if the metallic compounds resist solution; otherwise this zone is depleted of its ore material by solution, and preparation is made for deposition elsewhere. Solution also continues to take place varyingly as the water descends below this zone of dominant solution, and extends prob- ably to the full depth of water circulation; but in the deeper circuit, precipitation also takes place, and with the waters taking up and throwing down material at the same time, it is difficult to estimate the balance of results. It is probable, however, that the result of ~ these processes is to promote the development of the higher ore values at levels near enough the surface to be accessible, and along the main lines of ground-water circulation. Influence of contacts. As many ore-deposits depend on a dis- solving state of the waters followed by a depositing state, it is obvious that conditions which favor changes of state and the com- mingling of different: kinds of water, are apt to be favorable to ore production. At any rate it is observed that many important ore- deposits occur at the contact of unlike formations, as for example at the contact of igneous rock with limestone. It is not to be in- ferred that such contacts are generally accompanied by workable ore-deposits, but merely that a notable proportion of workable ore-deposits occur at such junctions. It is rational to suppose that where the chemical nature of the two formations is in contrast, the waters that percolate through the one are likely to be mineralized very differently from those that course through the other, and that on mingling at the contact, reactions are liable to take place. When a valuable metallic substance is present, it may be involved and, by chance, suffer precipitation. Reactions are the more probable because the contact plane of formations is, in some cases, a plane of crustal movement, and hence more or less open and accom- panied by fractures, zones of crushed rock, and other conditions that facilitate circulation and offer suitable places for ore formation. The effect of igneous intrusions. A special case of much im- ORE-DEPOSITS 47 portance arises where lavas are intruded into sediments that have previously been partially enriched in the ways described above. The igneous intrusion not only introduces new contact zones, and more or less fracturing, but it brings into play hot waters with their intensified solvent work, their more active circulation, and the reaction between waters of different temperatures. The special efficiency of these agencies is believed to be important in many cases. Furthermore the intruded lava may be rich in metallic substances, and so be a favorable site for later concentration. The magmatic waters themselves appear to be a source of important ore-deposits, as already noted, and the present tendency is to attach more and more importance to them. Ores deposited by magmatic waters are, in a sense, the product of magmatic segrega- tion (p. 42). The influence of rock walls. The rock walls themselves are thought to be a factor, in some cases, in the reactions which pre- cipitate ores. It appears that the effect of the wall may be to with- draw some constituent of the passing solution, and destroy its equilibrium in such a way as to cause the precipitation of metallic constituents. Once deposited on the walls, ore aids the further accretion of matter of the same sort. The effect of the rock wall here noted is sometimes called mass action. The special forms assumed by ores deposited from solution underground (veins, beds, etc.), are incidental to the local situation in which the precipitation takes place. SUMMARY All in all, ground-water is to be looked upon as a most important geological agent. When it is remembered that a very large part of all the water which falls on the surface of the earth, either in the form of rain or snow, sinks beneath the surface; that some of it sinks to a great depth; that much of it has a long underground course before it reappears at the surface; that it is everywhere and always active, either in subiracting from the rock through which it passes, in adding to it, in effecting the substitution of one mineral substance for another, or in bringing about new chemical combinations; and when it is remembered that these processes have been going on for untold millions of years, it will be seen that the total result accomplished must be great. The rock formations of the earth to the depths to which ground-water penetrates, are to Brg GROUND-WATER be looked upon as a sort of chemical laboratory through which waters are circulating in all directions, charged with many sorts of mineral substances. Some of the substances in solution are de- posited beneath the surface, and some are brought to the surface’ where the waters issue. Much of the material brought to the sur- face in solution is carried to the sea and utilized by marine organ- isms in the making of shells. Without the mineral matter brought to the sea by springs and river, many shell-bearing animals of great importance, geologically, would perish. Biologically, there- fore, as well as geologically, ground-water is of great importance. It is also of prime importance in the development of ores. SPRINGS AND ARTESIAN WELLS Springs. The term spring is applied to any water which issues from beneath the surface with volume enough to form a distinct current. If water issues so slowly as merely to keep the surface moist, it is seepage but not a spring. Many springs issue from the sides of valleys (Fig. 28), the bot- toms of which are below ground-water level. T hey are especially likely to issue at the surface of relatively impervious layers, and where the valley slopes cut joints, porous beds, or other structures which allow free flow of ground-water. Springs are classified in various ways, and the several classifi- cations suggest characteristics worthy of note. They are some- times classed as deep and shallow, but the idea involved in this Fig. 28. Diagram showing conditions favorable for springs, in the side of a valley. P, porous rock, and I, impervious. Rain-water sinks to I, and, se along its surface, comes out as springs at S and S. grouping would be better expressed by strong and feeble. They are also classed as cold and thermal, the latter term meaning that the temperature is such as to make the springs seem warm or hot. The temperature of thermal springs ranges up to the boiling-point of water. Again, some springs are continuous in their flow, while others are intermittent. Most intermittent springs flow after periods of rain, but dry up during droughts. Springs are also classified as SPRINGS AND GEYSERS 49 mineral and common. Mineral springs, in the popular sense of the term, are of two types: (1) Those which contain an unusual amount of mineral matter, and (2) those which contain some unusual min- eral. All springs which are not mineral are common. This classi- fication is not very significant, for all springs contain more or less mineral matter, and many springs which are ‘‘common”’ contain more mineral matter than some which are ‘‘mineral.”’ Mineral springs are themselves classified according to the kind and amount of mineral matter they contain. Thus saline springs contain salt; sulphur springs contain compounds (especially gaseous) of sulphur; calcareous springs contain abundant lime carbonate, etc. Medicinal springs are those which contain some substance which has, or is supposed to have, curative properties. Geysers. Geysers are intermittently eruptive hot springs. They occur only in volcanic regions (past or present), and in but few of them, being known only in the Yellowstone National Park, Iceland, and New Zealand. The cause of the eruption is steam. The surface-water sinks down until, at some unknown depth, it comes in contact with rock sufficiently hot to boil it. The source of the heat is not open to inspection, but it is believed to be the uncooled part of extruded or intruded lava. From what was said earlier in this chapter it is clear that geysers do not have their origin in water which sinks down to the zone of great heat, where the downward increment of heat is normal. The water of a geyser issues through a tube of unknown length. Whether the tube is open down to the source of the heat is not deter- minable, but water from such a source finds its way to the tube. Water may enter the tube from all sides and at various levels. The heating may precede or follow its entrance into the tube, or both. So far as the water is heated after it enters the tube, the point of most rapid heating may be at the bottom of the tube, or at some point above. If the water were converted into steam as fast as it enters the tube, steam would escape continuously, and there would be no geyser; but if the rock is only hot enough to bring the water in the tube to the boiling-point after some lapse of time, and after a good deal of water has accumulated, an eruption is possible. The exact sequence of events which leads to an eruption is not known, but a definite conception of the principles involved may 50 GROUND-WATER be secured by a definite case. Suppose a geyser-tube full of water and heated at its lower end. As the water is heated below, con- vection tends to distribute the heat throughout the column of water above. If convection were free and the tube short, the result would be a boiling spring; but if the tube is long, and especially if Fig. 29. Giant Geyser, Yellowstone National Park. (Wineman.) convection is impeded, the water at some level below the surface may be brought to the boiling-point earlier than at the top. If even a little water in the lower part of the tube is converted into steam, the steam will raise the column of water above, and it will overflow. The overflow relieves the pressure on all parts of the column of water below the surface. If before the overflow there was any considerable volume of water essentially ready to boil, the relief of pressure following the overflow might allow it to be converted into steam suddenly, and the sudden conversion of a considerable quantity of water into steam would cause the eruption of all the water above it (Fig. 29). The height to which the water SPRINGS AND GEYSERS 51 would be thrown depends upon the amount of steam, the size and straightness of the tube, etc. It is clear that everything which impedes convection in the geyser tube will hasten the period of eruption, since im- peded circulation will have the effect of holding the hot water down, and so of bringing the water at some level below the top more quickly to boiling. It follows that anything which chokes the tube, or which in- creases the viscosity of the water, hastens an eruption.! Some. geysers build up crater-like basins or cones (Figs. 30 to 32) about them- selves, the cone being of mate- rial deposited from solution Fig. 30. The cone of Lone Star Gey- (p. 37). The brilliant polos apne National Park. (U. S. of some of the deposits about the springs in the Yellowstone Park are attributed to the little plants which cause the deposition. When the water from any geyser or hot spring ceases to flow, the plants die and the colors disappear. The heating of geyser water must cool the lava or other source of heat below. As this takes place, the time between eruptions becomes longer and longer. In the course of time, therefore, the geyser must cease to be eruptive, and when this change is brought about, the geyser becomes a hot spring. Within historic time several geysers in the Yellowstone Park have ceased to erupt and new ones have been developed. There are something like 3,000 vents of all sorts in this park, hot springs which are not eruptive greatly outnumbering geysers. A few geysers have somewhat definite periods of eruption. Of such ‘‘Old Faithful” is the type; but even this geyser, which formerly erupted at regular intervals of about an hour, is losing the reputation on which its name was based. Not only is its period of eruption lengthening, but it is becoming irregular, and the 1Weed. Am. Jour. Sci., Vol. XX XVII, 1889, pp. 351-59. 52 GROUND-WATER irregularity appears to be increasing. In the short time during which this geyser has been under observation its period has changed from.a regular one of 60 minutes, or a little less, to an irregular one of 60 to go minutes. In the case of some geysers, years elapse be- tween eruptions, and in some the date of the last eruption is so remote that it is uncertain whether the vent should be looked upon as a geyser or merely a hot spring. Fig. 31. Cone (or crater) of Grotto Geyser, Yellowstone Park. (Detroit Photo. Co.) ; Artesian wells. The terms artesian well and flowing well were synonymous originally; but any notably deep well is now called artesian. The artesian well which does not flow does not differ from a common well in principle, while the flowing well is really a gushing spring, the opening of which was made by man. Flowing wells ' depend upon certain relations of rock structure, water supply, and elevation. Generally speaking, a flowing well is possible in any place underlain by any considerable bed of porous 1 Chamberlin. Geol. of Wis., Vol. I, pp. 689-97, and Fifth Ann. Rept., U. S. Geol. Surv., pp. 131-73. The former a brief, and the latter an elaborate, exposi- tion of the principles involved. ARTESIAN WELLS 53 rock, if this rock outcrops at a sufficiently higher level in a region of adequate rainfall, and is covered by a layer or bed of relatively impervious rock. This statement involves four conditions, all of which are illustrated by Fig. 34, where a is the bed of porous rock. It is not necessary that the beds of rock form a basin, nor is it neces- Fig. 32. Deposit from a hot spring in Yellowstone Lake. (Fairbanks.) Fig. 33. Hot springs and geysers. Norris Geyser Basin, Yellowstone Park. 34 GROUND-WATER sary, commonly, to take account of the character of the rock be- neath the porous bed which contains the water. The bed of porous rock is the ‘‘reservoir” of the flowing well. Sand or sandstone, and gravel or conglomerate, most commonly Fig. 34. Diagrams illustrating conditions favorable for artesian wells. In A, the porous bed a is in the form of a basin; in B, it merely dips. serve as the reservoirs. In order that they may contain abundant water they must have considerable thickness, and their outcropping edges must be so situated that water may enter free- ly, and be replenished by rain as the water flows out at the well. A relatively imprevious layer of rock above the reservoir (a, Fig. 34) is most important; otherwise the water in the reservoir will leak out, and there will be little or no ‘‘head”’ at the well site. Thus if the rock overlying stratum a were badly broken, the fractures extending up to the surface, the conditions would be unfavorable for flowing wells, for though wells might get abundant i wee ete : water, they would not be (U. & e. Se eee Drees Ole likely to flow. If the stra-. | tum next below the reser- voir is not impervious, some lower one probably is. No layer of ARTESIAN WELLS 55 rock is more impervious than one which is full of water, and the substructure of any bed which might serve as a reservoir is usually full of water. If the outcrop of the reservoir is notably above the site of the well, and if it is kept full by frequent rains, the ‘‘head”’ will be strong, though the water at the well will not rise to the level of the outcrop of the reservoir. Experience has shown that an allowance of about one foot per mile of subterranean flow should be made. Thus if the site of a well is 100 miles from the outcrop of the water- bearing stratum, and 200 feet below it, the water will rise something like 100 feet above the surface at the well. This rule is, however, not applicable everywhere. The failure of the water to rise to the level of its head is due chiefly to the friction of flow through the rock. The more porous the rock the less the friction. The height of the flow is also influenced by the number of wells drawing on the same reservoir, on the degree of imperviousness of the confining bed above, etc. Flowing wells, many of them relatively shallow, are frequently obtained from unconsolidated drift. Map work. See Plates XC to XCIV of Professional Paper 60, U. S. Geological Survey, and Exercise IV, The Interpretation of Topographic Maps, a laboratory manual by Salisbury & Trowbridge. CHAPTER IV THE WORK OF RUNNING WATER Kivers are estimated to carry about 6,500 cubic miles of water to the ocean annually.! Since the average height of land is nearly half a mile, the waters which flow from it to the sea fall, on the average, nearly half a mile in their flow. Their total energy is therefore great, and they are.the great carriers of sediment from land to sea. The sediment which they carry is composed largely Fig. 36. Spokane River, 4 miles above Spokane, during flood. (Photo. by Tolman.) of decayed rock, but undecayed rock. is sometimes worn away, especially where streams are very swift. Though the flow of some streams is so gentle that they do not appear to work great changes in their valleys, others wear away their banks so rapidly that.the changes they produce may be seen from year to year, or, when the stream is in flood (Fig. 36), from day to day. Flooded streams occasionally sweep away dams, bridges, and buildings on their banks. The strong rods and beams of bridges and the steel rails of railways are bent almost as if they were twigs by the force of the occasional flood (Fig. 37). 1 Murray, Scot. Geog. Mag. Vol. ITI. p. 70. 56 SOURCE OF RIVER WATER 57 That the source of river water is the rain and snow which fall from the atmosphere may be inferred from various familiar phe- nomena. Thus (1) streams are more numerous in regions where Fig. 37. ‘Scene in the freight- ati of Kansas City after the flood of 1903. (U. S. Weather Bureau.) the rainfall is abundant than in those where it is scarce (Figs. 38-39); (2) multitudes of small streams spring into being with each heavy fall of rain and with each period of rapidly melting ee $s" SEY GASESURNY Fig. 38 Fig. 39 Fig. 38. Map showing the many streams of a humid region. Central Ken- tucky. The area is about 225 square miles. Fig. 39. Map showing the few streams of an arid region. Northern Arizona. The area is as great as that shown in Fig. 38. «8 WORK OF RUNNING WATER snow; (3) streams are notably swollen after rains, and most after heavy ones; and (4) many small streams which flow during wet weather dry up in times of drought, while others shrink. It is true that lakes, glaciers, and springs feed the rivers, but the lakes, glaciers, and springs derive their supply of water from precipitation. If the slope of a surface were perfectly even, the immediate run-off (the water which flows off without sinking beneath the surface) would flow in a sheet. There are slopes so smooth that water runs off them in this way; but on most slopes, even those which appear to be regular, there are small unevennesses, so that, although the run-off may start as a sheet, it is soon concentrated into rills and streamlets which follow the depressions. The smallest streamlets unite to form larger ones, and the little rills, after many unions with one another, reach valleys which have permanent streams. Streams which flow but part of the time, as after a rainstorm, dur- ing wet weather, or during but a part of the year, are temporary or intermittent streams. Every permanent stream and many temporary ones flow in depressions called valleys. Valleys are therefore about as numerous as streams. The very small de- pressions in which water runs. after showers only are called gullies if they are very small (Fig. 40), or ravines if oe ; somewhat larger. ie Cue a gully developed by a single shower. ae Ane valle. and just as the tiny streamlets unite to form creeks and these to form rivers, so the little gullies, in which the smallest temporary streams flow, generally unite to form wider and deeper ones (Fig. 41). These, in turn, join one another and become ravines, which are but larger depres- sions of the same sort, and ravines lead to valleys just as gullies EROSIVE WORK 59 lead to ravines. Valleys, like streams, usually end at the ocean or a lake; but in arid regions many of them end on dry land. There is, as a rule, some relation between the size of a valley and the stream which follows it, though this relation is not one which can be stated in mathematical terms. The large stream and the Fig. 41. Slope with numerous gullies, the smaller ones joining the larger ones. Scott’s Bluff, Neb. (U.S. Geol. Surv.) large valley go together so commonly, however, that the combina- tion cannot be accidental. EROSIVE WORK OF RUNNING WATER Wherever water flows over the land, it erodes the surface on which it flows, and the faster it flows, the greater its power of wear. ‘The rate of flow is determined chiefly by (1) the gradient (slope), (2) the amount and especially the depth of water, and (3) the amount of sediment (load) itis carrying. The steeper the gradi- ent, the deeper the water, and the less its load, the faster it flows. When it flows off in a sheet, as on a smooth surface, the depth of the water is slight, the flow not very swift (unless the slope is very steep), and the wear correspondingly slight. Such wear is sometimes called sheet erosion. 60 WORK OF RUNNING WATER The Development of Valleys The growth of gullies. 1. If the slope of the surface is not uni- form the effect is very different. If there is, for example, a slight depression near the base of the slope (Fig. 42), more of the descend- ing water flows through it than over other parts of the surface. The greater volume of water in the depression gives it greater velocity; greater velocity causes greater erosion, and greater erosion deepens the depression. The immediate result is a gully or wash (Fig. 40). The gully, once started, tends to concentrate drainage in itself still more, and it is thereby enlarged. The water which enters it from the sides widens it; that which enters at its head lengthens it by causing its upper end to advance up the slope; and all which flows through it deepens it. The enlarged gully will gather more water to itself, and, as before, increased volume means increased velocity and increased erosion. As the gully grows, therefore, its increased size becomes the occasion of still further growth, and the gully is transformed into a ravine, which is no more than an enlarged gully. But growth does not stop with the ravine. Water from every shower gathers in it, and growth con- tinues until it becomes a valley. It was assumed in Mae a> the preceding para- Fig. 42. Diagram showing a slight meridional graph that the single Seba! in the surface of an otherwise even-sloped depression in the slope ew was meridional (Fig. 42) and low on the slope; but almost any sort of depression in almost any position would bring about a similar result, since it would lead to concentration of the run-off. Had the original surface been marked by a single ridge instead of a depression, the effect on valley development would have been much the same, for a ridge, like a depression, would cause the concentration of the run-off along cer- tain lines, and therefore lead to the development of valleys. Under the conditions represented in Fig. 42 the lengthening of the drainage depression is effected chiefly at its upper end, the head of the valley working farther and farther back into the land. This method of lengthening is known as head erosion. But the lengthening of the valley is not always wholly by head erosion. The gully begins normally where concentration of run-off begins, DEVELOPMENT OF VALLEYS 61 and if this is not at sea-level, the gully may be lengthening at both ends at the same time. This would have been the case, for exam- ple, had the depression of Fig. 42 been half-way up the slope. Val- leys developed under the control of surface slope are consequent valleys, and their streams are consequent streams. 2. If the surface material of a slope is of unequal resistance, the water flowing over it will develop irregularities of slope, even if the slope was uniform at the outset. If the material of one part of a slope is less resistant than that elsewhere, the run-off will erode most there. The depression thus started will grow, and, as before, the gully may develop into a valley. In the presence of sufficient rainfall, therefore, either heterogeneity of slope or of material will cause the development of valleys. The permanent stream. It appears from the foregoing dis- cussion that a valley may be developed by the run-off of successive showers. If supplied from this source only, surface streams would cease to flow soon after the rain ceased to fall, and a valley might attain considerable size without possessing a permanent stream. The permanent stream is, as a rule, dependent on ground-water. When a valley has been deepened until its bottom is below the ground-water surface (p. 31), water seeps or flows into it from the sides. The valley is then no longer dependent on the run-off of showers for a stream. When the bottom of a valley is below the ground-water level of a wet season, without being below that of a dry one, it will have an intermittent stream. Many valleys are now in that stage of development where their streams are intermittent. As the valley of an intermittent stream becomes deeper, the periods when it is dry become shorter, and when it has been sunk below the ground-water level of droughts, it will have a permanent stream (3, Fig. 43). Since a valley normally develops headward, its lower and older portion is likely to have a permanent stream while its upper and younger part has only an intermittent one. So soon as a valley gets a permanent stream, the process of valley- enlargement goes on without the interruption to which it was sub- ject when the supply of water was intermittent. In general, a permanent stream at one point in a valley means a continuous stream from that point to the sea or lake to which the valley leads; but to this rule there are many exceptions, as where a stream heads in a region of abundant precipitation, and flows thence through an arid tract where the ground-water level is low 62 WORK OF RUNNING WATER and evaporation great. In such cases, evaporation and absorption may dissipate the water gathered above, and the stream disap- pears (Pl. II). A stream like the St. Lawrence, which carries water Fig. 43. Diagram to illustrate the intermittency of streams due to fluctuations of the ground-water level. The water level aa would be depressed next the valley 2-2, by the flow of water into the valley. The profile of the ground-water surface would therefore be aca and bdb rather than aa and 0b. from a great lake, does not depend on ground-water for its con- tinuous flow. Again, a stream which carries the water of a melting glacier may be permanent, even though not fed by springs. Other modes of valley development. Not all valleys are developed from gullies in the manner outlined above. 1. The out- Figs. 44 and 45. Diagram to illustrate one mode of valley lengthening. In Fig. 44 there are two small valleys, a and 6, and the former ends at the base of the steep slope. In Fig. 45, valley } is represented as having been lengthened so as to join a, and the two have become one. flow of a lake would develop a valley, and the valley might be in process of excavation all the way from the lake basin to the sea at the same time. A valley developed in this manner is not simply a gully grown big by head erosion, and the valley would not pre- cede the stream. If a narrow coastal plain is limited landward by a steeper slope, valieys might develop as shown in Figs. 44 and 45. Again, in some mountain regions valleys are formed by the up-folding of DEVELOPMENT OF VALLEYS 63 parallel mountain ridges, leaving a depression between (Fig. 46). Drainage will appropriate such a valley, so that it becomes in some sense a river valley; but it is not a river valley in the sense in which the term has been used in the pre- ceding pages. It is rather a structural val- ley. A river valley may be developed in its bottom (a, Fig. 46) and it may be in process of development throughout the whole length of the struc- tural valley at the same time. These illustrations do not exhaust the list of conditions under which valleys develop, but they suffice to show that valleys origi- nate and develop in different ways. Limits of growth. There are limits in depth, length, and width, beyond which a valley does not grow. A stream flowing to the sea tends to erode its valley to sea-level,’ but actually reaches the sea- level only near the coast. In length, the valley will grow as long as its head continues to work inland. If but a single valley affected a land area, the limit in length toward which it would tend would be the length of the land area in the direction of the valley’s axis. In general, valleys are limited in length by other valleys. The head of a valley works back until it reaches a point where erosion toward the valley in question is equal to erosion in the opposite direction. Here the divide becomes permanent (Fig. 47). i ee The width of a valley is in- so Se Op een creased chiefly by the side cut- Fig. AG racine , rare the . Owering Of a divide without shiiting it. ting of the stream, by the wash The crest of the divide is at a, b, and ¢ of the rain which falls on its successively. If the erosion was unequal slopes, and by the action of © the two sides, the divide would be AN ; shifted. gravity which tends to carry down to the bottom of the slope the material which is loosened above by any process whatsoever. The widening of valleys is limited Fig. 46. Structural valley with a river valley developing in its bottom. 1 Great rivers, like the Mississippi, cut their channels somewhat below sea-level, for miles above their debouchures. 64 WORK OF RUNNING WATER much as their lengthening is. Adjacent valleys grow wider until the tops of the intervening divides are reduced to lines. Then, if erosion is equal on the two sides, the divide is lowered without being shifted in position. The development of tributaries. Most considerable valleys have numerous tributaries. So soon as a gully is started, the water flowing into it from either side wears back the slopes. Any slight inequality of slope or material makes the erosion of the slopes unequal at different points, and unequal erosion in the slopes results in the development of tributary gullies. Some of these gullies develop into ravines and valleys, the same as their mains. Every new valley facilitates the run-off of the water which falls on the land, and so helps along erosion. Struggle for existence among valleys and streams. It is not to be inferred that every gully becomes a valley, nor that every small valley becomes a large one. The number of little gul- lies which develop on a slope may be very large (Fig. 41); but Fig. 48. Diagram illustrating how one gully takes the history of many others as a result of lateral erosion. The lines 1-4 of them is short. If represent, in cross-section, four stages in the develop- ‘ : ment of gullies a, b, and c. adjacent gullies are of unequal depth, the growth of the larger finally removes the divide between them, and they become one (Fig. 48). Again, a good map of the north shore of Lake Superior or the west shore of Lake Michigan shows a large number of small valleys and gullies (Pl. III). No equal stretch of coast has so great a number of large valleys. It therefore seems evident that of these many small valleys a few only will attain considerable size. Some young valleys work their heads back into the land faster than others, because of inequalities of slope and material. Ii valleys develop in ways other than by head erosion, the chances are also against their equal growth. If two streams, such as @ and c, Fig. 49, develop faster than the intermediate stream 8, it is clear that their tributaries may work back into the territory which at the outset drained into 6, so as to cut off the supply of water from the latter stream (compare a’b’c’, Fig. 50). As a result, DEVELOPMENT OF VALLEYS 65 the growth of 6 will be checked, and ultimately stopped. Sim- ilarly other valleys, such as f (Fig. 49), will get the better of their neighbors, and many of the competitors, as 0’, d’, e’, and g’ (Fig. 50), will soon drop out of the race. Between the stronger streams a @ 0 e 0 e pe bie 2 Fig. 50 eas es a 3 Fig. 51 Figs. 49, 50, and 51. Diagrams illustrating successive stages in the struggle for existence among streams. competition still goes on. If a’ and f’ (Fig. 50) develop faster than c’, its prospective drainage territory will be pre-empted by them (compare Figs. 50 and 51). Thus as the result of the unequal rate at which valleys are lengthened, the larger number of those which come into existence are arrested in their development. Piracy. Notall streams hold permanently the courses which they 66 WORK OF RUNNING WATER establish for themselves in youth. Thus the Potomac River deepened its valley across the Blue Ridge (Fig. 52) faster than Beaverdam Creek deepened its valley. The head of the young Shenandoah River worked back and tapped Beaverdam Creek, A= KIT TATINNY Fig. 52 Fig. 53 Figs. 52 and 53. ‘The capture of the head of Beaverdam Creek by the Shenan- doah River. Virginia-West Virginia. (After Willis.) diverting its head waters to the Potomac (Fig. 53). The Shenan- doah was a pirate, and Beaverdam Creek was beheaded. ‘The stream to which waters are diverted is increased in size, and the beheaded stream is correspondingly diminished. A Cycle of Erosion From what has preceded it is clear that the topography of a region undergoing erosion will change greatly from time to time. The first effect of erosion by running water is to roughen the sur- face by cutting out valleys, leaving ridges and hills. The final effect is to make it smooth again by cutting the ridges and hills down to the level of the valley bottoms. When this has been done the plain resulting is called a base-level. The time necessary to produce a base-level is a cycle of erosion. Base-level, peneplain, grade. The development of a base-level may be illustrated further in the light of the preceding discussion. Suppose a land surface affected by a series of parallel young valleys without tributaries (@ and b, Fig. 54). On either side of them CYCLE OF EROSION 67 there are upland plains or plateaus. The profile of the surface about two adjacent valleys is represented in cross section by the wwe ELTA SSA LD a OO OS a ee Gyr SES, ee ea, —— Se = Fig. 54. Diagram to illustrate the leveling of the surface by valley erosion. The profile represented at the top shows two young valleys, 1 and 1, in an otherwise flat surface. In time these valleys will develop the cross-sections represented by 2 and 2, and later those represented by 3 and 3, 4 and 4, etc. The divide between them may finally reach 5, and the surface is then nearly flat. uppermost line in Fig. 54. As the valleys a and 6 are widened to a’ and 0’, the adjacent uplands are narrowed correspondingly. When the valleys have attained the form represented by 3-3, the intervening upland has been narrowed to a ridge, and the valley flats have become wide. With continued erosion the ridge will be lowered still more, and in time the surface will approach a plain. In this condition it is known a peneplain. When the ridges are obliterated the peneplain passes into a base- leveled plain. Tributaries are almost sure to develop along each main valley and their heads work back across the up- Fig. 55. aes aoe tributaries in | lands between the main val- 2” ¢atly stage of development. leys, dissecting them into secondary ridges (Fig. 55). Tributaries develop on the tributaries, and these tertiary valleys dissect the secondary ridges into ridges of a lower order. This process of tributary development goes on until Sslomnesieh Ahi au the fourth, fifth, sixth, and higher orders BES a are ate (Fig. 56). Since the process of valley devel- opment under such circum- stances is also the process of ridge dissection, a stage is presently reached where the ridges are cut into such short. : ; ee Se on) qe Peteee tion of sections that they cease to a surface much dissected by the deveap- be ridges, and become hills ment of numerous tributaries. R= Te Fie ap 68 WORK OF RUNNING WATER instead. Even then the processes of erosion do not stop, for rain- water falling on the hills washes the loose material from their surfaces, and starts it on its seaward journey. Thus the “‘ever- lasting hills’ are lowered, and, given time enough, will be carried to the sea. The base-leveled surface is not absolutely flat. The area reduced by each stream will have a slight slope down-stream, and from its sides toward its axis. The low divides between streams flowing in the same direction may, however, disappear altogether, for when valleys have reached their limits in depth, their streams ee F 5 do not cease to cut laterally. ” Poe Meandering in their flat- peat + UES bottomed valleys, they may Soa > a reach and undercut their divides (P1.IV, and Fig. 57). By lateral planation, there- fore, the divides between streams may be entirely eaten away. ' The terms ‘‘grade,” and ) 3 \ “‘oraded plain,” and ‘‘base Fig. 57. Diagram showing streams in level” and ‘‘base-leveled adjacent valleys, undercutting the divide plain,’ are somewhat vari- between them. They may, in time, cut the divide away. ously, and therefore some- what confusingly, used. ‘‘A graded valley is one in which there is a condition of essential balance between corrasion and deposition.’”! Its angle of slope is variable and is dependent on the capacity of the stream for work, and on the work it has todo. A small river must have a higher gradient than a large one; a stream with much sediment must have a higher gradi- ent than one with little, and a stream with a load of coarse material must have a higher gradient than one with a load of fine. Thus the graded valley of the lower Mississippi has an inappreciable angle of slope; but the graded valleys of some of its small mountain tributaries have slopes of hundreds of feet per mile. Since both the size of the stream and the amount and coarseness of its load at a given place vary from time to time in the course of a cycle of erosion, it is clear that the inclination of the graded valley in a given place must vary from time to time. With the changing conditions of 1Davis. Jour. of Geol., Vol. X, p. 87. Fic, 1 — Youthful topography in a region of slight relief. The valley of Maple Creek is narrow, and much of the area is unaffected by erosion. Seale, about 2 miles per inch, Contour interval, 20 ft. (N. D., U. S. Geol. Surv.) Irie AOE «Aa ane a) (et Springs, yf , BS “i Sprs nn Inspirgtion g Z L Point g® Veg Silver Cord x Hot Sprs Surf G VU; PP CG rece Fig. 2.—A young valley in a region of great relief. Scale, about 2 miles per inch, Contour interval, 100 feet. (U.S. Geol. Surv.) PLATE VI Fic. 1—A region in a mature stage of erosion. Scale, about 2 miles per inch. Contour interval, 100 feet. (Kentucky, U. 8S. Geol. Surv.) Co. EN tn ae ; ~~ fs FRANCISCO Cc. Fic. 2.—A coast line developed chiefly by wave erosion. Seale, about 1 mile per inch. Contour interval, 25 feet. (Tamalpais, Cal., Sheet, U.S. Geol. Surv.) t. Bonita CYCLE OF EROSION 69 advancing years, the slope of a graded valley normally decreases. The same principles apply to graded surfaces outside of valleys. When a stream has brought the bottom of its valley to grade, it may be said to be at the level of base-level if the gradient is low; but a narrow valley flat at this level is not a base-level. This term, in the sense of a base-leveled plain, is applied to extensive areas only. Any extensive area degraded by running water to essential flatness is a base-level. Under later conditions of erosion, even with- out uplift, a base-leveled surface may be reduced (slightly) to a lower base-level. There is no sharp distinction between a base- level and an extensive graded surface of low gradient, if the latter was reduced by running water. The ocean may be looked upon as a barrier which in a general 4 Fig. 58. A shallow river valley in a plain. Cerro Gordo Co., Ia. Contrast with Fig. 59. (Calvin.) way limits the down-cutting of running water. Other barriers, such as lakes, and outcrops of hard rock in ‘a stream’s bed, have a comparable, though more local and temporary, effect on the development of valley plains above themselves. Plains thus de- veloped have been called temporary base-levels. Stages in a cycle of erosion. Since river valleys have a begin- ning and pass through various stages of development before the country they drain is base-leveled, it is convenient to recognize their various stages of advancement. Nor is this difficult. An old valley and a young one have different characteristics, and the one would no more be mistaken for the other by those who have learned to interpret them, than the face of an aged man would be mistaken for that of a child. Youth. ‘The cycle begins with the beginning of valley develop- ment, and at that stage drainage is in its infancy. The type of the 70 WORK OF RUNNING WATER infant valley is the gully or ravine (Fig. 40). It has steep slopes and a narrow bottom. Plate III represents somewhat .older ravines, in contour. Asa valley is widened, lengthened, and deepened, it passes from infancy to youth. In this stage also the valleys are relatively narrow, and the divides between them broad. The valleys may be deep or shallow according to the height of the land in Fig. 59. Canyon of the Yellowstone below the falls. Yellowstone Park. which they are cut, and the fall of the water flowing through them; but in any case the streams flowing through them have done but a small part of the work they are to do before the country they drain is base-leveled. Figs. 58 and 59, respectively, represent youthful valleys in regions of slight and great relief. Fig. 1, Pl. V, shows youthful valleys in a region of slight relief, and Fig. 2, Pl. V, CYCLE OF EROSION a in a region of great relief. The uppermost line in Fig. 54 likewise represents topographic youth, as shown in cross-section. Not only are narrow valleys said to be young, but the territory affected by them is said to be in its topographic youth, since but a small part of the time necessary to reduce it to base-level has elapsed. An area is in its topographic youth when considerable portions of it are still unaffected by valleys. Thus the areas (as a whole), as well as the valleys, represented on Plate V, are in their topographic youth. It is often convenient to recognize Fig. 60. A valley much older than that shown in Fig. 59, Gray Copper Gulch, southwestern Colorado. (U.S. Geol. Surv.) various sub-stages, such as early youth, middle youth, and late youth, within the youthful stage of valleys and topographies. Youthful streams, as well as youthful topographies, have their distinctive characteristics. They are usually swift; their cutting is mainly at the bottom rather than at the sides, and their courses are often marked by rapids and falls. As valleys approach base-level, they develop flats. As valleys and their flats widen, and as their tributaries increase in number and size, a stage of erosion is presently reached in which but little of the original upland surface remains. The country is reduced largely to slopes, and in this condition the drainage and the topography which it has determined are said to be mature. Mature topography is shown in contours in Fig. 1, Pl. VI, where slopes rather than upland or valley flats, predominate. Mature 72 WORK OF RUNNING WATER topography is also shown in Fig. 60, which illustrates the universal tendency of rivers in regions of notable relief to develop new flats well below the former surface of the region. The same processes which have made young valleys mature will in time work further changes. When the gradients of the valleys have become low and their bottoms wide, and when the intervening ridges and hills have become narrow and small, the drainage and the drainage topography have reached old age. This is illustrated by Fig. 1, Pl. VII, and in section by the third and lower lines in Fig. 54. Topographic old age may have a different expression; this Fig. 61. A peneplain near Camp Douglass, Wis. (Atwood.) is shown in Fig. 61, where most of the surface has been brought low. ‘The elevations which rise above the general plain are small in area, but have steep slopes. This expression of old-age topography is usually the result of unequal resistance of the rock degraded. The marks of old streams are as characteristic as those of young ones. They have low gradients and are sluggish. Instead of lowering their channels steadily, they cut them down in flood, and fill them up when their currents are not swollen. They meander widely in their flat-bottomed valleys (Pl. VII) and their erosion, except in time of flood, is largely lateral. The preceding discussion, and the illustrations which accom- pany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow in youth and maturity depends on the height of the land and its distance from the sea. The higher the land, and EROSION TOPOGRAPHY 73 the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. A plateau in the heart of a continent would have less relief in maturity than one of equal elevation near the sea, since the grade-plain is higher in the former position than in the latter. Characteristics of river-shaped topographies. With the char- acteristics of river valleys clearly in mind, it is easy to say whether rivers have been the chief agents in the development of a given topography. River valleys are distinguished from other depressions on land surfaces by their linear form, and, leaving out of consider- ation the relatively insignificant inequalities in streams’ channels, by the fact that any point in the bottom is lower than any other point farther up stream in the same valley, and higher than any point farther down stream. The second point might be otherwise stated by saying that every valley excavated by erosion leads to’ a lower valley, to the sea, or to an inland basin. Streams which dry up, or otherwise disappear as they flow, constitute partial exceptions. If, therefore, the depressions on a land surface are linear, lead to other and deeper valleys, and finally to an inland basin or the sea, and if the elevations between these valleys are such as might have been left by the excavation of the valleys, it is clear that rain and rivers have been the chief factors in the development of the topography. If, on the other hand, a surface is characterized by topographic features which streams cannot develop, such as enclosed depressions, or hills and ridges whose arrangement is independent of drainage lines, other agents besides rain and surface streams have been concerned in its development. Note. For laboratory work see p. 120. ANALYSIS OF EROSION ! Erosion is the term applied to all processes by which earthy matter or rock is loosened or removed from one place to another. It consists of several sub-processes, namely, weathering, transporta- tion, corrasion, and corrosion. Weathering. Weathering is the term applied to nearly all those 1 An excellent discussion of this subject is given by Gilbert in The Henry Mountains, pp. 99 et seq., and more briefly in the Am. Jour. Sci., Vol. XII, p. 85, et seq., 1876. 74 WORK OF RUNNING WATER natural processes which tend to loosen or change the exposed sur- faces of rock. The inscriptions on exposed marble become fainter and fainter as time goes by, and finally disappear, because the rock in which the letters were cut has weathered away. In this case the weathering is effected partly by air and partly by water, two important agents of weathering. : The rain which falls upon the surface of exposed rock, and that which sinks through the soil to the solid rock below, dissolves slowly some of the constituents of the rock. This tends to make the rock crumble, much as mortar does when the lime carbonate which cements the sand is dissolved. The chemical changes effected by ground-water and the gases dissolved in it, also help to disintegrate the rock, as we have seen (p. 38). There are processes of weathering not due directly either to the atmos- phere or to water. Thus the roots of trees frequently grow in cracks of rocks (Fig. 62), and, increasing in size, act ciate siti like wedges. Water freezing in cracks Fig. 62. Tree growing in works in the same way. From the race Pstteda Tae meee "S faces of steep cliffs masses of rock are loosened frequently by the wedge-work of roots or ice, or by expansion and contraction due to changes of temperature. The quantities of debris at the bases of many cliffs, forming slopes of ¢alus (Fig. 63), testify not only to the importance of weathering, but also to the effectiveness of gravity in getting loosened material down. The importance of weathering in erosion is shown in many ways. Where the mantle rock is the product of the decay of the solid rock beneath, and this is the case over a large part of the earth’s surface, the soil and subsoil represent the excess of weathering over trans- portation. Since most of the earth’s surface is covered with soil and subsoil, it is clear that, on the whole, weathering keeps ahead of transportation. The loosening of rock by weathering greatly in- creases erosion, not only by running water, but by all other agents of erosion. ‘Though weathering is the first step in most erosion, it 75 ANALYSIS OF EROSION Talus slope, Utah. Fig. 63. Shows the downward creep of soil and slaty rock under the influence (U. S. Geol. Surv.) Fig. 64. of gravity. 76 WORK OF RUNNING WATER is not the only one, and under some conditions erosion takes place without it. Transportation. A second element of erosion is transportation. The transportation of sediment is to be distinguished from the transportation of ma- terials in solution. In so far as mineral mat- ter is dissolved, it be- comes a part of the fluid of the stream. The quantity dissolved is too small to influence the mobility of the water sensibly. The sediment transported by a stream is either rolled along its bottom, or carried in suspension above the bottom. The coarser materials (gravel and sand) are carried chiefly in the former posi- tion, and the finer (silt and mud) largely in the latter. Transporting power and velocity. The transporting power of running water depends on its velocity. Swift streams have much greater power of transportation than sluggish ones, but transpor- tation does not always keep pace with transporting power. The Niagara at its rapids is a stream of great transporting power, but it carries little sediment, because there is little to be had. The velocity of a stream depends chiefly on three elements — its gradient, its volume, and its load. The higher the gradient, the greater the yolume, and the less the load, the greater the velo- city. The relation between gradient and velocity is evident; that between volume and velocity is illustrated by every stream in time of flood, when its flow is greatly accelerated. The relation between velocity and load is less obvious, but none the less definite. Every particle of sediment carried by a stream makes a draught on its energy, and energy expended in this way reduces the velocity. A muddy stream is never so swift as a clear stream of the same size would be, flowing in the same channel. How sediment is carried. Coarse materials, such as gravel- stones, are rolled along the bottom of the swift streams which carry them. ‘Their movement is by the impact of the water. The same is true to a large extent of sand grains. So far as concerns the material rolled along the bottom, it is to be noted that a stream’s transporting power is dependent on the velocity of the water at its bottom, which is much less than the velocity at the surface, and less than the average velocity. Fig. 65. Diagram of a valley, the top of which is ten times the width of the stream. TRANSPORTATION 77 Particles of fine sediment, such as silt and mud, are carried by streams quite above their bottoms, as shown by the muddiness of many streams. Most particles of mud are small bits of mineral matter, the specific gravity of which is between two and three times that of water. come to rest at the bottom. Yet they do not sink through the water and A particle of sediment in running water is subject to two prin- cipal forces, that of the current which tends to move it nearly hori- zontally down stream, and that of gravity which tends to carry it to the bed of the stream. As a result, the particle tends to move in the direction which represents the re- sultant of these forces (Fig. 66). If a river were the simple straightfor- ward current which it is popularly thought to be, a particle in suspen- sion would reach its bottom in the time it would take to sink through an equal depth of still water; for the Ya Fig. 66. Diagram to illustrate the relative strength of the two forces acting on a particle in sus- pension. The arrows represented by full lines show the relative strength of the two forces when the stream’s velocity is about 5 miles per hour. No account is taken in the diagram of the viscosity of the water, or of the acceleration of velocity of fall. descent would be none the less cer- tain and scarcely less prompt because of the forward movement of the water. The current would simply be a factor in determining the position of the particle when it reached the bottom, not the time of reaching it. Very fine particles, like those of clay, sink less readily than coarser ones, because the former expose larger surfaces, relative to their mass, to the water through which they sink. But even such particles, unless of extraordinary fineness, would pres- ently reach the bottom if acted on only by a horizontal current and gravity. Since even sediment which is not of exceeding fineness is kept in suspension, it is clear that some other factor is involved. This is found, in part at least, in the subordinate upward currents in a stream. Where a bowlder occurs in the bed of a stream (Fig. 67) a part of the water which strikes it is forced up over it. If there are many bowlders, the process is repeated frequently, and the number of upward currents is great. Any roughness will serve the same pur- pose, and every stream’s bed is rough to a greater or less extent. Roughnesses at the sides of a channel start currents which flow 78 WORK OF RUNNING WATER toward the center, and the varying velocities of the different parts of a stream serve a similar purpose. A river is therefore to be $e OOK Gee ee tude of currents, some rising from the bottom jie toward the top, some ee . VAnw. bm en Din descending from top to Fig. 67. Diagram to illustrate the effect of bottom, some diverging irregularities, a and b, in a stream’s bed, on the from the center to- current striking them. ward the sides and some converging from the sides toward the center. The sum of the upward currents is of course always less than the sum of the downward, so that the aggregate motion of the water is down slope. Sediment in suspension is held up chiefly by the upward currents, which, locally and temporarily, overcome the effect of gravity. The particles in suspension are constantly tending to fall, and fre- quently falling; but before they reach the bottom, many of them ar¢ carried up by subordinate currents, only to sink and be carried up again. Even if they reach the bottom, as they do frequently, they may be picked up again. It is probable that every particle of sedi- ment of such size that it would sink readily in still water is dropped and picked up many times in the course of any long river journey, and its periods of rest often exceed its periods of movement. Corrasion. ‘The mechanical wear effected by running water is corrasion. So long as the materials to be moved are incoherent, it is easy to understand how running water moves them. The water which flows over the surface of a cultivated field gathers earthy mat- ter, and the process is continued all the way to the channel of the stream. Thus sediment is gathered at the very sources of flow, and the stream gathers load from its bed wherever it flows with sufficient velocity over loose material. Streams also undercut their banks, and receive new load from the fall of the overhanging material. The larger part of the sediment of streams is made up of mate- rial loosened in advance by weathering; but many rivers wear rock which is not weathered, for the principal valleys of the earth are in solid rock, and many of them in rock of great hardness. How does the stream wear the solid rock? | When a stream flows over a rock bed, the wear which it accom- plishes depends chiefly on the character of the rock, the velocity — CORRASION 79 of the stream, and the load it carries. If the rock is much divided by bedding planes and joint planes, the water of a clear stream of even moderate strength may dislodge bits of the rock. This con- dition of things is seen where streams run on beds of shale or slate. If the rock is hard and without bedding planes or joints, or if its layers are thick and its joints few, clear water is much less effective. If massive hard rock presents a smooth surface to a clear stream, the mechanical effect of even a swift current is slight. This general principle is illustrated by the Niagara River. Just above the falls the current is swift. When the river is essen- tially free from sediment, the surface of the limestone near the bank beneath it sometimes is distinctly green from the presence of the one-celled plants (fresh-water alge) which grow uponit. The whole force of the mighty torrent is not able to sweep them away. Were the stream supplied with a tithe of the sand which it is capable of carrying, it would not take many hours, and perhaps not many minutes, to remove the last trace of the vegetation. This illus- tration furnishes a clue to the method by which the erosion of solid rock in a stream’s bed is effected. The gravel rolled along the channel wears even solid rock, and as the moving stones wear the stream’s bed, they are themselves worn by impact both with the bed and with one another, and are reduced to rounded, water-worn forms. The particles broken off may make grains of sand, or, if very fine, particles of silt or mud. In the course of time the pebbles and cobbles rolled along may be literally worn out. The sediment carried in suspension, as well as that rolled along the bottom, wears the rock bed of a stream. ‘The coarser the sedi- ment and the stronger the current, the greater the wear. The gravel, sand, and mud carried by a stream are therefore the tools with which it works. Without them it is relatively impotent, so far as the abrasion of solid rock is concerned; with them, it may wear any rock over which it passes. Swift and slow streams corrade their valleys differently. The erosion of a swift stream is chiefly at the bottom of its channel. The sluggish stream lowers its channel less rapidly, or not at all, and lateral erosion is relatively more important. The result is that slow streams increase the width of their valleys more than the depth, while swift streams increase the depth more than the width. It follows that slow streams develop flats, while swift ones do not. 80 WORK OF RUNNING WATER Not only is a slow stream more likely to have a flat, and therefore a better chance to meander, but it is more likely to take advantage of opportunities in this line, for a slow stream gets out of the way for such obstacles as it may encounter, while a swift stream is much more likely to get obstacles out of its way. Corrosion. In most cases the solution (and other chemical changes) effected by a stream is much less important than its me- chanical work. Only when conditions are unfavorable for the latter is solution the chief factor in the excavation of a valley. This may be the case where a stream’s bed is over soluble rock, such as limestone, and where the stream is clear, or its gradient so low that its current is sluggish. The solvent power of water is not influenced by the presence of sediment, though the presence of sediment offers the water a greater surface on which to work. CONDITIONS AFFECTING THE RATE OF EROSION With a given amount of water, the declivity, the character of the rock, and climate, are the principal factors influencing the rate of erosion. Declivity. In general, the greater the slope the more rapid the rate of erosion by running water, whether in the stream’s channel or on the slopes above. But high declivity does not favor every ele- ment of erosion. It favors some phases of weathering and hinders others, but it favors both transportation and corrasion. Both corrasive power and transportive power increase rapidly with in- crease of velocity, and under these circumstances, corrasion also will be increased if the water has tools to work with, and trans- portation will be increased if there is material which can be carried. Since high declivity greatly increases both the transporting and the corrading power of running water, and favors certain elements of weathering, it is clear that its aggregate effect is to favor erosion. Rock. The physical constitution, the chemical composition, and the structure of a rock formation influence the rate at which it is broken up and carried away. Physical constitution. ‘The constituents of clastic rocks may be firmly or weakly cemented. The less the coherence the more ready the disintegration, and the finer the particles the more easily they are carried away. If the materials carried are harder than the bed over which they pass, corrasion of the latter is favored. Chemical composition, Something also depends on the chemical RATE OF EROSION 81 composition of the rock, since this affects its solubility and its rate of decomposition. The more soluble the rock, the larger the pro- portion of it which will be taken away in solution; but it does not follow that the most soluble rock will be most rapidly eroded, since the rate of erosion depends on abrasion as well as solution, and a rock which is readily soluble, as rocks go, may be less easily abraded than one which is made of discrete and insoluble par- ticles bound together by a soluble cement. In such rocks, for example a conglomerate in which the pebbles are cemented together by lime carbonate, the solution of the cement sets free a considerable quantity of gravel, so that a small amount of solution prepares a large amount of sediment for removal. A stream might cut its valley much more rapidly in such rock than in a compact lime- stone, though the latter is, as a whole, the more soluble. Structure. The structure of rock has much to do with the rate of its erosion. Other things equal, stratified rock is more readily eroded than massive rock, since stratification planes are planes of cleavage, and therefore of weakness. Taking advantage of these planes, the water has less breaking to perform to reduce the material to a transportable condition. For the same reason, a thin-bedded formation is eroded more easily than a thick-bedded one. The beds of stratified rock may be horizontal, vertical, or in- clined, and inclined strata may stand at any angle between hori- zontality and verticality. In indurated formations the rate of erosion is influenced both by the position of the strata and by the ~ relation of the direction of the flowing water to their dip and strike (Chapter X). In general, strata which are horizontal, or but slightly inclined, are probably less favorably situated for rapid erosion than those which are vertical or inclined at considerable angles. Joints have somewhat the effect of bedding planes, so far as erosion 1s con- cerned. Influence of climate. Climate has both a direct effect on erosion, chiefly through precipitation, changes of temperature, and wind; and an indirect effect, chiefly through vegetation. Like declivity and rock structure, climate does not affect all elements of erosion equally. Direct effects. The effects of variations of temperature on rock weathering have been discussed in Chapter II. Since high tem- perature favors chemical action, the weathering of rock by decom- position is at its best where the temperature is uniformly high, and 82 WORK OF RUNNING WATER moisture abundant. The climatic conditions favoring chemical weathering are therefore different from those favoring mechanical weathering (p. 25). So long as the water of the surface and that in the soil remain unfrozen, temperature affects neither corrasion nor transportation. But in middle and high latitudes the surface is frozen for some part of each year. During this time corrasion is at a minimum, for although the streams continue to flow, there is relatively little water running over the surface outside the drainage channels, and that little is relatively ineffective. Under some conditions, there- fore, temperature affects both corrasion and transportation. The humidity of the atmosphere has an influence on the rate of erosion. A moist atmosphere favors oxidation, carbonation, hydra- tion, and the growth of vegetation, all of which promote certain phases of rock weathering. On the other hand, humidity tends to prevent sudden and considerable variations in temperature, thus checking the weathering effected by this means. Precipitation, the most important single factor in determining the rate of erosion, is dependent on atmospheric humidity. Its amount, its kind (rain or snow), and its distribution in time, are the elements which determine its effectiveness in any given place. Other things being equal, the greater the amount of precipita- tion the more rapid the corrasion and transportation. Much, how- ever, depends on its distribution in time. A given amount of rainfall may be distributed equally through the year, or it may fall during a wet season only. The maximum inequality of distribution would occur if all the rainfall of a given period were concentrated in a single shower. With such concentration the volume of water flowing off over the surface immediately after the downpour would be greater than under any other conditions of precipitation, and since velocity is increased with volume, and erosive power with velocity, it follows that the erosive power of a given amount of water would be greatest under these circumstances. Further- more the largest proportion of the precipitation would run off over the surface under these circumstances, for less of it would sink beneath the surface, and less would be evaporated. If erosive power and rate of erosion were equal terms, the maxi- mum concentration of rainfall would be the condition for greatest erosion; but we have seen (p. 79) that erosive power and rate of erosion do not always correspond. If the water falling in this way EFFECT OF CLIMATE ON EROSION 83 could get all the material it could carry, erosion would be at a maxi- mum; but if the amount of material available for transportation is slight, a large part of the force of the water could not be utilized in erosion. While, therefore, it is not possible to say what distribu- tion of rainfall favors most rapid erosion without knowing the nature of the surface on which it is to fall, enough has been said to show that the problem is not a simple one. Some of. the most striking phases of topography developed by erosion, such as those of the Bad Lands (Figs. 68 and 69), are developed where the rain- fall is distributed unequally in time, and too slight or too infrequent to support abundant vegetation. Erosion in arid regions differs from that in regions of abundant rainfall in several ways. It is obvious that the valleys will develop more slowly in the former, that they will remain young longer, that the period necessary for the dissection of the surface is greater, that the water-courses will be less numerous, and that fewer of them will have permanent streams. If the arid region is high and composed of heterogeneous strata, the topography which erosion develops is more angular (Fig. 70) than that of the humid region. This is because there is less rock decay, and less vegetation to hold the products of decay. The more resistant beds of rock, therefore, come into greater prominence, especially on slopes, where they develop cliffs (Figs. 70 and 73). These general principles find abundant illustration in the plateaus of the western part of the United States,! where cliffs are by no means confined to the imme- diate valleys of the streams. _ Indirect effects. Through vegetation, climate influences erosion in ways which are easily defined qualitatively, but not quantita- tively. Both by its growth (wedge-work of roots) and by its decay (supplying COs, etc., to descending waters), vegetation favors cer- tain phases of weathering; but, on the other hand, it retards corra- sion and transportation both by wind and water. ‘This is well shown along the banks of streams and on the faces of cliffs com- posed of clay, sand, etc. Its aggregate effect is probably unfavor- able to erosion by mechanical means, and favorable to that by chemical processes. Winds have much to do with the rate of evaporation and the distribution of rainfall, so that their indirect effect on erosion is important. 1Dutton. Tertiary History of the Grand Canyon District, Mono. II, U.S. Geol. Surv. 84 WORK OF RUNNING WATER RATE OF DEGRADATION The amount of mechanical sediment which the Mississippi River carries to the Gulf of Mexico was estimated many years ago to represent a rate of degradation for the Mississippi basin of about one foot in 5,000 years. But the mechanical sediment carried to the Gulf does not really represent the total degradation of the basin, for the water which sinks beneath the surface is dissolving more or less rock substance, especially lime carbonate. This material is carried to the sea in solution, and does not appear in the sediment on which the above estimate is based. More recent studies, based on fuller data, indicate that the average rate of degradation for the United States is about 1 foot in 9,000 years." The sediment carried to the Gulf by the Mississippi River is gathered from nearly all parts of its basin, but much more of it comes from some places than from others. On the whole, the rate of erosion is probably greatest toward the margins of the basin, where the land is in its topographic youth or early maturity. It is notably less in the middle courses of the valleys, and is exceeded by deposition in some places along the lower courses of the Mississippi and some of its main tributaries. The average elevation of North America is probably not far from 2,000 feet. If it is being degraded at the rate of one foot in 9,000 years, and if this rate were to continue, it would take some- thing like 18,000,000 years to bring the continent to sea-level. But this rate of degradation could not continue to the end, for as the continent became lower, the streams would become sluggish and erosion less rapid. Long before the continent reached base-level, the rate of degradation, so far as dependent on mechanical erosion, would become so slow that the time necessary to bring the continent to sea-level would be prolonged almost indefinitely. Furthermore, it is quite possible that the land is suffering, or is liable to suffer, uplift, relative or absolute. If the rate of rise were equal to the rate of degradation, the average height of the continent would of course not be affected. FEATURES RESULTING FROM SPECIAL CONDITIONS OF EROSION Running water develops many striking topographic and scenic features. Some of them depend primarily on the conditions of 1 Water Supply Paper 234, U.S. Geol. Surv., pp. 78-83. PLATE VII f Fe Brang fey, ¢ Fic. 1—A meandering stream. The Mis- [ic. 2.—A stage in the develop- souri River. Scale, about 2 miles per ment of a meander. Schell inch. (Marshall, Mo., Sheet, U. S. Geol. River. Scale, about 2 miles Surv.) per inch. (Butler, Mo., Sheet, U.S. Geol. Surv.) | ais A co q] Fig. 3.—A plain in old age. Scale, about 2 miles per inch. Contour in- terval, 50 feet. (Abilene, Kan., Sheet, U. S. Geol. Surv.) PLATE VIII ( jowl and Mylls Cushetunk and Round Mountains, New Jersey; examples of isolated moun- tains left by the removal of less resistant surroundings. Scale, about ‘1 mile per inch. Contour interval, 20 feet. (High Bridge Sheet, U. S, Geol. Surv.) BAD LANDS 80 erosion, such as climate, altitude, etc., while others depend largely on the structure and resistance of the rock. Bad lands. A type of topography developed in early maturity in certain high regions where the rock is but slightly, though un- Fig. 68. Bad lands of South Dakota. Oligocene formation. (Williston.) equally, resistant, is termed bad-land topography (Figs. 68 and 60). Bad-land topography is found in various localities in the West, but especially in western Nebraska and Wyoming, and the western Fig. 69. Bad-land topography north of Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.) — j WORK OF RUNNING WATER 86 (‘AINS *TO94) °S ‘Q) “SouUOF]) "}2] 94} We pey oq 0} SI IBA Oy} Jo asdus y ‘OpeIojoD 9Y} Jo uoAURD pueID oy} Jo Jed & jo yowys "04 “B17 CANYONS 87 parts of the Dakotas. Many of the formations here are sandstone or shale, alternating with beds of unindurated clay. Climatic factors also are concerned in the development of this topography. = 277 PD eee . 7 PUP =< SS SS PQS Fig. 87. Cross-section of a portion of the Appalachian Mountains to illustrate the relations of mountain ridges to anticlines and synclines, and the phenomena of erosion cycles. (Rogers.) 1 Willis, The Northern Appalachians, in Physiography of the United States. STRUCTURAL ADJUSTMENT 97 In the structural adjustment which goes with the erosion of tolds, it happens in many cases that valleys come to be located on the anticlines after the latter have been worn down, while the outcrops of the hard layers on the flanks of the anticlines, or even in the original synclines, become the mountains (Fig. 87). ADJUSTMENT OF STREAMS TO ROCK STRUCTURES Valleys (gullies) are located at the outset without immediate regard to the hardness and softness of their beds. It is primarily che slope about the head of a gully which determines its line of growth, and, once established, streams tend to hold their courses; but the streams on the weaker rock will deepen their valleys more rapidly than others, and have an advantage over them. Being deeper, their tributaries may be lengthened until their heads reach the other valleys, with the results shown in Figs. 88-90. Even where several streams cross the same resistant bed, piracy is likely to take place among them, for some are sure to deepen their valleys faster than others, because of inequalities of volume, load, or hard- ness. This is illustrated by Figs. 91-93. (See also Figs. 52 and 53.) Piracy may take place where streams do not flow over rock of un- equal resistance, but it is more common where they do, for greater resistance of rock puts the stream which crosses it at a disadvantage as compared with the stream which crosses less resistant rock. The changes in the courses of streams by means of which they come to sustain definite and stable relations to the rock structure beneath, are known as processes of adjustment.' Since streams and valleys adjust themselves to other conditions as well, this phase of adjustment may be called structural adjustment. Struc- tural adjustment is not uncommon among rivers flowing over strata which are vertical or highly inclined, since in these positions, strata of unequal resistance are most likely to alternate with one another at the surface. The processes of adjustment go on until the streams flow as much as possible on the weaker beds, and as little as possible on the stronger. Adjustment is then complete. This amounts to the same thing as saying that the outcrops of resistant layers tend to become divides. In many cases an area is so situated that there is no escape for its drainage except across resistant rock. In tnis case drainage is completely adjusted when as few streams as possible cross the resistant rock, and these by the shortest routes. 1 Campbell, Jour. Geol., Vol. IV., pp. 567, 657. 98 WORK’ OF RUNNING WATER. Adjustment has been carried to a high degree of perfection in many parts of the Appalachian system. Here, as in all other mountains of similar structure, strata of unequal hardness were iolded into ridges. The folds were then truncated by erosion, SUL EO MEE OLDT 6S! VG 4 apie: - Figs. 88-90. Figs. 91-93. Figs. 88-90. Diagrams illustrating piracy, where the stream which does not flow over rock of superior hardness captures those which do. Fig. 89 represents a iurther development of the drainage shown in Fig. 88, and Fig. go represents a still later stage. Figs. 91-93. Diagrams to illustrate piracy where the competing streams all cross a hard layer. The diagrams represent successive stages of development. exposing the more and the less resistant beds (H and S, Fig. 86, respectively) in alternate belts along the flanks of the truncated folds (truncated at ab and cd). The streams, especially the lesser ones, now flow along the strike of the weaker beds much more INFLUENCE OF JOINTS 99 commonly than elsewhere, and where they cross the hard layers it is in most cases at right angles to the strike. This is shown in Fig. 94, where the arrows indicate a “10° the direction of strike. s As base-level is approached, the outcrops of hard rock are brought low. When the resist- ant beds have been reduced to base-level, streams may flow without regard to the resistance of the rock beneath, for down- ward cutting has ceased. It happens in some cases eh that rocks of unequal resistance are covered by beds of uniform hardness. A consequent stream (p. 61) developed on the latter 37°00 may find itself out of structural adjustment when its channel Fig. 94. Adjusted drainage in a region is sunk to the level of the of folded rocks. The many nearly paral- heterogeneous beds Bratch lel streams are flowing with the strike. a stream is said to be superimposed (Fig. 95) on the underlying structure. Structural adjustment is likely to follow in time. INFLUENCE OF JOINTS ON EROSION It has been pointed out that joint planes have somewhat the same influ- ence upon erosion that bedding planes have when the beds are tilted at a high angle. Most rocks are affected by joints, and many of them are nearly vertical. Two sets are generally present, and in some places more. When there are but two, they Fig. 95. Diagram to illustrate superimposition. The consequent stream on the upper formation usually meet at a large was superimposed on the underlying structures angle (Fig. 2). These when the upper bed had been cut through. 100 WORK OF RUNNING WATER Fig. 96. Figure showing crenate river bank, the re-entrants being determined by joints. Dells of the Wisconsin River, near Kilbourn, Wis. (Atwood.) joints allow the ingress of water, roots, etc., which help to weather and disrupt rocks. Their effect on erosion may be seen along many streams which flow in rock gorges. In such situations, the outlines of the banks are in some cases angular, and in some crenate (Fig. 96), the re-entrants being located at the joints. By working into and widening joints, running water in some places isolates masses of rock as islands (Fig. 97). Fig. 97. An island formed by river erosion in jointed rock; Lower Dells of the Wisconsin. (Atwood.) PLATE IX Fic. 1.—Entrenched Meanders. terval, 20 feet. Scale, about 1 mile per inch. Contour in- (Harrisburg, Pa., Sheet, U. S. Geol. Surv.) faect>, =. »> Ling Los Angeles ‘\oJunetion oa » \ " w ny ¥ w ny ny Pl Lae Fa? = “ = *. eee +4277 ise # — a) Fee. . ON sf Pecrecse sss. s Oceanside “G Fic. &. KY 2.—A Section of the California Coast, showing lands near the coast, which have recently emerged. Scale, about 1 mile per inch. Contour interval, 20 feet. (Oceanside, Cal., Sheet, U. S. Geol. Surv.) aR 5 DN — 2° o o 7) 2) prey ro) rob) a QD aS S&S oO t oo =| =) = 3S oO ss eat cS) t= a o jor ) ura = re ~Y = e) Ke ins o Lead 3 oO NM A piedmont alluvial plain or compound alluvial fan in Southern California. CHANGES OF LEVEL IOI In a region free from mantle rock, or where the mantle rock is meagre, joints have determined the courses of many valleys by directing the course of surface drainage. This is well shown in many parts of the arid West. In regions where the rocks are faulted the courses of some streams are controlled by the faults. It is probable that joints and fault planes have been more important in locating valleys, especially where the mantle rock is thin, than was formerly recognized. Joints in rocks may occasion the development of natural bridges. If above a waterfall, for example, there is an open joint in the bed of the stream (as at b, Fig. 98), some portion of the water will de- Rrkeh GEEMbN cer tes ence ; Diagram to illustrate the initial stage in the scend through it. development of a natural bridge. Longitudinal section After reaching a at the left, cross-section at the right. lower level it may find or make a passage through the rock to the river at the falls. If even alittle water takes such a course, the flow will enlarge the passageway through the joint to the valley at ae poten ta : the falls (bcde, Fig. 99. A stage later than that shown in Fig. 98. Fig. 98). This passageway may in time become large enough to accommodate all the water of the river. The entire fall will then _be transferred from the position which it previously occupied (f) to the position of the enlarged joint (6). The fall will then recede. The underground channel between the old falls and the new will then be bridged by rock (bf” and f’”’, Fig. 99). The natural bridge near Lexington, Va. (Fig. 100), almost 200 feet above the stream which flows beneath it, is believed to have been developed in this way. It is not to be understood that all natural bridges ! have had this history. EFFECT OF CHANGES OF LEVEL Rise. If, after being base-leveled, or notably advanced in an erosion cycle, a region is uplifted so as to increase the gradients and velocities of its streams, they are said to be rejuvenated. Renewed youth differs from first youth, in that the streams + Cleland, Pop. Sci. Mo., May ’11, and Bull. G. S. A., Vol. XXT, p. 313. I02 are already in existence. Fig. Virginia. (U. S. Geol. Surv.) is, the old meanders are entrenched. WORK OF RUNNING WATER The rejuvenated streams erode their valleys after the manner of youthful streams. They excavate new valleys in the bottoms of older ones (Figs. tor and 102), deep- ening them until they reach the new grade plane. Young val- leys in the bottoms of old ones are one of the evidences of re- juvenation. The new valley in the old one may be developing all along its course at the same time, or it may begin at the de- bouchure of a stream and work headward. In either case, the tributaries are rejuvenated when their main is lowered at the point of union. Another evidence of rejuve- nation is found in entrenched meanders. When an old wind- ing stream is rejuvenated, the deepened channel follows the course of the stream before re- juvenation. The result is that a new winding gorge is cut; that Entrenched meanders are rather common in the Appalachian Mountains (Fig. 1, P]. 1X), and are known in other parts of the world.!. With rejuvenation of the drainage, a new cycle of ero- sion is begun, whether the pre- ceding one was complete or not. The principles involved in the recognition of cycles of ero- sion, separated by uplifts, are Fig. 1o1. Cross-section of a wide valley, ab, in the bottom of which a younger valley, cd, has been excavated, as the result of uplift. illustrated by Fig. 103, which represents an ideal profile of consider- able length (say 20 miles). The points a, a’, and a” have about the same elevation. Below them there are areas 0, 0’, and 6’, which 1 Davis. The Seine, the Meuse, and the Moselle. Nat. Geog. Mag., Vol. VII, pp. 181-202, and 228-238. CHANGES OF LEVEL 103 have a nearly common elevation, below which are the sharp valleys d, d’, andd’’. The points a, a’, and a” represent the tops of ridges formed by the outcrops of layers of hardrock. If the crests of the ridges are level, the points a, a’,and a” must represent remnants of an old base-level, sznce at no time after a ridge of hard rock becomes deeply notched does it acquire an even crest, until it is base-leveled.1 After the cycle represented by the rem- nants a, a’, and a’’ was completed, the region suffered uplift. A new cycle represented by the plain 3, 0’, and 6’ was well advanced, though not completed, when the region was again elevated, and the rejuvenated streams began to cut their valleys d, d’, and d” in the plain of the previous incomplete cycle. The elevations, ¢ and c’ (intermediate between a, a’, 4} / /} and a”, and 6, 6’, 6”), may represent Fig. 102. Diagram to illustrate either remnants of the first base-level an ideal case of rejuvenation as the plain, lowered but not completely result of uplift. The black area removed while the plain 8, 0’, b” was 2 the bottom represents the sea. developing; or they may represent a cycle intermediate between that during which a, a’, a’ and 3, b’, b’’ were developed. x Fig. 103. Diagram to illustrate cycles of erosion where the beds are tilted. If the strata involved are horizontal, the determination of cycles may be less easy. . Thus in Fig. 104, it is not possible to say whether a and a’ represent remnants of an old base-level, or whether they represent the original surface from which degradation started. So, too, the various benches below a, such as 3, b’, and 6’, might well be the result of the superior hardness of beds at this level. For the determination of successive cycles in the field, it is necessary 1 Other views have been entertained. See Tarr, Am. Geol., Vol. XXI, pp. 351- 370, and Daly, Jour. of Geol., Vol. XIII, pp. 105-125. 104 WORK OF RUNNING WATER to consider areas of considerable size, and to eliminate the topo- graphic effects of inequalities of hardness. It is by the application of the preceding principles that it is known that the Appalachian Mountains, after being folded, were reduced to a peneplain (the Kittatinny peneplain) from the Hudson River to Alabama. The old peneplain surface is indicated by Fig. 104. Diagram to illustrate cycles of erosion where the beds are horizontal. the level crests of the Appalachian ridges. The system was then warped (not folded) up, and in the cycle of erosion which followed, broad plains were developed at a new and lower level, corresponding in a general way to the plains 0, 6’, and 6” of Fig. 103. The plains were located, for the most part, where the less resistant strata come to the surface. Above them rise even-crested ridges, the outcrops of the resistant layers, isolated by the degradation of the weaker beds between. It is the outcrops of these layers which constitute many of the present mountain ridges corresponding to the high points of Fig. 103. The evenness of their crests testifies to the com- pleteness of the first peneplanation. The evenness of the crests is, however, interrupted (1) by notches cut by the streams in later cycles, and (2) by occasional elevations (monadnocks) above the common level. Most of the monadnocks are rather inconspic- uous, but there is a notable group of them in North Carolina and Tennessee, of which Mount Mitchell and Roan Mountain are examples. When long distances are considered, the ridge crests depart somewhat from horizontality. This is believed to be due, in part at least, to deformation of the old peneplain during the uplift which inaugurated the second cycle of erosion. The extent to which the second cycle of erosion recorded in the present topography had proceeded before its interruption by up- warp is indicated by the extent of the valley plains (Fig. 103) below the mountain ridges. While these plains were being devel- oped on the weak rocks, narrow valleys (water-gaps) only were cut in the resistant rocks which stood out as ridges. Similar valleys, whether shallow or deep, from which drainage has been diverted, | are sometimes called wind-gaps. The second cycle of erosion, still incomplete, was interrupted by CHANGES OF LEVEL 105 uplift (relative or absolute), and a third cycle was inaugurated. The third cycle began so recently that it has not yet advanced far. Some of the features just described are illustrated by Fig. 8r. The even mountain crest in the background is the Kittatinny Mountain of New Jersey and its continuation in Pennsylvania. In common with other corresponding crests, it is a remnant of the oldest recorded base-level (or peneplain) of the region. Below the mountain crest there is another plain, developed in a subsequent cycle of erosion, while the valley plain in the foreground represents the work of a still later cycle. Many of the peculiarities: of the drainage of the Appalachian Mountain system are intimately connected with the history just outlined. Thus three great rivers, the Delaware, the Susquehanna, and the Potomac, have their sources west of: the Appalachians ‘proper, cross the system in apparent disregard of the structure, and flow into the Atlantic. The James and Roanoke head far to the west, although not beyond the mountain system, and flow east- ward, while the New River (leading to the Kanawha) farther south, heads east of the mountain-folds, and flows northwestward across the alternating hard and soft beds of the whole Appalachian system, to the Ohio. The French Broad, a tributary to the Tennessee, has a similar course. Such streams are clearly not in structural adjust- ment, and afford good opportunities for piracy. Their courses were apparently assumed during the time of the Kittatinny peneplain, when the streams had so low a gradient as not to be affected by structure. Elevation rejuvenated them, and they have held their courses in succeeding cycles across beds of unequal resistance, though smaller streams have become somewhat thoroughly adjusted. Crustal deformations have also helped them to hold their courses, for the peneplain seems to have been tilted to the southeast at its northern end, and to the southwest at its southern, when the suc- ceeding cycle began. | Streams which hold their early courses in spite of changes which have taken place since their courses were assumed are said to be antecedent. They antedate the crustal movements which, but for pre-existent streams, would have given origin to a different arrange- ment of river courses. As a result of crustal movements, therefore, a consequent stream may become antecedent. Master streams are more likely to hold their courses, and therefore to become ante- cedent, than subordinate ones. . 106 WORK OF RUNNING WATER The uplift of base-leveled beds, especially if the beds are tilted so as to bring layers of unequal resistance to the surface at frequent intervals, affords conditions favorable for extensive adjustment. The numerous wind-gaps in the mountain ridges, representing the abandoned courses of minor streams, and the less numerous water- gaps, which indicate the resistance of large streams to structural adjustment, are instructive witnesses of the extent to which ad- justment has gone. So extensive has it been among the streams of the Appalachian Mountains that there is probably no considerable stream in the whole system which has not gained or lost through its own or its neighbors’ piracy. Sinking. The land on which a river system is developed may be depressed relative to sea-level. In this case the sea occupies the lower ends of valleys, converting them into bays and estu- aries. A valley in this condition is said to be drowned. Of drowned valleys there are many examples along the Atlantic coast. Thus Fig. 105 Fig. 106 Fig. 105. Chesapeake Bay and its surroundings. ‘The bay is a drowned river valley, and the lower ends of its tributary valleys are also drowned. Fig. 106. The drainage of the region about Chesapeake Bay as it would have been but for drowning. CHANGES OF LEVEL 107 the St. Lawrence is drowned up to Montreal, and the Hudson up to Albany. If the drowned portion of the latter valley were not so narrow, it would be a bay. Delaware and Chesapeake bays, as well as many smaller ones, both north and south, are likewise the drowned ends of river valleys (Figs. 105 and 106). Successive rising and sinking. Another peculiarity of valleys and streams resulting from changes of level is illustrated by Pl. IX, Fig. 2. The main valleys of this part of the coast were developed when the land stood higher than now. Later, the sinking of the coast converted the lower ends of the valleys into bays. The bays were then transformed into lakes or lagoons by deposition at their mouths. Subsequent rise of the land or sinking of the sea al- lowed the drainage from the lakes to cut across the deposits which had converted the bays into lakes. The result is an older, wider valley above, succeeded by a younger one near the debouchure. Differential movement. Warping. A land surface on which a river system is established may suffer warping, some parts going up and others down. Above an upwarp which notably checks its flow, a stream is ponded. If a stream holds its course across a notable uplift athwart its valley, it becomes an antecedent stream. The Columbia River has been thought to hold its antecedent course across areas which have been uplifted (differentially) hun- dreds and even thousands of feet.1 A lesser stream would have been diverted, as many of its tributaries have been. AGGRADATIONAL WORK OF RUNNING WATER We have seen that rivers carry mud, sand, gravel, etc., from land to sea, and that their goal is the degradation of the land to base-level. We have seen also that rivers do not always carry their sediment directly to the sea. In many cases it is dropped for a time on land, perhaps to be picked up and carried on again when conditions for its transportation are more favorable. We have now to inquire more particularly into the causes and results of deposition. Causes of deposition. When running water drops its load, or any part of it, it is generally because the current has lost velocity. Decrease of gradient is the commonest cause of loss of velocity. The loss may be (1) sudden, as when the water passes from a steep slope to a gentle one, or into a body of standing water; or (2) slow, as in following a valley whose gradient decreases gradually. We 1 Russell. Rivers of North America, p. 279. 108 WORK OF RUNNING WATER therefore look to the places where these changes in velocity occur for the principal deposits of running water. Streams also become slower wherever their channels become wider, even if volumes and gradients remain constant. Decrease of volume is a less common cause of decrease of velocity. Most streams increase in size as they flow, but to this general rule there are exceptions. (1) If a stream flows through a very dry re- gion it receives few tributaries, while evaporation is great and the thirsty soil and_ rock through which it flows absorb some of its water. In such a region a stream may diminish as it flows, and may even disappear altogether (Pls. II and X). (2) In some places certain streams break up _ into LAKE ST. CLAIR several (Fig. 107), and in therefore the velocity of each is less than that of the original stream. (3) —_— ALLUVIAL DEPOSITS 109 Fig. 108. An alluvial cone. (U.S. Geol. Surv.) of talus. In the latter, gravity brings the material down with little aid from water, but between the two types of cones there are all gradations. Conspicuous alluvial cones are common at the bases of steep slopes in semi-arid regions. The rainfall there is fitful, and the Fig. 109. Deposition at the bases of valley slopes, tending to give the valley a U-shaped base. Unaweep Canyon, Colorado. (Cross, U. S. Geol. Surv.) occasional heavy showers, which give rise to temporary and power- ful torrents, favor the development of great cones. At the bases of the mountain ranges in the Great Basin, some of the talus and alluvial cones are 2,000 or 3,000 feet high. An alluvial fan is the same as an alluvial cone, except that it 110 WORK OF RUNNING WATER has a lower angle of slope. The term fav is more appropriate than cone for most alluvial accumulations at the bases of slopes. The lower angle of the fan may be due to the less abrupt change of slope where it is developed, to the larger quantity of water con- cerned in its deposition, to the smaller amount of detritus, or to its greater fineness. Less change of slope, more water, and less and finer material, all favor the wider distribution of the sediment, and so the development of fans rather than cones. Nearly all young rivers descending from mountains build fans where they leave the mountains. Thus, the rivers descending from the Sierras to the great valley of California build great fans at the base of the range. Many rivers descending from the Rockies to the Great Plains have done the same thing. The fans of some streams descending from the mountains are many miles across. That of the Merced River in California, for example, has a radius of about 40 miles. The fans made by neighboring streams may spread laterally until they merge. The union of such fans makes a compound alluvial fan, or a piedmont alluvial plain (P|. X). Such plains exist at the bases of most considerable mountain ranges. Sheet wash, as well as streams, contributes to them. The depth of alluvial material in such plains is, in some cases, hundreds of feet. The great spread of these land deposits is remarkable. East of the Rocky Mountains they extend out more than a hundred miles in some places. This wide spread appears to be the result of the long- continued action of running water. The cone or fan, as first built, i ee CLP MP LE LP BP GE LE BPR a Se 2 Fig. 110. Diagram to illustrate the spreading of alluvial deposits in a piedmont position. The deposits may first take the position represented by the line 1-1’. At a later stage, as a result of erosion and redeposition, they take the position repre- sented by the line 2-2’, being spread farther from the mountain and having a lower surface slope. At a still later time, they take the position 3-3’, with a still lower slope and a still wider spread. ALLUVIAL DEPOSITS III is degraded later, and its materials spread more widely, as suggested by Fig. r1o. Deposits of this sort have probably been far more important in the past than has been generally recognized. Much of the material of the Coastal Plain of the Atlantic and Gulf slopes of the United States appears to have been deposited in this way. A large part of the Great Plains is covered with wash from the Rocky Moun- tains, and similar deposits are of great extent and depth east of the Andes and south of the Himalayas. They are, indeed, of signifi- cant extent and depth on the plains about almost every mountain range which has been carefully studied. It seems clear that similar deposits must have been made at all stages in the past history of the earth, whenever and wherever mountainous lands bordered plains. Formations of this general sort, made at the bases of high lands, have now been recognized among the ancient formations of the Fig. 111. A branching stream. Junction of the Cooper and Yukon rivers, Alaska. Shows also bars, etc. (U.S. Geol. Surv.) earth, as well as among the recent ones, and some of the ancient beds of sediment deposited in this way attained thicknesses of hundreds and even thousands of feet. They probably attained their greatest thickness, as now, in basins. 2. In valley bottoms. A stream which makes deposits in its channel, makes the channel smaller. In time it may become too small to hold all the water. A part then breaks out, and follows II2 WORK OF RUNNING WATER a new course over the valley flat. This process may be repeated again and again (Fig. 111). Some streams deposit bars in their channels, especially in low water. The bars may be swept away in time of flood, but some of them become more or less permanent islands. The profiles of the bottoms of most valleys are curves, the curva- ture becoming less as the lower end of the stream is approached NOR cay MAL VALLEy PROFILE EVEL SEA LE SEA Fig. 112. Profile of a normal valley, showing decreasing slope down stream. (Fig. 112). It therefore happens that as a stream descends its valley it generally reaches a point where its reduced gradient so diminishes its velocity that it must abandon some of its load. In this way sediment is distributed for long distances along valley bot- toms. It is left in the channels of streams in low water, and spread ——] over their flood plains in high water, aggrading them and making them alluvial plains. Deposition in a valley which has no flat tends to develop one (Fig. 113). Alluvial deposits Fig. 113. Flat developed by aggrada- On valley flats are ‘usually but tion — diagrammatic. a few feet, or at most a few scores of feet thick; but in rare cases they reach hundreds of feet. Natural levees (Fig. 114) are developed on flood plains aggraded by occasional floods. At such times the current in the main chan- nel is swift; but as the water escapes its channel and spreads over Fig. 114. Levees of the Mississippi in cross-section, four miles north of Donald- sonville, La. Vertical scale X50. The horizontal line represents sea-level. The bottom of the channel is far below sea-level at this point. the adjacent flat, its velocity is checked promptly, because its depth suddenly becomes less. It therefore abandons much of its load then and there. Repeated deposition in this position, in excess of that over other parts of the flood plain, gives rise to the levees. Scour-and-fill.| Aggrading streams deepen their channels period- 1 Hill, Erosion and Deposition by the Indus. Geol. Mag., July, 1910. ALLUVIAL DEPOSITS 113 ically to a notable extent, and the deepening of the channel takes place at the very time when the flood-plain is being aggraded. In other words, the stream in flood aggrades its plain, and degrades its channel. This follows from the fact that the current is slow on the plain, where the water is shallow, and rapid in the channel, where it is deep. After the flood subsides, the channel, deepened while the current was torrential, is filled up again by sediment from Pa eee ae ob ¥ ase “Peep ae Fig. 115 Fig. 116 Fig. 115. Diagram illustrating an early stage in the development of river meanders. ‘The dotted area represents the area over which the stream has worked. Fig. 116. A later stage in the Cevelopment of meanders. the feebler current. This alternate deepening and filling is scour- and-fill. It is well illustrated by the Missouri River. At Nebraska City, scour reaches depths of 70 to go feet occasionally. At Blair, about 25 miles above Omaha, the same river is believed to cut to bed-rock (about 40 feet below the bottom of the channel in low water) during floods. All streams similarly situated do a like work. The material thus eroded is shifted down-stream, some of it for short. distances only, and some of it to the sea. An aggrading stream, therefore, is not without erosive activity; it is a stream whose fili exceeds its scour, not one which has ceased to erode. Materials of the flood-plain. Asa result of its varying velocities in flood and low water, a stream may deposit coarse material at 114 WORK OF RUNNING WATER one time and fine at another. Many flood-plain deposits are, therefore, very heterogeneous, ranging from the finest mud, through sand, to gravel, and even bowlders. In general they become finer down-stream. Flood-plain meanders. A stream with an alluvial plain is likely to meander widely (Pls. XI and VII). In general terms this may be said to be the result of low velocity, which allows the stream to be turned aside easily. Were the course of such a stream made straight, it would soon become crooked again. The manner of change is illustrated by Figs. 115 and 116. If the banks are less resistant at some points than at others, as is always the case, the stream will cut in at those points. If the configuration of the chaanelat is such as to direct a current against a given point, : b (Fig. 115), the result is the same, even without inequality of material. Once a curve in the bank is started, it is in- creased by the current which is directed into it. Further- more, as the current issues from the curve, it impinges against the opposite bank and develops a curve at that point. The water issuing from this curve develops another, and so on. Once started, the curves or meanders tend to become more and more pronounced (Fig. } 116). In the case represented by Fig. 1, Pl. VI, the narrow neck of land between curves is almost cut through. A later stage in the process is shown in Fig. 2. When the stream has cut off a meander, the aban- | ag: doned part of the channel may Fig. 117. Meanders and cut-offs in the Temain unfilled with sediment. Ree ree Valley below Vicksburg. The [f it contains standing water, gure shows the migration of the meanders : down stream, and their tendency to in- ie sgertety do, it becomes a lake crease. (Fig. 117). Some such lakes RiverBanks (883 Bars etc. 1883 River Banks 18956 ALLUVIAL DEPOSITS 115 NTCHARTRAIN yAKE PO Fig. 118. Delta of the Mississippi. The dotted line outside the land represents the 3-fathom line. have the form of an oxbow, and so are called ox-bow lakes (Fig. 117, and Pls. VII and XI). 116 WORK OF RUNNING WATER 3. Atdebouchures. Where a swift stream flows into sea or lake, its current is checked promptly and soon destroyed altogether, and its load is dropped. If not washed away by waves, etc., the deposits of river-borne sediment in such places make deltas. A delta has some features in common with an alluvial fan. In both cases the principal deposit is concentrated at the point where bi gece Fig. r19. A delta in a lake. The village is Silva Plana, in the Engadine, Switzerland. (Robin.) the velocity is checked. In the case of the delta, however, the cur- rent is checked more completely, and the debris accumulates (at the outset) below the surface of standing water. Though started below water, deposition on the surface of a delta may build it up to, and even above, the water-level. That part of the delta above water is like a flat alluvial fan. In profile, the delta differs from the alluvial fan in that its edge has a steep slope (compare Figs. 121 and 122). Much land has been made by delta-building. Thus the Colo- rado River has built a great delta many square miles (above water) in area at the head of the Gulf of Ufo) California (Fig. 123). The delta yb Sirs} has been built quite across the ze . si . "a : * aa a f eS “ ° = A : I sists SLE Aes] off its head. In thearid climate of Fig. :20. The delta of the Nile. the region, this shut-off head has gulf near its upper end, shutting — — i PLATE XI Sie Wik The Aiaocnir: and Big bout 2 miles per a Geol. Surv.) "Se a Sioux Rivers Neb. Sheet Ta. Z SS MQ 4S qa o— js) A, av, | i ml be a) aS) ~~ S . ss . a ® ~ ~~ M PLATE XIl it Y, Gf ; x ; { A t Cs I | ard i £ bi It : =" ; pi 4 4 A uf . Sa f (se Tes ss f ~ wr LS j y i : f 3 GLACIERS YA Itt LZII-Zz SPEAK) ie . ( Wee B | Black Mt, Ee NOW <4 SS) + : *& bas * Glaciers on Glacier Peak, Washington. Scale, about 2 miles per inch. (Glacier Peak Sheet, U. S. Geol. Surv.) ALLUVIAL DEPOSITS 117 become a nearly dry basin, the lowest part of which is about 300 feet below sea-level. The Skagit River, in Washington, has built its delta out so as to surround what were high islands in Puget Sound, thus joining them to the mainland. The deltas of the Mississippi (Fig. 118), the Nile and the Hoang-Ho Rivers are FREE SE. ~ eG G AA. LEO OES Te, Cg LOLS a Fig. 121. Diagrammatic profile and section of a delta. well-known. The united delta of the Ganges and Brahmaputra is also a great one, having an area (above water) of some 50,000 square miles. The Po has built a delta 14 miles beyond the former port of Adria, which gave its name to the Adriatic Sea. The Rhone River (France) has advanced its delta some 15 miles in as many centuries. The effect of delta-building is to increase the area of the land; but it is to be noted that the processes which lead to delta- ‘building reduce the volume of the land-masses, even though they increase their area. The outline of some deltas is determined by the surroundings in which they are built. When, for example, a delta is built inte a bay, the form of the bay-head determines the shape of the delta. Fig. 122. Diagrammatic profile and section of an alluvial fan. The normal form of a delta built on an open coast is somewhat semicircular, though there is in many cases a fringe of delta fin- gers which together have some resemblance to the Greek letter A. which gave these terminal deposits of streams their names. 118 WORK OF RUNNING WATER ALLUVIAL TERRACES Stream terraces ! are bench-like flats or narrow plains along the sides of valleys (Fig. 124) and above their bottoms. Most of them are narrow, but some of them have great length. AN ot REC AM ATTACH SEAVICE US ae “peUikE MAR OR Tae “LOWER COLORADO RIVER. SROWANG IRRIGABLE LANDS sdamente y TPR ener UNITED STATES & MEXICO, 4 MAAR ROE Fig. 123. Relief. -map of an area about the head of the Gulf of California, show- ing the delta of the Colorado River, outlined, in a general way, by dotted lines. The Salton Sink is shown at the north, and the Imperial Valley lies south of the sink. (U.S. Rec. Serv.) 1 For discussions of terraces see Gilbert’s Henry Mountains, p. 126; Davis, Bull. of the Mus. of Comp. Zool. Geol. Series, Vol. V, pp. 282-346; and Dodge, Proc. Boston Soc. of Nat. Hist., Vol. XXVI, pp. 257-273. ALLUVIAL TERRACES 119 Most river terraces are remnants of former flood-plains, below which the streams which made them have cut their channels, but the details of their history are various. Normal alluvial terraces. Alluvial terraces are developed in the normal course of every stream’s history, because the first graded plain which a stream develops in its valley is above the level to which the:stream can cut at a later time. After the stream has sunk its channel well below the former flood-plain, such parts of the latter as still remain are alluvial terraces. Where a stream’s deepened channel is in the middle of its flood-plain, there is a terrace on either Fig. 124. Terraces on the Fraser River at Lilloet, B.C. (Photo. by Calvin.) side; but wherever the deepened channel is at one margin of its flood-plain, a terrace remains on the other side only. In some valleys there are several alluvial terraces at different levels. The second terrace (regarding the highest as the first) is developed in the same way as the first, for after the stream has developed a second flood-plain, below the level of the first, it may cut its channei still lower, leaving the remnants of the second flood-plain as terraces. This process may continue until several sets of terraces have been developed. Alluvial terraces developed by the normal activi- ties of a stream are always low, and ordinarily would not be conspicuous. They are not very long-lived, for all processes of sub- aérial erosion conspire to destroy them. A stream is likely to mean- 120 WORK OF RUNNING WATER der on its second and later flood-plains, as on its first and highest one. Wherever the meanders on its second flood-plain undercut the first terrace, the terrace at that point is subject to destruction, and since the meanders are continually migrating, terraces are continually disappearing. Again, tributary streams cut through the terraces of their mains, and new gullies develop in them, dissecting them still further. At the same time, sheet erosion and other phases of slope wash tend to drive the scarps of the terraces back toward the bluff beyond. By the time a second set of terraces is well developed, no more than meagre remnants of the first may remain.! Other river terraces. There are valley terraces which do not represent necessary stages in a valley’s history. (1) Some are due to inequalities of hardness (Fig. 80). (2) Again, if an alluvial flood- plain has been built as the result of an excessive supply of sedi- ment (p. 112), the exhaustion or withdrawal of the excessive sup- ply would leave the stream relatively clear, and free to erode where it had been depositing. It would forthwith set to work to carry away the material which it had temporarily unloaded on the plain. The valley plains built up in many valleys in the northern part of our continent during the glacial period, when drainage from the ice owed through them, have been partially destroyed since and their remnants are terraces. (3) A notable increase in the volume of a stream, without corresponding increase in load, as when one stream captures another, may occasion the develop- ment of terraces by allowing the enlarged stream to deepen its chan- nel. (4) The uplift of a region in which there are well developed river flats, would rejuvenate the streams, and parts of their old flood-plains would be left as terraces. Other occasional causes which need not be mentioned here, develop terraces from flood plains. In conclusion, it is to be emphasized that many river terraces, mostly very low, are normal features of valley development, coming into existence at definite stages in a valley’s history. They are generally composed, in large part, of river alluvium. Others result from more or less accidental causes, working singly or in conjunc- tion, and to this class belong many of the more conspicuous terraces developed from flood-plains. Laboratory work. See excercises III-IX, in laboratory manual Interpretation of Topographic Maps; also Professional Paper 60. U.S.G.S. Pls. XXIII-LXXXIX. 1 For a fuller statement of the manner in which alluvial terraces are devel- oped, see the authors’ Geologic Processes. CHAPTER: V THE WORK OF SNOW AND ICE Ice beneath the surface. The wedge-work of ice in the crevices of rock has already been mentioned (p. 25). When the great areas where water freezes during some part of the year are con- sidered, it is clear that the aggregate effect of its freezing in the pores and crevices of rock must be great in long periods of time. Even the freezing of water in the soil is not without effect. This is Shown by the disturbance of the walls of buildings if their founda- tions are not below the depth of freezing, and by the working up of stones and bowlders through the soil of the fields, as freezing and thawing succeed each other. Frozen water in the soil makes it solid, and temporarily retards or prevents surface erosion. Ice on lakes and ponds. Since fresh water is densest at 30° Fahr., ice does not commonly form on the surface of a lake until the temperature from top to bottom is reduced to this point. .Cooled below 39°, the surface water fails to sink, and cooled to 32°, it freezes. If the lake is small and shallow, it will freeze over completely where the temperature is notably below 32° for any considerable period of time. It is under these circumstances that lake ice becomes most effective. Let us suppose a lake in temperate latitudes, where the range of winter temperature is considerable, to be frozen over when the temperature is 25° Fahr. If now the temperature is lowered to —1o-, and sucha temperature is not uncommon in the northern part of the United States, the ice contracts. In contracting, it either pulls away from the shores, or cracks. If the former, the water from which the ice is withdrawn quickly freezes; if the latter, water rises in the cracks and freezes there. In either cases, the ice-cover of the lake is again complete. If the temperature now rises to 25° the ice expands, and the solid cover becomes too large for the lake, and must either crowd up on the shores, or arch up (wrinkle) elsewhere. , If the water near the shore is very shallow, the ice freezes to the 121 122 WORK OF SNOW AND ICE sand, gravel, and bowlders at the bottom. If the land at the shore is very low, the ice in expanding may shove up over it, carrying the debris frozen in its bottom, and it may push loose gravel, sand, etc., in front of its edge. Where bowlders are frozen to the bottom of the ice, the shoreward thrust as the ice expands shifts them toward the shore, and they may be shoved up a little above the normal water-level. The concentration of bowlders at the shore-line, year Fig. 125. Shore of Wall Lake, Iowa. (Photo. by Calvin.) by year, gives rise to the ‘‘walled” lakes (Fig. 125), which are not uncommon in the northern part of the United States. The ‘‘wall’’ does not commonly extend entirely around a lake. If a lake is bordered by a low marsh, the ice and frozen earth of the latter are really continuous with the ice of the lake, and the push of the latter may arch up the former into distinct ridges (anticlines), the frozen part only being involved in the folds (Fig. 126). A succession of colder and less cold periods may give rise to a succession of such anticlines.! If the shore is steep, the crowd- ing of the ice against a low cliff of yielding material, such as clay, disturbs all above the shore-line (Fig. 127). Where the cliff is sufficiently resistant, it withstands the push of the ice, and the ice itself is warped and broken. On rivers. Rivers also freeze over in cold climates, and when the ice breaks up in the spring, the stones and bowlders to which it was frozen in the banks may be floated miles down the river. At 1 Buckley, Wis. Acad. of Sci., Vol. XIII, Pt. I, r9g00. A study of ice ramparts formed about the shores of Lake Mendota, Wis., in 1898-99. RIVER ICE 123 Fig. 126. Shove of shore ice where the shore was marshy. The ice of the frozen marsh is pushed up into ridges. (Buckley, Wis. Geol. Surv.) Montreal stone buildings 30 to 50 feet square, projecting so as to have river ice form about them, have been moved by the ice of the St. Lawrence. When the river ice breaks up, masses of it are carried down- Fig. 127. The shove of ice on the shore of Lake Mendota, Wis. (Photo. wy Buckley.) 124 WORK OF SNOW AND ICE stream, and in some cases accumulate in vast ‘‘jams” behind obstructions in the river. Where a jam forms above a bridge, the bridge may be swept away. Some jams occasion disastrous floods above their sites, and when they break, the waters accumulated above may sweep down the valleys with destructive violence. Poleward-flowing rivers are especially subject to such floods. The snows of their upper basins melt while the lower parts of the streams are still frozen over. The free discharge of the upper waters is thus prevented for a time, and freshets follow. On the sea. In high latitudes, ice is formed along the sea- shore. Unlike fresh water, sea-water condenses until it freezes, at a temperature of 26° to 28° Fahr., the variation being due to the amount of salt in the water. In polar regions the sea ice attains a depth of eight or ten feet at least. Floating ice of much greater thickness is sometimes seen, but it is doubtful if it represents ice formed by the freezing of undisturbed sea-water. The geologic importance of ice formed on the sea is slight. Lee Snow-fields. Over the larger part of the land, the snow of winter does not endure through the summer, and when it melts, the water follows the same course as rain; but in cold regions where the fall of snow is heavy, some of it remains unmelted from year to year, and constitutes perennial snow-fields. High mountains and the lands of high latitudes are the common habitats of snow-fields. In North America there are numerous small snow-fields in the western mountains, from Mexico to Alaska, their number and size increas- ing to the north. In the United States there are few snow-fields south of the parallel of 36° 30’, and most of the many hundreds north of that latitude (excluding Alaska) are small. Snow-fields comparable to those of the northwestern part of the United States and British Columbia occur in the higher mountains of Europe and Asia, while in South America there are snow-fields of small size even in equatorial latitudes. Small snow-fields occur on the highest peaks of tropical Africa, and in the mountains of New Zealand. For reasons which will APH SAS later, much of every large snow-field is really ice. Besides these fields Re snow in mountain regions, there are idlds of much greater extent in polar regions. The greater part of Green- land is covered with a single field of ice and snow, the size of which is estimated at 300,000 to 400,000 square miles (Fig. 128),—an area 400 to 600 times as large as the snow-and-ice-covered area of Swit- SNOW TO ICE 125 zerland. Numerous islands to the west of North Greenland are also partly covered with snow. In Antarctica there is a still larger field, the largest of the earth. Its area is not known, but its ex- tent is at least 6 or 8 times as great as that of Greenland. The only condition necessary for a snow-field is an excess of snow-fall over snow-waste. The lower edge of a snow-field, the snow-line, is de- pendent chiefly on temperature and snow-fall. It does not depart much from the summer isotherm of 32°, though where the snow-fall is light, it may be above this isotherm. That the snew-line is not a function of temperature only, is shown by its position in various places. Thus in the equatorial portion of the Andes, the snow-line has an altitude of about 16,000 feet on the east side of the mountains, where the precipitation is heavier, and of about 18,500 feet on the west side, where it is lighter. For the same reason the snow-line in the Himalayas is lower on the south side than on the north. Though ees Map showing the temperature and snow-fall are the icé-cap of Greenland. Only the most important factors controlling borders (shaded parts) of the island the position of the snow-line, both “~ peer Ra ae humidity and movements of air are of some importance, since both affect the rate of evaporation of snow and ice. Change of snow to ice. Snow does not lie on the surface long before it undergoes obvious change. The light flakes are trans- formed into granules, and the snow becomes ‘‘coarse-grained.”’ The granular character, so pronounced in the last banks of snow in the spring, is even more distinct in perennial snow-fields. This granular snow is called névé. Where the thickness of the snow is great, the névé becomes compact below, and grades into porous ice. Ice is found in some snow-fields at no great depth from the surface. Structure of the ice. The ice of a snow-field is in some sense stratified. It is made up of successive falls of snow which tend to 126 WORK OF SNOW AND ICE retain their individuality. Thus the snow of one season may have been considerably changed before the next season. Again, the sur- face of the snow-field at the end of the melting season is generally soiled by a little earthy matter, some of which was blown up on the surface during the melting season, and some of which was concen- trated at the surface by the melting of the snow in which it was originally imbedded. In many places this earthy matter is sufficient to ‘define snows of successive years, giving the ice a somewhat stratified appearance. In addition to its stratification, the ice of the deeper portions may take on a stratiform structure which may be called foliation, to distinguish it from the stratification which arises from deposition. Foliation appears to be akin to slaty or schistose cleavage, and to result largely from the shearing of one part of ice over another, as it moves forward. , Texture. Ice formed from snow is composed of interlocking crystals. The crystalline character is assumed by the snow-flakes when they form, and the subsequent changes which the snow under- goes seem to modify the original crystals by building up some and destroying others. By the time the snow is converted into névé, the granules have become coarse, and wherever the ice derived from the névé has been examined, the granular crystalline texture is present. The individual crystals in the ice are usually larger than those of the névé, and more closely grown together. In compact ice, the crystals are so intimately interlocked that they are not seen readily by the eye; but when the ice has been honeycombed by partial melting, the granules become partially separated and may be seen easily. It is therefore legitimate to assume that a granu- lar crystalline condition persists throughout all stages of the history of ice formed from snow. Inauguration of movement. When the ice beneath a snow-field becomes very deep, motion is developed. The exact nature of the motion has not been demonstrated to the satisfaction of all who have studied the problem, though much is known about it. Brittle and resistant as ice seems, it may, under proper conditions, be made to exhibit some of the characteristics of a plastic substance. A piece of ice may be made to change its form, and may even be mould- ed into almost any desired shape if subjected to sufficient pressure, applied steadily through long intervals of time.' These changes may be brought about without visible fracture, and have been GLACIERS 127 thought to point to a viscous condition of the ice. There is much reason, however, to question this interpretation. Whatever the real nature of the movement, its aggregate result in a field of ice is ‘comparable, in a superficial way at least, to that which would occur if the ice were capable of moving like a viscous liquid, the motion taking place with extreme slowness. This slow motion of ice in an ice-field is glacier motion, and ice thus moving is glacier ice. The cause of movement is gravity, which tends to bring the ice to lower levels, just as it tends to bring water, in similar positions, to lower levels. GLACIERS Types. The different shapes of glaciers have given rise to differ- ent names. If the surface on which the ice-sheet develops is plane, the ice will move outward in all directions, and ice spreading in all directions from a center is an ice-cap. The glacier covering the larger part of Greenland (Fig. 128) is a good example. The glaciers on some of the flat-topped peninsular promontories of the same island are examples of small ice-caps (Fig. 129). If ice-caps cover a large part of a continent, as some of those of the past have done, they are called continental glaciers. Where ice-caps lie on pla- teaus whose borders are dis- sected by valleys, tongues of ice from the ice-cap may ex- tend down the valleys. They Fig. 129. Ice-caps of small size. The . figure also shows some valley glaciers constitute one type of valley extending out from the main ice-sheet glacter. A second and more and from the local ice-caps. A portion familiar type of valley glacier oe Ean cms Spree aan of occupies mountain valleys, and is the offspring of mountain snow-fields. The former type, confined chiefly to high latitudes, are polar or high-latitude glaciers (Fig. 130); the latter are alpine glaciers (Figs. 131, 132). The distinctive feat- ure of high-latitude glaciers is their steep slopes at sides and ends. 1 For an account of experiments illustrating the mobility of ice see Aitkin, Am. Jour. Sci., Vols. V, 1873, Pp. 395; and XXXIV, 1887, p. 149, and Nature, Vol. XXXIX, p. 203. 128 WORK OF SNOW AND ICE Fig. 130. End of Bryant glacier, a high-latitude glacier of North Greenland. Fig. 131. The Rhone glacier. (Photo. by Reid.) GLACIERS _ TG Fig. 132. The medial moraine of the Roseg Glacier, Switzerland. » .When a valley glacier descends through its valley to a plain beyond, its end spreads. | If the deploying ends of adjacent glaciers merge, the resulting body of ice constitutes a predmont glacier (Fig. 133). Piedmont glaciers are confined to high latitudes. In some sy \y \ + N XY Ni y o> ish Py Yalrutat Bay Pactlic Ocear fla val Fig. 133. Malaspina Glacier, a piedmont glacier in Alaska. (After Russell.) 130 WORK OF SNOW AND ICE cases the snow-field that gives rise to a glacier is restricted to a relatively small depression in the side of a mountain, or in the escarpment of a plateau. In such cases the snow-field and glacier are hardly distinguishable, and the latter descends but little below the snow-line. Such a glacier, nestled in the face of a cliff, has been called a cliff glacier! (Fig. 134). Cliff glaciers may be as wide as Fig. 134. A cliff glacier, coast of North Greenland. The height of the cliff is perhaps 2,000 feet. The water in the foreground is the sea. long, and are always small. Between them and valley glaciers there are all gradations (Fig. 135). Occasionally the end of a valley glacier, or the edge of an ice-sheet, reaches a precipitous cliff, and the end or edge of the ice breaks off and accumulates like talus below. The fragments of ice may then become a coherent mass by regela- tion, and the whole may resume motion. Such a glacier is called a reconstructed glacier. The precipitous cliffs of the Greenland coast furnish illustrations. Of the foregoing types of glaciers, ice-caps far exceed all others in both size and importance, while valley glaciers outrank the remaining types; but since valley glaciers are the most familiar, the general phenomena of glaciers will be discussed with primary reference to them. 1 Jour. of Geol., Vol. III, p. 888. GLACIERS “131 Fig. 135. Glaciers intermediate in type between cliff glaciers and valley glaciers. Cascade Mountains, Wash. (Willis, U. S. Geol. Surv.) General Phenomena of Glaciers ' Dimensions. Some valley glaciers occupy only the upper parts of mountain valleys, others extend through them, and push out on the plain beyond. In length they range from a fraction of a mile to many miles. Their thickness is usually measured by scores or hundreds of feet rather than by denominations of a larger order, but the variation is great. The minimum thickness is that which is necessary to cause movement, and this varies with the slope, the temperature, and other conditions. There is also much variation in the thickness in different parts of the same glacier. Asa rule, it is thinnest in its terminal portion, and thickest at some point be- tween its terminus and its source. Cliff and reconstructed glaciers 1 The following list includes some of the more available articles and treatises on existing glaciers; others are referred to in the following pages. Alaskan glaciers: Reid, (1) Nat. Geog. Mag., Vol. IV, pp. 19-55; (2) Sixteenth Ann. Rept., U. S. Geol. Surv., Part I, pp. 421-461. Russell, (1) Nat. Geog. Mag., Vol. III, pp. 176-188; (2) Jour. of Geol., Vol. I, pp. 219-245. Glaciers in the United States: (1) Russell, Eighteenth Ann. Rept., U. S. Geol. Surv., Part II, pp. 379-409; (2) Glaciers of North America. Greenland glaciers: Chamberlin, Jour. of Geol., Vol. II, pp. 768-788; Vol. III, pp. 61-69, 198-218, 469-480, 565-582, 668-681, and 833-843; Vol. IV, pp. 582- 592. Salisbury, Jour. of Geol., Vol. IV, pp. 769-810. 132 WORK OF SNOW AND ICE are comparable in size to the smaller valley glaciers. An ice-cap is thickest, theoretically, at its center, and thins away to its borders; but its actual thickness is influenced by the topography of the sur- face beneath it. The Greenland ice-cap rises about 9,000 feet above the sea toward its southern end, and it probably rises higher in the unexplored center of the broader part of the island. The height of the rock surface beneath the ice is unknown, but it is unlikely that: it averages half this amount, and hence the ice is probably very thick at its center. Limits. The ice of a glacier is always moving forward, but the end of a glacier may be retreating, advancing, or remaining station- ary, according as waste exceeds, falls short of, or equals forward movement. The position of the lower end of a glacier is therefore determined by the ratio of movement to waste. Its upper end is generally ill-defined. In a superficial sense, it is where the ice emerges from the snow-field; but the lower limit of the snow-field is ill-defined, and in any case is not the true upper limit of the glacier. The snow-field is really an ice-field covered with snow, and there is movement from it to the tongue of ice in the valley. The ice so moving is, in reality, a part of the glacier. The lower end of a glacier is usually free from snow and névé in summer, but its upper end is covered with névé or snow, and finally merges into the snow-field without ceasing to be a glacier. The term glacier is, however, commonly used to mean merely the more solid portion outside (below) the snow-field. Movement. The advance of a glacier is too slow, as a rule, to be seen from day to day, but is detected in other ways. If its end advances, it overrides or overturns objects which were in front of it, or it moves out over ground previously unoccupied. But even when the end of a glacier is not advancing, movement of the ice may be established by means of stakes or other marks on its surface. If the position of these marks relative to fixed points on the sides of the valley is noted, they are found, after a time, to have moved down the valley. | Rows of stakes or lines of stones set across a glacier in its upper, middle and lower portions have revealed many facts concerning the movement of the ice. Generally speaking, the central part moves ‘faster than the sides, and the top faster than the bottom. In Switzerland the determined rates of movement range from one or two inches to four feet or more per day. Some of the larger glaciers MOVEMENT OF GLACIERS 133 in other regions move more rapidly, but it does not follow that large glaciers always move faster than small ones. The Muir glacier of Alaska has been found to move some seven feet per day,} and some of the glaciers of Greenland move, in the summer time, so or 60 feet per day; but these rates have been observed only where the ice of a large inland area crowds down into a comparatively narrow fiord, and debouches into the sea, and there only in the summer. In the case of the glacier with the highest recorded summer rate of movement (1oo feet per day), the advance was only 34 feet a day in April. The average movement of the border of the inland ice of Greenland is very small, probably less than a foot a week. Conditions affecting rate of movement. The rate of glacier movement depends on (1) the depth of the moving ice, (2) the slope of the surface over which it moves, (3) the slope of the upper surface of the ice, (4) the topography of its bed, (5) the temperature of the ice, and (6) the amount of waterit contains. Great thickness, steep slopes, smoothness of bed, a high (for ice) temperature, and abund- ance of water, favor rapid movement. Since some of these condi- tions, notably temperature and amount of water, vary with the season, the rate of movement of a glacier varies during the year. Other conditions vary through longer periods of time, and cause corresponding variations in the rate of movement. A sloping upper surface is essential to glacier motion, and the motion is down-slope. ‘There are short stretches where this is not the case; indeed there are places where the upper surface declines away from the direction of motion, as where the ice pushes up over a swell in its bed; but such cases are local exceptions and do not militate against the general truth of the statement that the upper surface of a glacier declines in the direction of motion. A declining lower surface is less necessary. In the case of a valley glacier, the bed does, as a rule, decline in the direction of motion; but the deep basins in rock which many such glaciers leave behind them when they retreat, show that the bottom of a valley glacier does not slope downward at all points. In the great continental glaciers of recent geologic times, the ice moved up slopes for scores, and even hundreds of miles; but in all such cases, the prevailing slope of the upper sur- face was down in the direction of movement. Fluctuations of glaciers. The lower ends of glaciers advance 1Reid. Natl. Geog. Mag., Vol. IV, p. 44. 134 WORK OF SNOW AND ICE and retreat at intervals', and the periods of advance follow a suc- cession of years when the snowfall was heavy and the temperature low, while the periods of retreat follow years when the snowfall was light and the temperature above normal. The periods of advance and retreat lag behind the periods of heavy and light snowfall, respectively, by some years, and a long glacier re- sponds less promptly than a short one. Likenesses and _ unlike- nesses of glaciers and rivers. Slope, roughness of bed, and volume affect the movement of glaciers somewhat as they af- fect the movement of rivers. The temperature of water, on the other hand, has little effect on its flow, so long as it remains unfrozen; but the effect of temperature on the motion of ice is important. In many cases, indeed, the tempera- ature, together with the water that is incidental to it, seems to be the chief factor in determin- ing its rate of movement. Its effects will be discussed later. From Fig. 136 it will be seen that a valley glacier is an elongate body of ice, following the curves of the valley in stream-like fashion. It has its origin in the snows collected on the mountain heights, and it works its way down the valley in a manner which, in the aggre- gate, is similar to the movement of a stiff liquid. The likeness to a river extends to many details. Not only does the center move faster than the sides, and the upper part faster than the bottom, as in the case of streams, but the movement is more rapid in the narrow parts of the valley and slower in the broader. These Fig. 136. Aletsch Glacier, Switzerland. 1 Reid. Variations of Glaciers. Occasional articles in Journal of Geology, Vo}. III and later volumes. ee MOVEMENT OF GLACIERS 136 and other likenesses, some of which are apparent rather than real, gave rise to the view that glacier ice moves like a stiff, viscous liquid. But while the points of likeness between glaciers and rivers are several, their differences are numerous and significant. The most obvious difference is the fact that the glacier is fractured readily, as the numerous gaping crevasses on many glaciers show. Some of the crevasses are longitudinal, some are transverse, and some are oblique. In the case of arctic glaciers, longitudinal crev- Fig. 137. Crevassed glacier, the cracking due to change in grade of bed. North Greenland. assing is especially conspicuous. Crvevasses appear to be de- veloped wherever there is appreciable tension, and the causes of tension are many. An obvious cause is an abrupt increase of gradient in the bed (Fig. 137). If the change of gradient is con- siderable, an ice-fall or cascade results, and the ice may be greatly riven. Some of the transverse crevasses at the margins appear to be the result of tension developed on curves. Oblique crevasses on the surface near the sides are commonly ascribed to the tension between the faster-moving center and the slower-moving margins, and in like manner cracks that rise obliquely from the bottom are attributed to the tension between the faster-moving parts above and the slower-moving parts below. All crevasses indicate strains. Liquids, whose pressures are equal in all directions, show nothing analogous to crevassing. Longitudinal crevasses may affect both the narrow part of a glacier and its deploying end, and are the result of tension developed by movement within the ice itself, to which, 136 WORK OF SNOW AND ICE again, rivers offer no analogy. All cracks show that the glacier is a very brittle body, incapable of resisting even very moderate strains brought to bear upon it very slowly. In its behavior under tension, therefore, a glacier is notably unlike a river. Surface moraines. ‘The surfaces of many glaciers are affected by rock debris, some of which is disposed in the form of belts or moraines (Fig. 138). The surface moraines may be lateral, medial, or ter- minal. A lateral moraine is any considerable accu- mulation of debris in a belt on the side of a gla- cier. A medial moraine is a similar accumula- tion at some distance from the margins, but not necessarily in or even near the middle. There may be several medial moraines on one glacier. In valley glaciers, the surface terminal moraine may connect two lateral moraines, making a loop roughly concentric with the end of the glacier. Besides the surface mo- raines, there may be Fig. 138. Lateral and medial moraines, scattered bowlders and the latter formed by the union of glaciers. ; > bits of rock of various sizes on the ice, and, in addition to the coarse material, there is in many cases some dust which has been blown upon the ice. Relief due to surface debris. The debris on the ice affects its topography by influencing the melting of the ice beneath and about it. Rock debris absorbs heat more readily than the ice. A thin piece of stone lying on the ice is warmed through by the sun’s rays, and, melting the ice beneath, sinks, just as a piece of black cloth would. Though a good absorber of heat, rock is a poor conductor, and so the lower surface of a thick mass of stone is not warmed = an RELIEF OF GLACIERS 137 notably, and the ice beneath, being protected from the sun, is melted less rapidly than that around it. The result is that the bowlder presently stands on a protuberance of ice (Fig. 139). When its pedestal becomes high, the oblique rays of the sun and the warm air surrounding it cause it to waste away, and the cap- ping bowlder falls. The same _ prin- ciples apply to mo- raines. A_ surface moraine protects the ice beneath from melting, and causes the development of a ridge of ice beneath itself. As the ice on either side is lowered by ablation, the mo- raine matter tends to slide down on either hand. So far does this spreading go, that in some cases the lower end of a glacier is completely covered with debris which has spread from me- dial and lateral mo- ~- Fig. 1309. A glacial table due to the protection of raines. the ice beneath the flat stone from the rays of ‘the sun. enceinslow the Taléfre Glacier. surface. Debris carried by a glacier is not restricted to its upper surface. Debris near the bottom is in some cases so abundant, espe- cially near the ends and edges of the ice, that it is difficult to locate the bottom of the glacier; for between the moving ice which is full of debris, and the stationary debris which is full of ice, there seems to be complete gradation. The debris in the lower part of arctic glaciers, and to some extent of others, is in many cases disposed in thin sheets between layers of clean ice. Debris also occurs to some extent in the ice far above its base, in some places in sheets and in 138 WORK OF SNOW AND ICE some places in bunches. These various relations are illustrated by Figs. 140 and 141. Drainage. Some of the water produced by surface melting forms little streams on the ice. Sooner or later they plunge into crevasses or over the sides and ends of the glacier. In the former case, they may melt or wear out well-like passages (moulins) in the Fig. 140. Side view of end of glacier. Southeast side of McCormick Bay, North Greenland. Shows foliated structure of ice as well as position of debris. ice, and hcles or ‘‘ wells” in the rock beneath. Much of the surface water sinks into the ice without forming streams. The depth to which: water penetrates is undetermined by observation, but it doubtless goes down to the zone of constant temperature in all cases, and still lower where there are crevasses, and where the temperature is not below freezing. Once within the glacier, the course of the water is variable. Exceptionally it follows definite englacial channels, as shown by the springs and streams which issue from some glaciers above their bottoms. More commonly it descends or moves forward through the irregular openings which the accidents of motion have made. If it reaches a level where the temperature is below 32° it freezes. Otherwise it remains in cavities or descends to the bottom. The STRUCTURE OF GLACIER ICE 139 water produced by melting within the glacier probably follows a similar course. So far as these waters descend to the bottom, they join those produced by basal melting, and issue from the glacier with them. In some alpine glaciers, the waters beneath the ice unite in a common stream in the axis of the valley, and hollow out a_ tunnel in the bottom of the ice. The Rhone River is already a consider- able stream where it issues from beneath the glacier. In high lati- tudes, subglacial tunnels are not common, and the drainage is in streams along the sides of the gla- ciers, or through the debris beneath and about them. At the end of the glacier, all waters, whether they have been superglacial, englacial, or subglacial, unite to bear away the silt, sand, gravel, and even small bowlders set free from the ice, and to spread them in belts along the border of the ice, or in trains stretching down the val- leys below, forming glacto-fluvial deposits. : Fig. 141. A part of the vertical The structure and the motion of — gide of a North Greenland glacier. glacier ice have been the subject of |The vertical or even overhanging faces are in some cases more than much discussion. Though univer- 100 feet high. sal agreement concerning them has not been reached, a brief outline of one of the current views is added. Mention also is made of other views, some of which are still held by various geologists. THE STRUCTURE OF GLACIER ICE The key to the structure and motion of glacier ice is based on the view that a glacier is a mass of crystalline rock of the purest and simplest type known. It is made of a single simple mineral, ice, which is always crystalline. It differs from other rock chiefly in that its one mineral is liquefied at a low temperature. The development of ice from snow. The fundamental conception of a glacier is best developed by tracing the growth of its constituent crystals. When 140 WORK OF SNOW AND ICE water solidifies from the vapor of the atmosphere, it takes the form of separate crystals (Fig. 142). The flakes are rarely perfect, but they are always crystals. Snow crystals may continue to grow so long as they are in the atmosphere; or if the air is warm or dry they shrink, from melting or evaporation. When they reach the ground, the processes of growth and shrinking continue, and the crystals increase or decrease according to circumstances. A glacier is a colossal aggregation of crystals grown from snowflakes to granules of greater size and more compact form. ‘The microscopic study of snowflakes shows how they change from flakes to granules. The slender points and angles of new- fallen flakes melt and evaporate more than the central portions. The water’ (and doubtless the water vapor) thus formed gathers about the centers of the flakes and, if the temperature is right, freezes there. These are first steps toward the pronounced granulation of snow which has lain long on the ground. Measured from day to day, the larger granules beneath the surface of coarse-grained snow are found to be growing. When the temper- ature of the atmosphere is above the melting-point, the growth is faster than when the air is colder, but there is an increase in the average size of the granules, and a decrease in their number, under all conditions of temperature. Part of the increase of the larger granules appears to be at the expense of the smaller ones; part doubt- less comes from the moisture of the atmosphere which penetrates the snow and condenses there, and part from the descent of water due to surface melting. Deep beneath the surface of a large body of snow, the larger part of the growth of the large granules is probably at the expense of the small ones. To understand how this takes place, it should be noted that the free surface of every granule is constantly throwing off particles of water-vapor (i. e., evaporating); that the rate of evaporation increases with the sharpness of the curve of the surface, and that the smaller the particles, the sharper the curve; that the surface of a granule is liable to receive and retain molecules evaporated from other granules, and that, other things being equal, the retention of particles is most common on surfaces of least curvature. It follows that the larger granules of less curvature will lose less and gain more, on the average, than the smaller granules. The result is that the larger granules grow at the expense of the smaller. Another factor affecting the growth of granules is pressure and tension. The granules are compressed at their points of contact, and under tension elsewhere. Tension increases the tendency to evaporation, and the capillary spaces adjacent to the points of contact probably favor condensation. Pressure reduces the melting-point, while tension raises it. Though the effect of this is slight, it is to be correlated with the much more important fact that compression produces heat ~ which may bring the temperature of the ice to the melting temperature at some points, while tension may reduce it to or below freezing temperature at adjacent points. There is therefore a tendency for the ice to melt at points of contact and compression, and for the water so produced to refreeze at adjacent points where the surfaces of granules are under tension. This process becomes effective beneath a considerable body of snow, and here the granules gradually lose the spheroidal form assumed in the early stages of granulation, and become irregular polyhedrons, interlocked into a mass of more or less solid ice. Whether these processes furnish an adequate explanation of the changes or not, all gradations may be observed from snowflakes to granular névé, and thence to the granules of glacier ice, ranging in size up to that of walnuts, and even 141 SNOWFLAKES Photographs of snowflakes, enlarged. (Bentley.) Fig. 142. 142 WORK OF SNOW AND ICE beyond. In coherence, these aggregations vary from the névé stage, where the grains are small and spheroidal, to the ice stage, where the cohesion is strong through the interlocking growths of the large granules. MOTION OF GLACIER ICE! Rotation and sliding of granules. There seems to be no escape from the con- clusion that the primal cause of glacier motion is one which may operate even under the relatively low temperatures, the relatively dry conditions, and the relatively granular textures at the heads of glaciers. These considerations lead to the view that movement there takes place by the movements of the grains upon one another. While they are in the spheroidal form, as in the névé, this would not seem to be difficult. They may rotate and slide over each other as the weight of the névé increases, and the motion between the granules might be comparable to that between shot in great quantities in similar positions. The amount of motion required of an individual granule is surprisingly small. In order to account for a movement of three feet per day in a glacier six miles long, the mean motion of the average granule relative to its neighbor would be roundly, todo of its own diameter per day; in other words, it should change its relations to its neighbors to the extent of its diameter once in about thirty years. A change of such slowness under the conditions of granular alteration can scarcely be thought improbable. Melting and freezing. After the granules become interlocked, as in the body of the glacier below the névé field, rotation and sliding must be more difficult. Then, if not earlier, the movement between granules is supposed to be effected chiefly by the temporary passage of minute portions of the granules into the fluid form at points of greatest compression, the transfer of the water thus produced to adjoining points, and its resolidification. The points of greatest compression are obviously those whose yielding most promotes motion, and the successive yielding of points which come in succession to oppose motion most (and thus to receive the greatest stresses), permits continuous motion. It is only necessary to assume that the gravity of the accumulated mass is sufficient to produce a little temporary iiquefaction at the points of greatest stress, the result being accomplished not so much by the lowering of the melting-point as by the development of heat by pressure. This is believed to be the largest single element in glacier motion. This conception of glacial movement involves the momentary liquefaction of minute portions of the ice, while the mass as a whole remains rigid, as its crystalline nature requires. Instead of assigning a slow viscous fluidity like that of asphalt to the whole mags, which seems inconsistent with its crystalline character, it assigns a free fluidity, momentarily, to a succession of particles that form only a minute fraction of the whole at any instant. This conception is consistent with the reten- tion of the granular condition of the ice, with its rigidity and brittleness, and with © its strictly crystalline character, a character which a viscous liquid does not possess, however much its high viscosity may make it resemble a rigid body. Accumulated motion in terminal part of glacier. However slight the relative motion of one granule on its neighbor, the granules in any part of a glacier partake in the accumulated motion of all parts nearer the source, and hence all except those at the head are thrust forward. Herein appears to lie the distinctive nature of glacial movement. Each part of a stream of water feels (1) the hydrostatic pres- - £or fuller discussion, see the authors’ Geologic Processes, pp. 308-321. MOTIONS OF GLACIER ICE 143 sure of neighboring parts (theoretically equal in all directions), and (2) the mo- mentum of motion, but vot the thrust of the water up stream. This is probably one of the fundamental differences between water flow and glacier motion. Lava streams are good examples of viscous fluids flowing in masses comparable to those of glaciers, on similar slopes, and, in the last stages of motion, at similar rates; but their special modes of flow and their effects on the sides and bottoms of their paths are radically different from those of glaciers. Forceful abrasion, and particularly the rigid holding of imbedded stones which score and groove the rock beneath, is unknown in lava streams, and is scarcely conceivable. There is, so sate Sees e er i mas “ Fig. 143. A well defined shearing plane in a Spitzbergen glacier. (Hamberg.) far as we know, no experimental or natural evidence that any viscous fluid, in the ordinary sense of that term, detaches and picks up fragments and holds them firmly as graving tools in its base so as to cut deep, long, straight grooves in the hard bottom over which it flows. It would seem that competency on the part of a “viscous body to do this peculiar class of work so distinctive of glaciers should be demonstrated before the viscous theory of glacial movement is accepted as even a good working hypothesis. In contrast with viscous movement, it is conceived that a glacier is thrust forward rigidly by internal elongation, and that it is sheared forcibly over its sides and bottoms, leaving its distinctive marks upon them. Shearing. In the terminal part of a glacier, where the thrusts are greatest, where the granules are fewest and their interlocking most intimate, shearing takes place within the ice. This is illustrated by Figs. 143 and 144. The shearing re- sults in the foliation of the ice, and in the dragging of debris along the planes of shear. Shearing is obs:rved chiefly where the ice below the plane of shearing is protected more or less from the force of the thrust, as in the lee of a hill or mass of 144 WORK OF SNOW AND ICE Fig. 144. Portion of the east face of Bowdoin Glacier, North Greenland, show- ing oblique upward thrust, with shear. débris. It perhaps occurs also at the top of the basal zone of ice so loaded with débris that it is incapable of ready movement. It is probable, also, that sharp differential strains and shearing are developed at the level where the surface water of the warm season, sinking into the ice, reaches the zone of freezing; for the expansion which attends the freezing may cause the expanding layer to shear over the part below. As the level of freezing descends with the advance of the warm season, the zone of shearing sinks. Expansion at the zone where descending water freezes not only leads to shear, but to the development of surface cracks, for the surface is stretched as the zone below expands. In the course of years, the cracks developed in this way may become wide crevasses, limited below by the depth of the zone of freezing in summer. | High temperature and water. Toward the:lower end of a glacier, the higher temperature and the greater abundance of water lend their aid to the fundamental — elements of movement. During the warm season, the ice here is bathed in water — all the time, so that the necessary changes in the crystals are facilitated. Under these conditions, movement takes place more readily than in the drier, colder and ~ more open, granular ice of lower temperature, near the source of the glacier. Application. The co-operation of these several factors appears to explain the ~ peculiarities of glacial movement. In regions of intense cold, where a dry state and low temperature prevail, as in the heart of Greenland, the snow-ice mass may Pe 1 The crystals of ice have a peculiar structure which has been thought by some to be an important factor in shearing, and so in the motion of glaciers ice. See the authors’ Geologic Processes, p. 312; also (10) p. 323. MOTIONS OF GLACIER ICE 145 attain extraordinary thickness. Here the burden of movement seems to be thrown almost wholly on compression, with the slight aid of molecular changes due to internal evaporation. Since the temperature in the upper part of the ice is very low most of each year, the compression must be great before it becomes effective in melting; hence the great thickness of ice necessary before motion is considerable. Similar conditions affect the heads of Alpine glaciers, though here the high gradients favor motion among the granules of ice. In the lower reaches of Alpine glaciers, where the temperatures are near the melting-point, and where the ice is bathed in water much of the time, movement may take place in ice which is thin and compact. If the views here presented are correct, there is also, at all points below the source, the co-operation of rigid thrust from behind, with the tendency of the mass to move on its own account. The latter is controlled by gravity, and conforms in tts results to laws of liquid flow. The former is a mechanical thrust. This thrust is different from the pressure of the upper part of a liquid stream on the lower part, because it is transmitted through a body whose rigidity is effective, while the latter is transmitted on the hydrostatic principle of equal pressure in all directions. Thrust would be most effective toward the end or edge of a glacier. Corroborative phenomena. ‘The conception of the glacier and its movement here presented explains some of the anomalies that otherwise seem paradoxical. If the ice is always a rigid body which yields only as its interlocking granules change their form by loss and gain, a rigid hold on the imbedded rock at some times, and a yielding hold at others, is intelligible. Stones in the base of a glacier may be held with great rigidity when the ice is dry, scoring the bottom with much force, while they may be rotated with relative ease when the ice is wet. In short, the relation of the ice to the bowlders in its bottom varies radically according to its dryness and temperature. A dry glacier is a rigid glacier. A dry glacier is neces- sarily cold, and a cold glacier 1s necessarily dry. It is difficult to explain the furrows and grooves cut by glaciers in firm rock if the ice is so yielding as to flow under its own weight on a surface which is almost flat. If the mass is really viscous, its hold on its imbedded debris should also be viscous, and a bowlder in the bottom should be rotated in the yielding mass when its lower point catches on the rock beneath, instead of being held firmly while a groove iscut. ‘This is especially to the point since viscous fluids flow by a partially rotary movement. On the view here presented, a glacier should be more rigid in winter than in summer. ‘The total thickness of a glacier should experience this rigidity of winter at its ends and edges, where the ice is thin enough to permit the low temperature to affect its bottom. The motion in these parts during the winter is, therefore, very small. In this view, also, may be found an explanation of the movement of glaciers for considerable distances up-slope, even when the surface of the ice, as well as its | bed, is inclined backwards. So far does this go, that a few superglacial streams run for some distance backwards, i. e., toward the heads of the glaciers, while in other places surface waters are collected into ponds and lakelets. Such a slope of the surface of ice is not difficult to understand if the movement is due to thrust from behind, or if it is occasioned by internal crystalline changes acting on a rigid body; but it must be regarded as very remarkable if the movement of the ice is that of a fluid body, no matter how viscous, for the length of the acclivity is in some cases several times the thickness of the ice. Crevassing and other evidences of 146 WORK OF SNOW AND ICE brittleness and rigidity find a ready elucidation under the view that ice is really a solid body at all times, and that its apparent fluency is due to the momentary ~ fluidity of small portions of its mass assumed in succession as compression demands. In addition to the considerations already adduced, it may be urged that a glacier does not flow as a stiff liquid because its granules are not habitually drawn out into elongated forms, as are cavities in lavas, and plastic lumps in viscous bodies. Flowage lines comparable to those in lavas are unknown in glaciers. All this is strictly consistent with our primary thesis, that a glacier is crystalline rock of the purest and simplest type, and that it never has other than the crystalline state. This strictly crystalline character is incompatible with viscous liquidity. Other views of glacier motion. While these views of glacial motion seem to us to accord best with the known facts, they are not to be regarded as established in scientific opinion, or as the views most commonly held. The main alternative interpretations that have been entertained are the following: (1) In the early days of glacial studies De Saussure thought that glaciers slid bodily on their beds. (2) Charpentier and Agassiz referred the movement to the expansion of descending water freezing within the glacier. (3) Rendu and Forbes, followed by many modern writers, believed ice to be viscous, and that in sufficiently large masses it flows under the influence of its own weight, like pitch or asphalt. (4) Others, realizing the fundamental difference between crystalline ice and a true viscous body, have fallen back on a vague notion of plasticity, which scarcely amounts to a definite hypothesis at all. (5) Tyndall urged that the movement was accomplished by minute repeated fracturing and regelation, appealing to the fact that broken pieces of ice slightly pressed together at melting temperatures freeze together, but neglecting the fact that this would destroy the integrity of the crystals. (6) Moseley assigned the movement to a bodily expansion and contraction of the glacier, analogous to the creeping of a mass of lead on a roof. (7) James Thompson demonstrated that pressure lowers the melting-point. and while this effect is so small as probably to be ineffectual, it is correlated with the very important fact that compression, by generating heat, may cause melting, which is not the case in most other rocks. He recognized that under pressure partial liquefaction took place, that the water so liberated might be refrozen as it escaped from pressure, and appears to have regarded this as a vital factor. (8) Croll held that the movement was due to a consecutive series of molecular changes somewhat like the chain of chemical combinations in electrolysis. (9) Hugi, Eli de Beaumont, Bertin, Forel, and others thought that the growth of the granules was the leading factor in ice movement. (10) McConnel and Miigge have made the gliding planes of the ice crystals serve an important function in glacial movement. It will be seen that the principle of partial liquefaction for which Thompson laid the basis, the crystallization of descending water urged by Charpentier and Agassiz, and the granular growth on which Hugi, Beaumont, Forel, and others founded their hypotheses, are incorporated in the view already presented. Prob- ably the agencies on which some of the other views are based may also be partici- pants in producing glacial motion, in some places as incidental factors, and in others perhaps as important ones. EROSION BY GLACIERS 144 THE WORK OF GLACIERS Erosion Glaciers abrade the valleys through which they pass, carry for- ward the material which they remove from the surface, and wear, grind, and ultimately deposit it. Like other agents of gradation, their work includes erosion, transportation, and deposition. Getting load. If the snow-field which is to become a glacier accumulates on a rough surface covered with rock debris, the glacier has a basal load when it begins to move, for the snow covers, sur- rounds, and includes such loose blocks of rock as project above the general surface, and envelops all projecting points of rock within its field. When the ice begins to move, it carries forward this debris in its bottom, and tears off the weak points of rock which project up into it. In addition to the basal and sub-glacial load which the glacier has at the outset, there may be surface debris which has fallen on the snow or ice from cliffs above. If debris descending to the glacier in this way is unburied, it is superglacial, but if it has been buried by subsequent falls of snow, it is englacial. Once in movement, the ice not only moves the debris to which it was originally attached, but it gathers new load, partly by the rasp- ing effect of its rock-shod bottom, and partly by its power of pluck- ing off or quarrying out considerable blocks of rock from its sides and bottom. ‘This plucking process is at its best where the ice passes over cliffs of jointed rock, but is not confined to such situations. The steep bed of a valley glacier may be worn more by plucking than by rasping. The advancing ice gets some material, too, espe- cially loose debris, by freezing to it, for the water in the soil freezes and becomes’ continuous with the ice above, and moves with it. Superglacial material may be acquired during movement, as well as before it, by the fall of debris from cliffs, or by the descent of ava- lanches. : Conditions influencing rate of erosion. (1) Ice wears a flat surface relatively little, since there is little for it to get hold of. Glaciers have been known to override such a surface, burying its soil and more or less of its herbaceous vegetation. Erosion is at its maximum, so far as influenced by topography, when the surface is rough enough to offer notable catchment for the base of the ice, but not so rough as to impede its motion seriously. Other conditions which influence glacial erosion are (2) the amount of loose or slightly 148 WORK OF SNOW AND ICE attached debris on the surface; (3) the slope of the surface; (4) the thickness of the ice; (5) its rate of movement; (6) the resistance of the rock; and (7) the amount and kind of debris the ice carries. The effect of most of these conditions is evident, but the last two call for a word of explanation. So far as concerns the resistance of the rock, it should be noted that resistance is not a matter of hardness simply. Rock which is affected by cleavage, whether joints or bedding planes or both, is eroded readily, expecially on steep slopes, even if very hard. In Fig. 145. Striz on bed-rock. Kingston, Des Moines County, Iowa. such situations, the removal of rock in large blocks (plucking) is probably more important, on the whole, than wear by the debris carried. : Clean ice passing over a smooth surface of solid rock would have little effect upon it; but a rock-shod glacier abrades the same surface notably. The effect of this abrasion is shown in the grooves and scratches (stri@) which the stones in the bottom of the ice inflict on the surface of the rock over which they pass (Figs. 145 and 146). At the same time, the stones in the ice are worn by abrasion both with the bottom, and with one another (Fig. 147). It does not fol- low, however, that erosion is greatest when there is most material in the bottom of the ice; for with increase of debris there may be EROSION BY GLACIERS 149 Fig. 146. Strie, grooves, etc., in a canyon tributary to Big Cottonwood Can- yon, Wasatch Mountains. (Church.) Fig. 147. Stones striated by glacial wear. Theirshapes, as well as their mark- ings, are characteristic. decrease of motion,! and decrease of motion retards erosion. When any considerable thickness of ice at the bottom of a glacier is full of debris, the loaded part may approach stagnancy, while the cleaner ice above shears over it. A moderate but not excessive load of debris, therefore, favors great erosion. Something depends, too, on the character of the load. Coarse, hard, and angular debris is 1 Russell. Jour. of Geol., Vol. III, p. 823. 150 WORK OF SNOW AND ICE more effective for abrasion than fine, soft, or rounded material. In plucking, rate of motion is probably more important than load. So far as concerns the ice itself, erosion is not most effective at the end of a valley glacier, or at the edge of an ice sheet, for here the strength of movement is too slight and the load too great; nor is it most effective at the source or near it, for the ice here moves slowly and its load is likely to be slight. Ice alone considered, erosion is most effective somewhere between the source and the terminus of a glacier, and probably much nearer the latter than the former. In summary it may be said that rapidly moving ice of sufficient thickness to be working under goodly pressure, shod with a sufficient but not excessive quantity of hard-rock material, passing over non- resistant formations possessing a topography of sufficient relief to offer some resistance, and yet too little to retard the progress of the ice seriously, will erode most effectively. Varied nature of glacial debris. From its mode of erosion it will be seen that a glacier may carry various sorts of material. At its bottom there may be (1) bowlders which the ice has picked up from the surface, or which it has broken off from projecting points of rock over which it has passed; (2) smaller pieces of rock of the size of cobbles, pebbles, etc., either picked up by the ice from its bed or broken off from larger masses; (3) the fine products (rock- flour) produced by the grinding of the debris in the ice on the rock- Fig. 148. A mountain valley in the Wasatch Mountains, not glaciated. (Photo. by Church.) EROSION BY GLACIERS Ist bed over which it passes, and similar products resulting from the rubbing of stones in the ice against one another; and (4) sand, clay, soil, vegetation, etc., derived from the surface overridden. Thus the materials which the ice carries (called drift) are of all grades of coarseness and fineness, from huge bowlders to fine clay. The coarser materials may be angular or round at the outset, and their forms may be changed and their surfaces striated as they are moved forward. Whether one sort of material or another predominates depends primarily on the nature of the surface overridden. The topographic effects of glacial erosion. In passing through its valley, an alpine glacier deepens it, widens its lower part, and smoothes its slopes up to the limit of the ice. It tends to make a Fig. 149. A mountain valley which has been strongly glaciated, Wasatch Mountains. (Photo. by Church.) V-shaped valley (Fig. 148) U-shaped (Fig. 149), and to make its head big, blunt, and steep-sided. Such a valley head is a cirque (Pl. XIII). The change in topography at the upper limit of glaciation is striking in many places (Fig. 150). The deepening of a valley by glacial erosion may throw its tributaries out of topographic adjustment. Thus if a main valley is lowered 100 feet by glacial erosion while its tributary is not deepened, the lower end of the latter will be too feet above the former when the ice disappears. Such valleys, called hanging val- leys (Fig. 151), are common in the western mountains of North America which were recently glaciated. 152 WORK OF SNOW AND ICE Ice-caps which overspread the surface irrespective of valleys and hills tend to reduce angularities of surface. Hills and ridges are cut down and smoothed (Figs. 152 and 153); but since valleys Fig. 150. Contrast between a glaciated rock surface below, and non-glaciated crests above. Kearsarge Pinnacles, Bubbs Creek Canyon, Cal. parallel to the direction of movement are deepened at the same time, it is doubtful if the relief is commonly reduced by the erosion of an ice-cap. Fiords. A valley glacier descending to the sea may gouge out Fig. 151. A hanging valley near Lake Kootenay. (Photo. by Atwood.) TRANSPORTATION OF DEBRIS 153 the head of a bay or the lower end of a valley to a very considerable depth. When the ice melts, the bay, if narrow, deep, and long, with high slopes, is called a fiord. Many of the fiords of coasts in high latitudes originated in this way, and some glaciers of. these coasts are now mak- ing fiords. Sinking accompanying or 44 erases Pere ey << following glaciation, is also a factor in . Fig. 152. Diagram representing a hill unworn by the makin go Photds ice, and the irregular contact of soil and rock. The positions in which debris is carried. Debris is carried in three po- sitions: (1) basal or subglacial, (2) engla- cial, and (3) super- glacial. ‘The material picked up or rubbed off from the surface over which the ice moves is normally carried forward in the bottom of the ice, and is therefore basal; that which falls on the surface is usually carried there, and is therefore superglacial. Either basal or super-glacial drift may become englacial. The basal load of a glacier is constantly being mixed with new drift from the ground over which the ice is passing. The superglacial material, on the other hand, may be borne from its place of origin to its place of deposition with- out such intermixture. Transfers of load. Superglacial debris obviously may become englacial or basal by falling into crevasses, or by being carried down by descending waters. Debris which is basal at the outset, may become englacial or super- glacial later. Thus when ice passes over a hill, the bottom of the ice rends debris from Merlo 10 the ‘lee of the hill the ice from either side may close in under that which came over the _ Fig. 154. Diagram illustrating one way in which a top. The debris de- glacier gets englacial material. rived from the top of a hill by the bottom of the overriding ice will then be well up in the ice (Fig. 154). Pes EF ars EE we — yg Se es =a en ee SS SS SS ae Pe ‘Fig. 153. Diagram showing the effect of glacial wear on a hill such as is shown in Fig. 152. 154 WORK OF SNOW AND ICE Englacial drift may become superglacial by surface ablation. In this case the drift does not rise, but melting brings the surface of the ice down to it. This occurs chiefly at the end or edge of the ice, where the surface melting is greatest. Englacial debris, es- pecially that near the bottom, also may become basal by the melting of the ice beneath it. Drift is sometimes transferred from a basal to an englacial and then to a superglacial position by upward movement. Such trans- Fig. 155. Taking debris from a protuberance of the bed. fer is the more remarkable because the specific gravity. of rock is about three times that of ice, so that the normal tendency of rock is to sink in ice. In arctic glaciers, and probably in others, some material which has been basal becomes englacial by being sheared forward over ice in front of it. So far as observed, this takes place chiefly where the ice in front of the plane of shearing lies at a lower level than that behind, as where the surface of an upland falls off into a valley, or where a boss of rock shelters the ice in its lee from the thrust of the overriding ice (Fig. 155). At the borders of many arctic glaciers the lower layers are turned up, as shown in Fig. 156. Where the layers turn up at the end of a TRANSPORTATION OF DEBRIS 155 , Fig. 156. End of a North Greenland glacier, showing the upturning of the layers of ice at the end. This structure is common in North Greenland. At one point, a few stones are seen on the surface of the ice where an upturned layer comes to the surface. glacier, basal and englacial debris are carried to the surface by actual upward movement, and a terminal moraine or a series of terminal moraines may be developed where the upturned layers of ice outcrop at the surface (Fig. 157). That the material of these moraines was originally basal is shown in many cases by the bruised and scratched __ Fig. 157. Surface terminal moraines due to upturning. Edge of the ice-sheet, North Greenland. condition of the bowlders and pebbles, or by the nature of the mate- rial itself. The upturning may affect the edges of glaciers (Fig. 158) as well as their ends, and the material thus brought to the surface gives rise to lateral moraines. In some cases, too, there is upturn- ing of the ice along a longitudinal zone well back from the lateral margins (Fig. 158), and the material brought to the surface in such a zone giyes rise to a medial moraine. This upturning of ice has 156 WORK OF SNOW AND ICE been observed only at or near the terminus of the ice. It perhaps is due in part to the resistance of frozen morainic or other material beneath and in front of the edge. ‘To this should probably be added the effect of the great rigidity of the outer part of the ice due to the low external temperature during the larger part of the year, while Fig. 158. Diagram to illustrate one method of formation of medial and lateral moraines. The horizontal line at the base represents sea-level, and the lower part of the glacier is under the sea. The layers of upturning ice bring debris up along the planes of movement, and it accumulates at the top as indicated. the interior, with its higher temperature, remains more fluent; but even this probably leaves the explanation incomplete. Wear of drift in transit. Drift carried at the bottom of the ice is much worn, for the materials in transit abrade one another and are abraded by the bed over which they pass. Englacial drift is subject to less wear, because it commonly is more scattered. Super- glacial drift is worn little or none while it lies on the surface of the ice; but in so far as superglacial or englacial drift is derived from basal drift, it may show the same evidences of wear as the basal drift itself. In many cases superglacial drift reveals its history in this way. Deposition During the advance of a glacier, deposition takes place both (1) beneath the body of the ice, and (2) beneath its end and edges. In the former position it takes place where the topography favors lodgment, or where the ice is overloaded. The topography favoring deposition is much the same as that favoring erosion, but the two processes are not favored at the same points. Erosion is greatest on the ‘‘stoss”’ side (the side against which the ice advances) of an obstruction, and deposition on the lee side (Fig. 159). Glacier ice Fig. 159. Crag and tail. The passage of glacier ice is likely to leave drift in the lee of the boss of rock, C. DEPOSITION OF DRIFT 157 is likely to be overloaded (1) just beyond a place where conditions have favored erosion, and (2) where the ice is thinning rapidly.- On the whole, deposition beneath the body of a glacier back from its iS Fig. 160. Glacier building an embankment. Southeast side of McCormick Bay, North Greenland. end or edge, is much more than balanced by erosion in the same position. At and near the end of a glacier, deposition goes on faster than elsewhere, chiefly because of the rapid melting, and therefore the thinning and weakening of the ice. If the end of the glacier is stationary in position, drift is being brought to it continually and Se NS ok LS aah Fig. 161. Embankment completed. Near the last. (Fig. 160.) 158 WORK OF SNOW AND ICE left there, for it is to be remembered that the ice is moving, though its end is stationary. Ifa glacier moves forward 500 feet per year, while its end is melted at the same rate, all the debris in the 500 feet of ice melted, is deposited, and all except that washed away is deposited at and beneath the end of the glacier (Figs. 160-161). Uf ice advances 500 feet per year and is melted back 600 feet in the Fig. 162. Illecillewat Glacier; Glacier, British Columbia. A lateral moraine at the right of the ice records its diminution. same time, all the debris carried by the 600 feet melted has been deposited, and largely in the narrow zone (100 feet) from which the ice has receded. If the end of a glacier advances 500 feet per year while it is being melted but 400 feet, all the drift in the 4oo feet melted is deposited, chiefly at or beneath the immediate margin of the ice. To the marginal and sub-marginal accumulations made in this way, the material carried on the ice is added whenever the ice is melted from beneath it. Deposition beneath the lateral GLACIAL DEPOSITS 159 F ig. 163. The moraines about the lower end of a glaciated mountain valley. Bloody Canyon, Cal. (U.S. Geol. Surv.) margins of a glacier is much the same as beneath its terminus (Fig. 163). Types of Moraines The terminal moraine. The thick accumulation of drift made at the end of a glacier or at the edge of an ice sheet, especially where its end or edge is stationary or nearly so for a long time, is the terminal moraine. ‘Terminal moraines of ice caps are of more im- portance, relatively, than those of valley glaciers, for streams are more effective in destroying the moraines of the latter. The topog- raphy of terminal moraines is rather distinctive, as illustrated by Fig. 168. | The ground moraine. When a glacier disappears, all its debris is deposited. All drift deposited beneath the body of the ice, and all deposited from its base during dissolution, constitutes the ground moraine. ‘The thickness of the ground moraine is notably unequal. In general, it is thicker toward the terminus of the glacier and thinner toward its source, but considerable portions of a glacier’s bed may be left without debris when the ice melts. As a rule, the ground moraine is thinner than the terminal moraine, and less irreg- ularly disposed. The ground moraines of valley glaciers are rela- tively unimportant as compared with those of ice-caps, since condi- tions for erosion under the body of a valley glacier are, on the average, better than under an ice-sheet, while those for deposition 160 . WORK OF SNOW AND ICE are less favorable. The topography of the ground moraine (Plate XIV) is, as a rule, less uneven than that of the terminal moraine (Fig. 168). Lateral moraines. Lateral moraines are the product of valley glaciers. The lateral moraines on such glaciers are let down on the surface beneath when the ice melts; but the lateral moraines in a valley from which the ice has melted are not merely the lateral moraines which were on the glacier. They are made up chiefly of drift accumulated beneath the sides of the glacier. This accumula- tion is the result of the lateral motion of the ice from the center to Fig. 164. A lateral moraine lett by a former glacier in the Bighorn Mountains of Wyoming. (Photo. by Blackwelder.) the sides of the valley. Such sub-lateral accumulations are akin to terminal moraines. Some of the lateral moraines of ancient valley glaciers, those like of the Uinta, Wasatch, and Bighorn mountains are several hundred feet high (Fig. 164), or even as much as a thou- sand. In northern Italy a lateral moraine is said to be more than 2,000 feet high. Distinctive nature of glacial deposits. The deposits made by glaciers are distinctive. In the first place, the ice does not assort its drift, and bowlders, cobbles, pebbles, sand, and clay are con- fusedly commingled (Fig. 165). In this respect, the deposits of ice differ notably from those of water. Furthermore, many stones of the drift show the peculiar type of wear which glaciers inflict. * Geikie. The Great Ice Age, 3d ed., p. 529. GLACIAL DEPOSITS 161 Fig. 165. Section of drift showing its heterogeneity: Though notably worn, they are not rounded like the stones carried by rivers. Many of them have sub-angular forms with planed and beveled faces, the planes being striated and bruised (Fig. 147). Absence of stratification, physical heterogeneity, and the striation of at least a part of the stones are among the most distinctive char- acteristics of glacial drift. A not less real though less obvious characteristic is the constitution of the fine material, for it is, as a rule, the product of rock grinding, not of rock decay. Glaciated rock surfaces. Another distinctive mark which a glacier leaves behind it is the- character of the surface of the rock -on which the drift rests. This is generally smoothed by the severe abrasion to which it has been subjected, and the smoothed surfaces (Figs. 145 and 166) are marked by grooves and striz, similar to those on the stones of the drift (Fig. 147). Other distinctive fea- tures of a glaciated area are rounded bosses of rock (roches mouton- 162 WORK OF SNOW AND ICE Fig. 167. Roches moutonnées, Engineer Mountain, Colo. (Hole, U.S. Geol. Surv.) GLACIAL TOPOGRAPHY 163 S) nées, Fig. 167), rock basins, ponds, and marshes, and the peculiar topographies resulting from the unequal erosion (Pl. XIII), and the still more unequal deposition (Fig. 168) of drift. Surface bowl- Fig. 168. Sketch of drift (terminal moraine) topography near Hackettstown, N. J. (N. J. Geol. Surv.) ders, in many cases unlike the underlying formations of rock, and sometimes in peculiar and apparently unstable positions (perched bowlders) are still another mark of a glaciated area (Fig. 169). Fig. 169. Perched bowlder, New Jersey. 164 WORK OF SNOW AND ICE GLACIO-FLUVIAL WORK The streams to which the melting of the ice gives rise are laden with gravel, sand, and silt derived from the ice. Where the mud is light-colored, the streams are sometimes described as ‘‘milky.”’ Where the amount of material carried is great, much of it is dropped at a slight distance from the ice, the coarsest being dropped first. Glacial streams are, as a rule, aggrading streams, and therefore develop alluvial plains, called valley trains (Fig. 170), or, where they Fig. 170. Diagram to illustrate the profile of a valley train, and its relations to the terminal moraine (m) in which it heads. enter lakes, bays, or other streams, deltas. In its transportation, the river-borne drift is assorted, and after its deposition it is strati- ite aS Sere Wea >.” Fig. 171. Esker of Punkaharju, Finland. fied. Glacial deposits in the upper part of a mountain valley are, therefore, generally connected with glacio-fluvial deposits farther down the valley. The silt, sand, and gravel of valley trains can, as a rule, be distinguished from valley deposits of non-glacial origin PLATE XIII GH) AN S APS ERK Wi Aa AW WY FS))/S won 9 == oD ee | . = sy Z, ee {YA oA, a | shi} Jp J, . Uy Hi 7 ‘Sj fe GSK SOG a A WY —FBESS BLN YY f ; = A portion of the Bighorn Mountains, showing glaciated valleys, the heads of which are in many cases cirques. Scale, about 2 miles per inch. (Cloud Peak, Wyo., Sheet, U. S. Geol. Surv.) PLATE XIV Characteristic surface of a glaciated plain, showing marshes, ponds, and lakes. Southern Wisconsin. Scale, about 1 mile per inch, (Silver Lake, Wis., Sheet, U. S. Geol. Surv.) oe a GLACIO-FLUVIAL DEPOSITS 165 by the fact that they are largely of undecayed rock material, espe- cially if deposited recently. Numerous streams flow from an ice-sheet, spreading their debris in front of the terminal moraine, forming a broad fringing sheet ‘of gravel and sand (outwash plain) along it. Outwash plains have much in common with piedmont alluvial plains. They differ from valley trains chiefly in being shorter, wider, and not confined to valleys. Where streams of considerable size form tunnels under the i ice, the tunnels may become more or less filled with water-worn debris, and when the ice melts, the aggraded channels appear as ridges of gravel and sand, known as eskers (Fig. 171). It has been thought that eskers represent deposits formed in superglacial channels; but this is probably rarely if ever the case, for most surface streams have high gradients, swift currents, and smooth bottoms, and hence give little opportunity for lodgment. Furthermore, ice-sheets, in con- nection with which eskers are developed, have no surface material except at their immediate edges. At the mouths of ice-tunnels or ice-channels, and in the re-entrant angles of the edge of the ice, sands and gravels are liable to be bunched in quantity, giving rise, after the adjacent ice has melted, to peculiar hills and hollows of knob-and-basin type. The hills and short ridges of stratified drift formed in this way are known as kames. Much stratified drift (gravel, sand, and silt) deposited by glacial streams has no distinctive topographic form, and therefore no special name. All fluvio-glacial deposits are stratified. Kames and eskers made in immediate association with the ice, and more or less affected Fig. 172. The end of a glacier in Spitzbergen. (Rabot.) 166 WORK OF SNOW AND ICE by its movements, are less perfectly and regularly stratified than valley trains and outwash plains. ICEBERGS Where glaciers advance into water the depth of which approaches their thickness, their ends are broken off (Fig. 172), and the de- tached masses float away as icebergs (Fig. 173). Many of the bergs ih ‘of MANE Fig. 173. Aniceberg. (Robin.) are overturned, or at least tilted, as they set sail. If this does not happen at the outset, it may later, as the result of melting, wave- cutting, etc., which shift the centers of gravity of the bergs. The great majority of them do not float far before losing all trace of stony and earthy debris; but the finding of glaciated pebbles in dredgings far south of all glaciers shows that bergs occasionally carry stones far from land. The importance of icebergs as agents of transportation has been greatly exaggerated, and the assignment of shoals, like the Banks of Newfoundland, to them, is without foundation. Map work. See Interpretation of Topographic Maps, Exercises XI to XIII, and Plates XCV to CXXIX, Professional Paper 60, U. S. Geological Survey. CHAPTER VI THE WORK OF THE OCEAN A few facts concerning the depth of the ocean and the distribu- tion of its water have been given on a preceding page (p. 5), and reference to the origin of the ocean basins and the ocean will be made later. We are concerned here chiefly with the geologic proc- esses now going on in the sea; but a few facts concerning the sea- water and its life, and the topography of the ocean’s bed,! may well precede the study of the processes now in operation. Mineral matter in solution. Every 1,000 pounds of sea-water contain about 34.40 pounds of mineral matter in solution. The principal substances in the water are the following: MMT CP GS he ye ee un BS a theres vy OG Cte: MMIC STIAULCSIUIN : ¢ 25... dy cw ey tte eee eee eens 10.878 PMMIPEMINTI AGIOS ye. or ae fa eM eben es el 4.737 LEER CPR ep, OU de eS a 3.600 IMIS OL MSSINTSN Slee 5 clef be bere a. gots oe oe wala aes ope ys 2.465 RE LECTION te Ca ay ih cic sie tldices ch wa vsive 0.345 SeEEPIIEE MA OTCSPUNT ey os. a Se Ad eve honk eh ole hae ete Ocary There are many other mineral substances in sea-water, and the gases oxygen, nitrogen, and carbon dioxide are present in quan- tity. -The amount of the last is estimated to be 18 times that in the atmosphere. The amount of sea-water is estimated at about 324,000,000 cubic miles, or about 15 times the volume of the land above sea- level. The volume and composition of the sea-water being known, the amount of its mineral matter may be calculated. Assuming the average specific gravity of the mineral matter to be 2.5, the 3.5% (nearly) by weight becomes about 1.4% by volume, and 1.4% of 324,000,000 cubic miles is 4,536,000 cubic miles. This represents approximately the volume which the mineral matter of the sea 1 Much information on these and other points is to be found in the following books: Wild’s Thalassa; Thompson’s Depths of the Sea; Barker’s Deep Sea Soundings, and Agassiz’s The Three Cruises of the Blake. ‘The Challenger Reports give more detailed information for certain regions. 2 Dittmar, Challenger Reports, Physics and Chemistry, Vol. I, p. 204. 167 168 WORK OF THE OCEAN would have if it were precipitated and compacted so as to have an average specific gravity of 2.5. This amount of mineral matter would cover the ocean bottom to a depth of about 175 feet. Its amount is equal to about 20% of that of all lands above sea-level. A large part of the mineral matter of the sea has come from the land, where it was dissolved chiefly by ground-water, and carried to the sea by rivers. But the mineral matter of the sea gives no more than a hint of the importance of the solvent work of water in the general processes of rock decay, for most of the mineral matter carried from the land to the sea in solution is taken from _sea-water about as rapidly as it is received. Calcium carbonate, for example, is about twenty times as abundant as sodium chloride in river-water, but it is only 1/200 as abundant in sea-water. This is because the calcium carbonate is used by animals and plants to make shells, skeletons, etc., while the salt remains in solution. From the amount of water discharged by rivers into the sea each. year (about 6,500 cubic miles), and from the amount of salt it carries, it is calculated that it would take about 370,000,000 years for the salt of the sea to have been contributed by rivers, at the present rate. This figure, however, must not be taken as the age of the ocean, for (1) the salt is not all brought in by rivers, (2) it is not probable that the rivers have always contributed salt at the present rate, and (3) much salt once in the sea has been precipitated. Never- theless the above figure gives some suggestion as to the order of magnitude of the figure which represents the age of the ocean. Topography of ocean basins. The ocean basins are convex upward. It is only when we remember that a level surface (6n the earth) is one which has the mean curvature of the earth, and that the deeper parts of the ocean basin are considerably below the mean sphere level, that the name basin seems appropriate. The bed of the ocean, like the face of the land, has elevations and depressions, and its deepest parts are about as far below its surface as the highest mountains are above it. If the water were drawn off, so that the bottoms of the ocean basins could be seen, three great features would appear: (1) Extensive tracts of low land (now covered by deep water); (2) other great, but less extensive tracts of higher land (now covered by shallow water); and (3) ridges and peaks of mountainous heights. These three principal divisions may be compared to the plains, plateaus, and mountains of the land, though mountain systems would be less numerous than GENERAL FEATURES 169 on land. In addition there are great depressions comparable to the great basins of the land. Apart from these general features, there is little in common be- tween the topography of the sea bottom and that of the land. If the ocean’s bed could be seen as the land is, its most impressive feature would be its monotony. The familiar hills and valleys which give the land its most familiar features are essentially absent. A large part of the ocean bottom is so nearly flat that the eye would not detect its departure from planeness. The reason for this difference is readily found. The dominant processes which shape the details of the surface of the land are degradational, and though the final result of degradation is flatness (base-level), the earlier result is roughness. In the sea, the domi- nant processes are aggradational, and tend to planeness. Distribution of marine life. Marine life has been of such im- portance in the history of the earth that the elementary facts concerning its distribution and the principles which control it are here recalled. Its distribution is influenced by many factors, chief among which are temperature and depth of water. It is more abun- dant in the warmer parts of the ocean than in the colder, the species inhabiting cold waters are different from those in warm, and few species range through great variations of temperature. Many forms are restricted to shallow water; many others, especially those living near the surface, swim about freely without reference to depth; while a few are restricted to great depths. Some species are influenced by (1) the salinity of the water, which varies con- siderably along coasts where the fresh waters from the land are dis- charged; (2) the character of the sediment at the bottom, some species preferring mud, others sand, etc.; (3) the movement of the water, some species preferring quiet water and others rough water; (4) the abundance and nature of the food-supply; and (5) the presence or absence of rival and hostile species. Subject to exceptions determined by temperature, etc., plant life abounds in the superficial parts of the ocean, and down to the bottom where the depth does not exceed too fathoms. Animal life is abundant in shallow water at all depths down to 200 or 300 fathoms, and in the surface-waters of temperate and tropical regions regardless of depth. The great body of the ocean-water lying below the depth of a few hundred fathoms has but little life, though animals exists sparingly at the bottom, even where the depth is great. 170 WORK OF THE OCEAN PROCESSES IN OPERATION IN THE SEA Diastrophism. So far as the lithosphere is concerned, the sea~ level is the critical level. At this level and above, many processes are in operation which are not effective below, while below sea-level some processes are effective which find no counterpart above. Warpings of the surface which do not involve the submergence of land or the emergence of sea bottom, are relatively unimportant compared with those which do. The rise of the bottom of the sea from a depth of 400 fathoms to a depth of 200 fathoms would not have important results, so far as the area itself is concerned, while an equal rise of the bottom beneath too fathoms of water, or an equal sinking of land 500 feet high, would be much more important. Fig. 174. Map showing the early stages in the simplification of a shoreline by deposition, and showing that at this stage the irregularities are increased. | If the land rose or the sea sank 100 fathoms, the coast-line would be regular. PROCESSES IN OPERATION iy ge {t follows that the changes effected by diastrophism are more obvious in shallow water than in deep. Emergence or submergence shifts the zone of contact of ocean and land, and so the areas of aggrada- tion and degradation, and changes the region concerned from one appropriate for sea life to one appropriate for land life, or vice versa. Over the continental shelves the water is shallow and the bottom relatively smooth. If the sea-level were drawn down, or if the con- tinental shelf were elevated evenly, the new shore-line on the smooth surface of the former submerged shelf would be regular relatively, even though the coast was notably irregular before the change. This is illustrated by Fig. 174. Subsidence of a coast-line (or rise of the sea-level) tends to the opposite result, for in this case the sea advances on a surface which has relief, and the water covers every low place sunk to its level. Thus the numerous bays at the lower ends of the streams along the Atlantic coast from Long Island Sound to Carolina are the results of recent sinking. From the present configuration of coast-lines it has been inferred that the present is an era of continental depression. Some river valleys, the lower ends of which are embayed, are found to be continuous with 15) Lol Fig. 175.. The ecbierced valley which has been interpreted as the continua- tion of the Hudson Valley. The position of the valley is indicated by contours. (Data from C. and G. Survey.) 172 WORK OF THE OCEAN submerged valleys beyond the coast-line (Fig. 175). Submerged river valleys show that the surface in which they lie was once land. The effects of diastrophism in the ocean and about its borders, may (1) make the water of any ocean, or of any part of it, shallower or deeper; (2) cause the emergence or submergence of land; (3) make coast-lines regular or irregular; (4) shift the habitat of many forms of life, and, through these changes, (5) influence the processes of gradation, especially at and near the contact of sea and land. Vulcanism. Vulcanism affects the sea-bottom much as it affects land. At the volcanic centers, where the great body of extruded matter accumulates, mounds and mountains are built up, and most of the mountain peaks of the sea-bottom had a volcanic origin. Where volcanic cones are built up near the surface of the sea, they may furnish a home for shallow-water life, such as polyps. Wher- ever they are built up so as to be within the reach of waves, grada- tional processes are stimulated. The number of active volcanoes on islands is about 200, but the number of active vents beneath the sea is unknown. A few sub- marine eruptions have been observed, but those observed are prob- ably but a small percentage of those which take place, for eruptions in deep water may not be seen at the surface. Oceanic volcanoes affect both the temperature and the composi- tion of the sea-water. Both the increase of temperature and the volcanic gases increase the solvent power of water, and both the change in temperature and composition affect the life of the adja- cent waters. Volcanoes in the sea have furnished much of the sediment now found on the bottom of the ocean. Some of it is very fine, like volcanic dust, and some of it is coarse. Both the fine and the coarse are distributed far from the volcanoes which emit them, are found indeed nearly everywhere on the bottom of the deep sea, though not in uniform abundance. It is therefore clear that the effects of oceanic volcanoes on the sea-water are considerable, when long periods of time are considered. Gradation. The gradational processes of the land and the sea are in striking contrast. On the land, degradation predominates, and aggradation is subordinate;.in the sea, aggradation predom- inates and degradation is subordinate. On the land, degradation is greatest, on the whole, where the land is highest, while aggrada- tion is of consequence only where the land is low, or where steep slopes give place to gentle ones. In the sea, degradation is vir- MOVEMENTS OF SEA-WATER 173 tually confined to shallow water, or to what might be called the highlands of the sea, while aggradation is nearly universal, though most considerable in shallow water, or where shallow water gives place to deep. Both the degradational and aggradational work of the sea are greatest near its shores. Though the gradational work on the land and in the sea are in strong contrast, they tend to a common end — the leveling of the surface of the lithosphere. The gradational processes of the sea-bottom are effected (1) by mechanical, (2) chemical, and (3) organic agencies. Mechanical gradation is effected chiefly by the movements of the water. These may be degradational where the water is shallow enough for the motion to affect the bottom, but elsewhere they are aggradational. Gradation by chemical processes is likewise partly degradational and partly aggradational. In lagoons and other small inclosures, the water may become saturated with mineral matter; with further evaporation, precipitation takes place, the precipitate accumulating as sediment on the bottom. On the other hand, solution results in degradation. Organic agencies are, on the whole, aggradational. Accumulations of coral, coral debris, shells, etc., help to build up the sea-bottom. In the aggradation effected directly by organic agencies, the sea is passive. Its only part is to support the life which produces the solid matter, and incidentally to float a part of it in its currents. MOVEMENTS OF SEA-WATER The movements of sea-water fall into several categories. There is (1) a general circulation of sea-water, determined by (a) differences in density in the sea-water, (b) differences of level, and (c) move- ments of the atmosphere; (2) periodic tidal movements; and (3) aperiodic movements due to earthquakes, volcanic explosions, land- slides, etc. For present purposes, all movements of the sea-water may be grouped into two main classes — (1) waves, with the undertow and the littoral currents they generate, and (2) ocean-currents. W aves Wave-motion.! The most common waves are those generated by winds. During the passage of a wave, each particle affected by 1 In the following pages concerning the waves and their work, Gilbert’s discus- sion of shore features, in the Fifth Annual Report of the U. S. Geol. Survey, pp. 80- 100, is freely drawn on. See also Fenneman, Jour. of Geol., Vol. X, pp. 1-32. 174 WORK OF THE OCEAN it rises and falls and moves forward and backward, describing an orbit in a vertical plane. If the passing wave is a swell, the orbit of the particle is a circle or an ellipse; but in the case of a wind-wave the orbit is not closed, for in such a wave the water, as well as the undulation, moves forward. On the crest of the wind-wave each particle of water moves forward, and in the trough it moves less rapidly backward, and the excess of the forward movement over the backward gives the water a slight advance. As a result of this advance, the upper part of the water is carried forward with refer- ence to that below, in the direction toward which the wind blows. The waves of any considerable or long-continued wind, therefore, generate asurface movement in the direction of the wind. Wave motion is prop- agated downward indefi- nitely, but the amount of mo- tion diminishes rapidly with increasing depth (Fig. 176). Engineering operations have ; shown that submarine struc- Fig. 176. Diagram illustrating the de- tures are little disturbed at creasing size of orbits of water particles in a depths of five meters in the wave, with increasing depth. Mediterranean, and eight meters in the Atlantic. On the other hand, debris as coarse as gravel, which is transported by rolling on the bottom, may be carried out to depths of 50 feet, and sometimes even to 150 feet. Fine sediment, like silt, is disturbed at still greater depths, for ripple-marks, which indicate agitation of the water, are said to have been found at depths of too fathoms. When a wave approaches a shelving shore, its habit is changed. The velocity of the undulation is diminished, while the velocity of the advancing particle of water in the crest is increased; the wave- length, measured from trough to trough, is diminished, and the wave-height is increased; the crest becomes acute, with the front © steeper than the back, and these changes culminate in the breaking of the crest when the undulation proper ceases. Waves of a given height break in about the same depth of water, and the line along which incoming waves break is the line of breakers. The line of breakers is in deepest water and farthest from shore when the waves are strongest. The return of the water thrown forward MOVEMENTS OF SEA-WATER 175 Fig. 177. Shore wave breaking on east wall of Hastings. (From Wheeler’s The Sea Coast; by permission of Longmans, Green and Company.) in the crests of waves is accomplished by a current along the bot- tom, called the undertow, which is sensibly normal to the coast when uninfluenced by oblique waves. When waves advance on shore obliquely, a shore-current is developed as illustrated by Fig. 178, where ad represents the direc- tion of the incoming wave, bc the direction of the shore (or littoral) current, and bd the direction of the (4 undertow. Where they strike the bor- ders of land, the wind-waves, there- fore, generate two other movements, the undertow and the littoral current. Any particle of water near shore may be affected by any two or by all three of these movements at the same mo- ment. The effect of littoral current and undertow is to give a particle of water on which both are working a direction between the two, as be. The effect of other combinations is readily inferred. These various combinations : : are of consequence in the transporta- aA cerapieteea ima penne tion of debris. Waves and the move- undertow, and shore-current. Lda LMM Wb \N i 176 WORK OF THE OCEAN ments to which they give rise (1) wear the shores, (2) transport the products of wear, and (3) deposit the transported materials. Erosion. In the dash of the waves against the shore, the wear is effected chiefly by the impact of the water and of the debris which the water carries, but lesser results are accomplished in other ways. When the land at the margin of the water consists of uncop- solidated material, or of fragmental material but slightly cemented, the dash of the water is sufficient to displace or erode it. If weak rock is associated with resistant rock within the zone of wave-work, the removal of the former may lead to the disruption and fall of the latter, especially when weak rock is washed out from beneath strong. The impact of the water is competent also to break up and remove rock which was once resistant, but which has been weakened by weathering. Rock affected by joints is attacked with success, for the blocks bounded by joints may be loosened and quarried out. Waves of clear water, even when their force is very great, have little effect on rock which is thoroughly solid. The effect of the impact of the waves is generally increased by the detritus they carry. The sand, the pebbles, and such stones ame 7 ahs Fig.179. Angular blocks of rock, fallen from the cliff above, as a result of under- cutting by waves; Grand Island, Lake Champlain. MOVEMENTS OF SEA-WATER 177 as the waves can move are used as weapons of attack, both against the shore and against one another. Masses of rock too large for the waves to move (Fig. 179) are worn by the detritus driven back and forth over them, and in time reduced to movable dimensions. They then become the tools of the waves, and, in use, are reduced still more. Thus bowlders are worn to cobbles, cobbles to pebbles, pebbles to sand, and sand to silt. The silt, held in suspension in agitated water, is carried out beyond the range of breakers, and Fig. 180. Showing blocks similar to those of Fig. 179, but reduced and rounded by wave-action. Shore of Lake Champlain. (Perry.) settles in water so deep as not to be agitated to its bottom. Thus one generation of shore bowlders after another is worn out, and the comminuted products come to rest in deeper water. The effectiveness of waves depends on their strength and on the concentration of their blows.!. The average force.of waves on the Atlantic coast of Britain has. been found to be 611 lbs. per square foot in summer, and 2,086 lbs. in winter, but winter breakers which 1 Willis, Jour. of Geol., Vol. I, p. 481. 178 WORK OF THE OCEAN exert a pressure of three tons per square foot are not infrequent. Exceptional storm-waves have moved blocks of rock exceeding 100 tons in weight. Waves are most efficient on bold coasts bordered by broad expanses of deep water, for here their force is expended almost wholly near the water line; where shallow water borders the land, the force of the waves is expended over a greater area. The direct effect of wave-erosion is restricted to a zone which is narrow both horizontally and vertically. There is no impact of breakers at levels lower than the troughs of the waves, though erosion may extend down to the limit of effective agitation. The upper limit of effective wave-action is the level of the wave-crests. The rise and fall of the water during the flow and ebb of the tides gives the waves a greater vertical range than wind-waves alone would have. The indirect work of waves is limited only by the height of the shore, for as the zone of excavation is carried land- ward, masses higher up the slope are undermined and fall. The fallen rock protects the shore against the waves temporarily (Fig. 179), but the fallen masses are themselves broken up eventually. The general result of wave-erosion is the advance of the sea on the land, the rate of advance being determined chiefly by the nature of the material attacked and the strength of the waves. Though examples of the retreat of coast-lines before the advance of the LG MYTULED ee Wy UTLEY Ms ANE Fig. 181 Fig. 182 Fig. 181. High sea cliffs, and a submerged terrace, due partly to wave-cutting and partly to building. Fig. 182. A low sea cliff. sea are numerous, the advance is not universal or uninterrupted. On the contrary, the land encroaches on the sea in some places, and the two things may go on side by side. At Long Branch, N. J., advance of the sea has been so rapid in recent times as to menace important buildings, while a few miles to north and south, land is advancing into the sea by the deposition of shore drift. The low coast of the Middle Netherlands has retreated two miles or more in historic times, but the land has advanced at other points in the WAVE-EROSION 179 same region. On the coast of England the sites of villages have disappeared by the advance of the sea within historic times,! but the coast of the same island affords illustrations of land advance. On the south side of Nantucket Island, the sea-cliff has been known to retreat before the waves six feet in a single year.?, Almost every considerable stretch of coast affords illustrations both of the advance of sea on land and of land on sea; but in the long run, the former exceeds the latter. Topographic features developed by wave-erosion. As _ the waves cut into the shore at and near the water-level, they develop a steep slope above the line of cutting. This steep slope is the sea- cliff (Figs. 181 to 184). The term /ake cliff is applied to the cor- responding cliffs of lakes. The height of the cliff depends on the height of the land along shore. Its slope may be steep or gentle (Figs. 181 and 182). Rapid A high sea cliff without a beach, La Jolla, Cal. Fig. 183. cutting and resistant material tend to produce steep cliffs; but steep cliffs may develop in incoherent materials, such as sand and clay, if cutting is rapid. The structure of the cliff-rock also in- fluences the slope and configuration of the sea-cliff. By working in along the joints of the rock, widening them and quarrying out the intervening blocks, pillars of rock. (‘‘chimney-rocks,” ‘‘ pulpit- 1Dana, Manual of Geology, 4th ed., p. 219. 2 Shaler, Sea and Land, p. 29. 180 WORK OF THE OCEAN rocks’”’), or even considerable islets are sometimes isolated by the waves (Fig. 185). Waves may excavate caves at the bases of cliffs. The bottoms Fig. 185. A chimney rock and an arch on the coast of France. (Neurdein.) PLATE XV ‘ (‘AINg "TOAD “Gg “AQ Yoo “ssepT ‘Avg U0jsog) “Joo 0% ‘[VAIoJUT AN04QUOD ‘your aed oir [ qnoqe ‘apvog .Yyoredq,, vw Aq pUv[UreUt vq} 0} pot} puvyst UWY—'T ° 0.44) Sseg Long Pr. Herring Pond sueyen aiyiq AM fa “Ais C = ea = *e52cet2e=% Fic. 2.—Coastal lakes formed by the bloc Contour interval, 20 feet. king of the ends of drowned (Marthas Vineyard, Mass., Sheet, U. S. Geol. Surv.) v about 1 mile per inch. Seale, valleys. The upper end of Seneca Lake, New York. The fiat between Montour Falls and Watkins is a delta which has been built out into the lake by the in-flowing creek. Scale, about 1 mile per inch. Contour interval, 20 feet. (Watkins, N. Y., Sheet, U. S. Geol. Surv.) WAVE-EROSION 181 and roofs of most sea-caves have a pronounced inclination land- ward, and if the cliff is low, the cave may be extended landward until its roof is pierced. Through such an opening in the top of the cliff the water of the incoming waves may be forced in the form of spray. On the New England coast, such holes are sometimes known as “‘spouting horns.” Similar openings may be made by the compression or rarefaction of the air in the cave as the wave enters or retreats. Sea caves, ‘“‘spouting horns,” ‘‘pulpit-rocks,”’ and other isolated islets, all are closely associated with the sea-cliff in origin. The bottom of the sea-cliff is bordered by a submerged platform over which the water is shallow. This platform, or at any rate its Fig. 186. Wave-cut terrace. The land has risen or the sea sunk since the terrace was cut. Seward Peninsula, Alaska. (U.S. Geol. Surv.) landward portion, represents the area over which the water has advanced as the result of wave-cutting, and is known as the wave- cut terrace. Such a terrace is the necessary accompaniment of the cliff. Wave-cut terraces may become land by elevation, or by the lowering of the level of the sea (Fig. 186). Elevated sea-cliffs with wave-cut terraces at their bases are among the best evidences of change of relative level between water and land. Wave-erosion and horizontal configuration. The structure of the rock along shore has much to do with the horizontal configura- tion of the wave-shaped coast. Wave erosion develops re-entrants in the weaker portions of the shore, leaving the more resistant parts as headlands (Fig. 2, Pl. VI. p. 69). It is to be noted that the resist- ance of rock to wave-erosion is not determined by its hardness alone. 182 WORK OF THE OCEAN Every division plane, whether due to bedding, to jointing, or to irregular fracture, is a source of weakness, and rock of great hard- ness may be so broken as to offer little resistance. A coast which is , VALLEY Fig. 220. Ranges of the Great Basin. Length of section, 120 miles. (Gilbert.) with continent-forming movements rather than with mountain-fold- ings, differing from the former chiefly in magnitude. Plateaus may be regarded as parts of a continental mass that have suffered additional movement. Plateaus standin some such relation to con- tinents as one fault block of a plateau does to the whole plateau. 3. Continent-forming movements. These are widespread movements affecting large masses of the body of the earth, if not its whole outer portion. Two or more continents may be affected by similar movements at the same time, and it is the view of many geologists that all continents are affected simultaneously by move- ments of a like kind, resulting in emergence or submergence, while the ocean basins are affected by movements of the opposite phase. These movements are regarded as reciprocal, and parts of a world- wide adjustment. While well supported both by observation and theory, this view is not universally accepted. Movements of this class seem to have started early in the history of the earth, and to have been renewed from time to time, rejuvenating the continents and deepening the ocean basins. Under the view that the earth is essentially solid throughout, these movements are regarded as extending down to great depths, while mountain folding is regarded as but the wrinkling of the earth’s skin to fit its changed body. Downward movements are regarded as the primary ones, and horizontal movements as a necessary result of them. The under- lying cause of movement is believed to be shrinkage due to an in- crease in the density of the earth, caused by gravity and by molec- ular and sub-molecular attractions. Cooling is probably a lesser cause of shrinkage. The master movement is thought to be the 3232 MOVEMENTS AND DEFORMATIONS sinking of the ocean basins, whose specific gravity is greater than that of the continents. If the ocean basins and the continents, respectively, be regarded as the surfaces of great segments of the earth all of which are crowding toward the center, the stronger and heavier segments may be conceived to take precedence, squeezing the weaker and lighter ones between them. The consequent swell- ing up of the lighter segments accounts for the relative protrusion of the continents. The area of the depressed segments is almost exactly twice that of the protruding ones, if we count the 10,000,000 square miles of the continental shelves as parts of the latter. In millions of square miles, the depressed segments are approximately as follows: the Pacific 60, the Indian 27, the South Atlantic 24, the North Atlantic 14, leaving 8 for minor depressions. The elevated segments are the Eurasian 24, the African 12, the North American to, and the South American 9, leaving 10 for the minor blocks. The downward movement of the larger segments and the crowding of the smaller and lighter segments between them involves deformation of the latter. The movements that spring from the deeper crowding affect the continental protuberances generally, or at least broadly, while the crowding of the more superficial parts affects the lands more locally. According to this view, it is obvious there should be special bowings on the borders of the continental segments, and this tallies with the archings common on borders of the continents, even where there is no folding. The shell of the earth is free at the surface, and as a result, folding and faulting are the modes of easiest accommodation there: while the deeper ee under great pressure, must be deformed throughout. The periodicity of the movements is assigned to the rigidity of the thick, massive segments which must be deformed to effect readjustment after shrinkage. Because of this rigidity, stresses accumulate for a time until they are equal to the resistance opposing them. A further increase of the stresses then causes yielding and readjustment. When masses under stress once begin to yield in the direction of their free surfaces, their attitudes for resistance be- come less favorable, and hence the yielding continues until the stress is eased. After this another period is required for stresses to ac- cumulate sufficient to produce another general deformation. Mean- time the minor stresses that may remain, or may be produced by the great deformations, tend to ease themselves and thus give rise SECULAR MOVEMENTS 222 to minor movements (p. 219). Other minor movements are doubt- less due to local causes. Extent of the movements. Between the highest elevation of the land and the lowest depth of ocean, there is a vertical range of nearly twelve miles. From the Tibetan plateau, where a con- siderable area exceeds three miles in height, to the Tuscarora deep, where a large tract exceeds five miles in depth, the range is eight miles. This represents fairly the vertical range of differential movement of large areas, though not areas of continental size. The average height of the continents is about three miles above the average bottom of the oceans, and this may be taken roughly as the differential vertical movement of the segments of continental dimensions. If the protruding portions of the lithosphere were graded down and the basins graded up to a common level, this level would lie about 9,000 feet below the surface of the sea. Referred to this datum plane, the continents have been squeezed up relatively about two miles, and the basins have sunk about one mile. The total down- ward movement, representing the total shrinkage due to increase of density, is quite unknown, but from theoretical considerations, it would appear to be far greater than the differential movement. This would mean that all segments have probably moved toward the center, the basin segments about three miles more than the con- tinental. The extent of the /ateral movements of the shell has a peculiar interest, for it has a theoretical bearing on the extent of the down- ward movements. Every mile of descent of the crust represents more than 6 miles (6.28=27) shortening of the circumference. If the vertical movements were limited to the relative ones just named, the mile of descent of the ocean basins would give but little more than 6 miles excess of circumference for lateral thrust and the crumpling of the shell. How far does this go in explaining moun- tain folds? The shortening represented by the folds of the Alps has been estimated at 74 miles;! the shortening for the Appalachians in Pennsylvania, not including the crystalline belt on the east, at 16 miles;? that of the Laramide Range in British America at 25 miles.* 1 Heim, Mechanismus der Gebirgsbildung, p. 213. 2 Chamberlin, R. T., Jour. Geol., vol. 18, p. 255, 1g10. 3’ McConnel, Geol, Surv. of Canada, p. 33 D, 1886. 224 MOVEMENTS AND DEFORMATIONS These estimates cannot be taken as measurements, but they are sufficiently close approximations to make it clear that the shortening of the shell involved in mountain folding is large. These estimates represent only that shortening of the circumference effected at cer- tain times and places; the whole shortening of a circumference involves the shortening implied by all the transverse folds on a given great circle. Usually a great circle does not cross more than one or two strongly folded tracts of the same age, from which it is — inferred that the shortening on each great circle at any one time was concentrated largely in a few tracts running at large angles to each other. If the folding of one of the main mountain ranges be doubled, it may perhaps represent roughly the shortening for the circle at right angles to it, for its own period of folding. If one is disposed to minimize the amount of folding, the estimate of the shortening may perhaps be put at 50 miles on a circumference, for each of the great mountain-making periods; or, if disposed to make the estimate large, the shortening may be put at 100 miles. For the whole shortening since the beginning of the Paleozoic era, perhaps twice these amounts might suffice. Assuming the cir- cumferential shortening to have been 50 miles during a given great mountain-folding period, the appropriate radial shortening is 8 miles. For the more generous estimate of roo miles, it is 16 miles. If these estimates are doubled for the whole of the Paleozoic and later eras, the radial shortenings are 16 and 32 miles, respectively. If these or similar figures are correct, it is clear that the surface of the earth has sunk toward the center by an average amount greater than that of the highest mountains above mean sphere level, since the beginning of the Paleozoic era. The shortening for earlier eras can hardly be estimated from present data. Causes of Secular Movements The volume of the earth is affected by two sets of forces, acting in opposition to one another, (1) the concentrating forces, consisting of (a) gravity and (b) molec- ular and sub-molecular attractions, and (2) the forces which resist concentration consisting of (a) heat and (b) molecular and sub-molecular resistances. 1. The centripetal forces. The best known of the concentrating forces is gravity, which tends to bring all parts of the earth as near the center as possible, the heavier beneath the lighter. The gravitative force of the earth causes a pressure of about 3,000,000 atmospheres at its center, and lesser pressures at lesser depths. Gravity acts all the time, and tends to bring about greater density wherever molecular movement permits. In addition to gravity, there are attractive forces between molecules, atoms, \ SECULAR MOVEMENTS 225 ions, and electrons, which co-operate with gravity in accordance with laws of their own. Their general effect is to make matter denser. The extent of their opera- tion is undetermined, but there is ground for thinking that the density of the interior still may be increasing by their action. It is known that substances which crystallize in a given way under surface pressures may be changed into denser forms under higher pressures. Re-aggregation in the interior thus probably means increased density, and it may be going on constantly. While knowledge on this point is inconclusive, it is permissible to entertain the view that gravitational, molecular, atomic, and sub-atomic forces have been and are still at work tending to increase internal density. It is even conceived that this may be a chief (if not the chief) cause of earth-shrinkage. 2. The resisting agencies. The condensing agencies are more or less held in check by resisting agencies. Of these heat is the most familiar. It is abetted by the molecular and atomic arrangements which exist at any given time, and which resist change, and by factors in the ultimate structure of matter, not well understood. It has been usual to regard the primitive state of the earth as one of intense heat, and to assign its subsequent reduction of volume almost solely to loss of heat; but this is not the view here favored. On the contrary, the heat of the earth is supposed to have been developed chiefly by reduction of volume and by radio-activity, and the heat thus developed is one of the forces which check further decrease of volume. Loss of heat is, of course, a cause of shrinkage, but its effect is thought to be less than that of molecular and sub-molecular rearrangements of the material of the earth, resulting in greater density. The loss indeed may not be greater than the new heat generated in the shrinkage. Observed temperatures in deep excavations. As the earth is penetrated below the zone of seasonal changes, by wells, mines, tunnels, and other excavations, the temperature is almost invariably found to rise, but the rate of rise is far from uniform. If we set aside as exceptional the unusually rapid rise near volcanoes and in other localities of obvious igneous influence, the highest rates are more than six times the lowest, the range being from 1° F. in 20 feet, to 1° in 135 feet,! with an average of 1° in 50 to 60 feet. The recent deep borings in which temperatures have been carefully recorded, indicate a slower rate of rise, say 1° for 80 feet. It is not probable that the observed rates of increase continue to the center. One degree in 60 feet, continued to the earth’s center would give a temperature of 348,- ooo° Fahr., and 1° Fahr. in 100 feet would give 209,000° Fahr. It is probable that the rate of increase diminishes with depth, and that the temperatures cited above are far in excess of those actually existing at the center. Amount of loss of heat and shrinkage. The amount of loss of interior heat may be estimated from that which is observed to be passing outward through the rocks, or by computations based on the estimated temperature gradients and with the known conductivity of rock. Estimates of the loss of heat in 100,000,000 years range from 10°C. (18° Fahr.) (Tait) to 45° C. (81° Fahr.), for the whole earth. This is an exceedingly small result, and emphasizes the low conductivity of rock. With this amount of cooling, the shrinkage resulting has been calculated. For a loss of 10° C., the circumferential contraction is calculated to be 1.6 to 2.35 miles; for a loss of 45° C., 7.27 to 10.5 miles. These results are so small (cf.p. 223) 11° F. for 250’ down to 8,000 feet, is reported from the Rand.,S. Af. Mining World, Jan. 7, 1911, p. 2. 226 MOVEMENTS AND DEFORMATIONS that unless there is serious error in the estimates, cooling would seem to be a very inadequate cause for the shrinkage implied by mountain folds, overthrust faults, and other crustal deformations. This inadequacy has been urged strongly by various students of the problem.!' In view of the apparent incompetency of external loss of heat, the possibilities of distortion from other causes deserve consideration. Shrinkage from denser rearrangement of material already has been referred to (p. 225), and the transfer of heat from deeper to more superficial parts will be discussed in Chapter X.. A lowering of the average temperature of the inner half of the earth 500° C., and a raising of the temperature of the outer half an equal amount, would cause a lateral thrust of about 83 miles. Some transfer of this kind is among the theoretical possibilities under the planetesimal hypothesis. The process could not continue indefinitely; but computations imply that it still may be in progress. The rise of lavas. Vf lavas are forced out from beneath the surface, a com- pensatory sinking of the outer shell will follow. The great lava-flow of the Deccan is credited with an area of 200,000 square miles, and a thickness of 4,000 to 6,000 feet. This would form a layer about 5 feet thick if spread over the whole surface of the globe. The compensatory sinking would cause a lateral thrust, on any great circle, of about 31 feet only. It requires a very generous estimate of the lavas poured out since the beginning of well-known geological history to cause a horizontal thrust amounting to any appreciable part of that involved in the folding of a typical mountain system. The case is different, however, if we go back to Archean times when the amount of extrusion was very large. Notable distortion - may have arisen from the extravasation of the lavas of that era. Intrusions of lava rising from lower to higher levels in the earth would have a dynamic effect similar to that of extrusions, so far as the outer part of the earth is concerned, and the amount of intrusive rock is probably far greater than that of extrusive. There are other possible factors in deformation which will not be discussed here. References on crustal movements. Dana, Manual of Geol., 4th ed., p. 345 et seq.; Willis, The Mechanics of the Appalachian Structures, 13th Ann. Report, U. S. Geol. Surv., Pt. IT (1893), pp. 211-282; LeConte, Theories of Mountain Origin, Jour. Geol., Vol. I (1893), p. 542; Gilbert, Jour. Geol., Vol. III (1895), p. 333, and Bull. Phil. Soc. of Washington, Vol. XIII (1895), p. 31; Van Hise; Estimates and Causes of Crustal Shortening Jour Geol., Vol. VI (1898), U. S. Geol. Surv. (1904), pp. 924-931; A. Geikie, Text- book of Geology, 4th ed., pp. 672-702; Chamberlin and Salisbury, Geologic Proc- esses and their results, Chapter IX; R. T. Chamberlin; The Appalachian folds of Central Pennsylvania, Jour. Geol. Vol. XVIII (1910) pp. 228-251. Map work. See Plates CLXV to CLXVII of Professional Paper 60, U. S. Geol. Surv., and Exercise XVII, Interpretation of Topographic Maps. 1 Fisher, Physics of the Earth’s Crust, Chap. VIII; and Dutton, Penn. M onthly, Philadelphia, May, 1870. Pernt ete, CHAPTER IX VULCANISM Vulcanism is the term applied to all movements of lava toward the surface of the earth, and is made to include certain other phe- nomena closely connected with these movements. In its rise, some lava reaches the surface, giving rise to eruptive or volcanic phenom- sae Fig. 221. A dike two feet wide, cutting through sandstone. Arran, coast ot Scotland. (H. M. Geol. Surv.) ena; and some intrudes itself into the outer formations of the earth and congeals there. The first gives rise to volcanic rocks, and the second to plutonic. ‘The first are extrusive; the second, intrusive; the first constitute eruptions; the second, irruptions. The fundamental nature of the two phases of vulcanism is the same. 229 228 VULCANISM I. INTRUSIONS Fluid rock forced into fissures and solidified there forms dzkes (Fig. 221); forced into chimney-like passages it forms pipes or plugs; insinuated between beds of other sorts of rock, it forms sills; and accumulated in considerable bodies which arch the strata up over them, it forms /accoliths (Fig. 222). If it breaks and lifts its cover, instead of arching it up, it is a bysmalith. Some laccoliths and bysmaliths are large enough to make good-sized mountains, of mound-like form. The Henry Mountains of Utah are laccoliths. Still more massive intrusions of igneous rock are sometimes called batholiths. The very great bodies of granite in Canada and along the axes of some of our western mountains are examples. The total amount of lava which has risen toward but not to the surface probably far exceeds all that has flowed out at the surface. In- trusions are usually seen only after erosion has removed the rocks which overlay them. There appear to be cases where intrusions come so near the surface as to develop explosive phe- nomena at the surface. At any rate, it is certain that occasional violent explosions take place where no lava comes to the surface. The explosion may be due to an intrusion of lava, or it may be due to the pene- Fig. 222. Ideal cross-section of . a laccolith with accompanying tration of surface-waters to hot rocks sheet and dikes. (Gilbert, U.S. that have remained uncooled from Geol. Surv. A ; : ) previous volcanic action. A case of this kind occurred in Japan in 1888, where there was a sudden and violent explosion which blew away a considerable part of the side of a volcanic mountain which had not been in eruption for at least a thousand years. The explosion filled the air with ashes and debris like a violent volcanic eruption. There was but one eruption, and within a few hours the cloud of dust had disappeared and the phenomenon was ended. No lava was extruded. Intruded igneous rock changes the rock into which it is forced. Thin dikes and sills produce little effect, but greater masses alter the adjacent rock notably. The metamorphism is effected by (1) the heat, (2) the pressure incident to the intrusion, and (3) the chemical changes stimulated by the heat, water, and gases issuing from the lava, and by pressure in the presence of ground-water. EXTRUSIONS 220 2. EXTRUSIONS When molten rock is forced to the surface it gives rise to the most impressive of all geological phenomena. The energies acquired in the interior under great compression here find sudden relief. En- closed gases may expand with extreme violence, hurling portions of lava to great heights and shattering them into fragments, special forms of which are called bombs, cinders, ash, etc., all of which con- stitute pyroclastic material. Much of the explosive violence of volcanoes has been attributed to the contact of the hot rising lava with ground-water. There are two phases of extrusion, and at their extremes they are contrasted strongly. The one is explosive ejection, attended in some cases with great violence; the other a quiet out-welling of the lava. More or less closely related to these two phases of extrusion are two classes of conduits, the one, restricted openings, such as pipes, ducts, or limited fissures, from which the amount of lava ex- truded is relatively small and forms cones; the other, great fissures out of which the lava pours in great volume and from which it spreads widely. ‘The extent of the spreading of lava into thin sheets is due more to the mass and fluidity of the lava than to the form of the outlet. The stupendous outflows of certain geologic periods appear to have issued mainly from extended fissures. Fissure Eruptions The chief known fissure eruptions of recent times are the vast basaltic floods of Iceland; but at certain times in the past there have been prodigious outpourings of lava, flow following flow, making formations thousands of feet thick and covering thousands of square miles. One of these occurred in Tertiary times in Idaho, Oregon, and Washington (Fig. 223), where about 200,000 square miles were covered with lava, aggregating in places some 2,000 feet in thickness. Still earlier, in the Cretaceous period, there were enormous flows on the Deccan, covering a like area to the depth of 4,000 to 6,000 feet. Still earlier, in the Keweenawan period, an even more remarkable succession of lava-flows in the Lake Superior region developed a series of igneous rocks of almost incredible thick- ness. In these cases there is little evidence of explosive or other violent action, and little pyroclastic material. For the most part these wide-spreading flows are composed of basic material. Massive outflows of this class are the greatest examples of extrusions, 230 VULCANISM though not now the dominant type. It has been thought that the A volcano is a cCir- cumscribed vent in the earth’s crust, out of which hot rock, gases, and va- pors issue. The ejected material is generally built up into mounds or cones (Figs. 224-225), which are often called volcanoes, though they are really the products of volcanoes. Fig. 223. Lava-flows of the northwestern So long as a volcano is part of the United States. active there is likely to be a depression, or crater (Fig. 226), in the top of its cone. The crater connects downward with the source of lava at unknown ose cose 0 OO ra oes Fig. 224. Cinder cone forming the summit of Mt. Vesuvius. depths. Craters may be a mile or more across, but most of them are smaller, some much smaller. After sufficient erosion, extinct vol- DISTRIBUTION OF VOLCANOES 231 canoes show that the former passageways leading down toward the sources of lava vary much in size and shape. The exact number of volcanoes now active is not known, because most volcanoes are active periodically only, and it is impossible to say whether a volcano which is now quiescent is extinct or only resting. It is safe to include 300 in the active list, and the number may reach 350 or more. The number that have been active so recently that their cones remain distinct is several times as great. Mavna Loa Scale of ‘Tiles eo So Fig. 225. Profile of the cone of Mauna Loa. Vertical scale same as horizontal. (U. S. Geol. Surv.) Fig. 226. Sketch of the crater of cinder cone near Lassen Peak, Cal., showing the peculiar feature of two rings. The funnel is 240 feet deep. (U.S. Geol. Surv.) Distribution. 1. J time. In the earliest known ages, igneous action appears to have been very widespread. No great area of the oldest (Archean) rocks is known where the formations are not largely igneous. From the Paleozoic to the present, the distribu- tion of volcanic action over the surface seems to have been, in.a general way, much what it is to-day; that is, certain areas were affected at times by volcanoes, while other and larger areas had few or none. This is not equally true of all periods, as will be seen in the historical studies that follow. There were periods when vol- canic activity was widespread and energetic, and others when it was limited. The known facts do not indicate a steady decline, but rather a periodicity; at least this is so for the portion of the globe that is now known well enough to warrant conclusions. 2. Relative to land and sea. Active volcanoes are located chiefly along the borders of continents, and within great oceanic basins (Fig. 227). On this account, the sea-water was formerly supposed to have some causal connection with their activity, and 232 VULCANISM the presence of chlorine in the volcanic gases has been urged in sup- port of this view. Volcanoes, however, are not distributed so equably and exclusively about the several oceans as to give this conclusion force. Volcanoes are numerous within and around the Pacific, the greatest of the oceans, and in and around the Mediter- ranean, a much smaller body of water; but they are not especially abundant in or about the Atlantic. On the other hand, there are existing or very recent volcanoes in the interior of Asia, Africa, and America. If volcanoes were dependent upon proximity to the sea, they should have had close relations to it in the past, as much as now; but in recent periods there has been much volcanic activity in western America, far from the sea, and in the heart of Asia and Africa. In older periods, it is still less clear that there was any con- nection between volcanoes and oceans. 3. Relative to crustal deformations. The distribution of present and recent volcanoes is more suggestively associated with those portions of the crust that have undergone movement in comparatively recent times, or are still moving. ‘The great mountain belt stretching from Cape Horn to Alaska and thence onwards along the east coast of Asia is dotted with active and recently extinct volcanoes. The tortuous zone of mountainous wrinkles about the Mediterranean, and thence eastward to the Polynesian Islands, is another notable volcanic tract. These two belts include the greater number of existing and recent volcanoes on the land. 4. In latitude. Volcanoes appear to have no specific relation to latitude. Mounts Erebus and Terror amid the ice-mantle of Antarctica, and Mount Hecla in Iceland, as well as the numerous volcanoes of the Aleutian chain, give no ground for supposing that volcanoes shun the frigid zones, while the numerous volcanoes of the equatorial zone imply that they do not avoid the torrid belt. 5. In curved lines. In the Aleutian and Kurile Islands, and elsewhere, there is a linear arrangement of volcanoes, with appre- ciable curvatures, the convexities of which are turned toward the adjacent ocean. In other cases there is a linear arrangement with- out appreciable curvature, as in the Hawaiian range. In some cases, volcanoes are bunched irregularly, as in some of the groups of vol- canic islands of the Pacific (Fig. 227). The relations of volcanoes. A significant feature in connection with volcanoes is the apparent sympathy between adjacent vents ‘In some cases, and their entire independence in others. The recent ae SP) SP) DISTRIBUTION OF VOLCANOES (Tlassny Joi Vy) BIONVOIOA ADNILXE ATLNIOSY + Kea ae RTS [Rte att NN Ae { 4 *SOOULI[OA JO UOT}NIUIysIp Surmoys dvpy “422 “SI oa ie a ae SSONVOIOA SAILOY @ EO ee w. oc , Y AA Ga sig? aoe + a 234 VULCANISM (1902) outbursts in Martinique and St. Vincent, and the symptoms of activity at the same time in other places, seem to point clearly to sympathy. On the other hand, the independence of some neighbor- ing vents, as those of Mauna Loa and Kilauea in Hawaii, is extraor- dinary. These two volcanoes are only about twenty miles apart, the one on the top and the other on the side of the same mountain mass. The crater of Loa is about 10,000 feet higher than that of Kilauea, and yet, while the latter has been in constant activity as far back as its history is known, the former is periodic. The case is the more remarkable because of the greatness of the ejections. The outflow of Mauna Loa in 1885 formed a stream 3 to ro miles in width and 45 miles in length, with a probable average thickness of too feet, and some of its other outflows were nearly as massive. Besides this massiveness, there were extraordinary movements of the lava within the crater, if the testimony of witnesses may be trusted. But throughout these great movements in the higher crater, the lava-column of Kilauea, 10,000 feet lower, continued its quiet action without sensible relation to its boisterous neighbor. No difference in specific gravity that could account for a difference in height of 10,000 feet has been observed or can be presumed. It seems a necessary inference, therefore, that the lava-columns in the two volcanoes have no connection with each other, or with a common reservoir. The tops of some lava-columns stand about 20,000 feet above the sea, while others emerge on the sea-bottom far below sea-level. ‘This range of elevation tells its own story as to the independence of vents. Eruptions seem to be somewhat more common when atmos- pheric pressure is high than when low, doubtless because the in- creased atmospheric weight on a large area of the crust, aids in forc- ing out lava and volcanic gases. This can be effective only when other forces have almost accomplished the result. Eruptions seem also to be more common when tidal strains favor them, for like rea- sons. In the same class are probably to be put the effects of heavy rains. Such factors are regarded as mere incidents, of no moment as real causes of vulcanism, but of some value in determining the moment of eruption. Periodicity. Most volcanoes are intermittent in their action, © long periods of dormancy intervening between periods of activity. Some volcanoes supposed to be extinct have renewed their activity with terrific violence. Their periodicity awaits an explanation, eRe 2 oe PRODUCTS OF VOLCANOES 235 Fig. 228. The edge of an old stream of lava, showing (1) its broken character due to movement after the outside had hardened, and (2) the steep slope of the stream of stiffened lava. Near Flagstaff, Ariz. (Fairbanks.) but the temporary quiet very likely means an exhaustion of the supply of gas or lava, or both. Products of volcanoes. 1. Pyroclastic material. The fragmental materials which are blown out of a volcano are, as a rule, portions of lava which solidified before ejection, or during their flight in the air. From masses of rock tons in weight, the fragments grade down to particles of dust. The dust particles (often called ask) are thrown high into the air in some cases, and, caught by the winds, are shifted incredible distances (p. 13). In some cases, beds of volcanic ash many feet in thickness (as those of Nebraska) are found far from any known volcanic center. The extremely fine ash from the great explosion of Krakatoa floated several times around the earth in the equatorial belt, and spread northward into the temperate zones. Liquid rock, lava. The term lava is applied to all kinds of liquid rock, and also to the solid rock formed when fluid rock congeals. The various phases assumed by lava, on solidification, have been noted in connection with igneous rocks. Lava never flows so freely as water, and is, in many cases, very stiff or viscous. The distance to which it flows depends on its liquidity, its amount, and the slope ef the surface on which it is poured out. ‘ 236 VULCANISM As lava flows, its upper surface may cool so much as to become hard while the interior is still fluid. The fluid part may then break out at the side or end of the hardened shell and flow away, leaving a hollow crust of solidified lava. On further cooling, the shell contracts and cracks, and perhaps caves in. The hardened surface of a lava-flow may be broken by the movement of the fluid lava below, and the solid fragments be displaced and upturned so as to give the surface a jagged appearance. 3. Gases and vapors. The gases and vapors which issue from volcanoes are of many kinds. Among the commoner ones are those of water (H2.O), carbon dioxide (COs), carbon monoxide (CO), chlorine (Cl), hydrochloric acid (HCl), sulphur dioxide (SOz), and hydrogen sulphide (H2S); but with these more important ones there are many others. Oxygen and hydrogen are generally present, per- haps produced by the dissociation of the elements of water. Some of the gases are poisonous, and, as in the case of Pelée, their tem- perature is in some cases so high as to be destructive to life. Formation of lava cones. Lava usually flows away from a vent in streams which solidify before running far. As the lava-streams flow in different directions at different times, the total effect is a low cone formed of radiating tongues of lava. The streams may congeal before they reach beyond the base of the cone, and not rarely while yet on its slope. The volcanic cones formed of lava have low slopes, since the fluidity of the lava prevents the development of high gradients. It is, however, the exception rather than the rule, that the cone is made up mainly of lava-streams, though the great Hawai- ian volcanoes are of this class. The form of the cone, when com- posed chiefly of lava, is also affected by the mass of the outflow and by the fluidity of the lava. Other things being equal, the larger the outflow at a given time, the more widely it distributes itself, and the flatter the cone. Cinder-cones. ‘The larger portion of the lava blown into the air by expanding gas-bubbles falls back in the immediate vicinity of the vent and builds up cinder-cones. This fragmental matter may be disposed more or less symmetrically, making a cone with steep slopes (Fig. 224). Minor cones. Small or temporary vents formed as offshoots from the main vents may give rise to secondary or ‘‘parasitic’’ cones. These may be numerous, as in the case of Etna, and so important that a volcanic mountain becomes a compound cone. VOLCANIC CONES 237 Fig. 229. Spatter-cone and cavern. Kilauea, Hawaii. (Photo. by Libbey.) A still more subordinate type of cone is the ‘‘spatter-cone”’ formed about small vents that eject little dabs of lava which form chimneys, cones, domes, etc. Spatter-cones (Fig. 229) may arise from the surface of the lava-flows themselves. From most existing volcanoes both lava-flows and fragmental ejecta are given forth, and the resulting cones are composite in material. Lava breaks through the side of a cone more frequently than it overflows its summit, and this gives rise to irregularities of form and structure. Cones also are subject to partial destruction both by outbursts of lava and by explosions. As a result, many volcanic regions show old, partially destroyed craters, as well as new and more perfect ones. In violent eruptions, steam, accompanied with much ash, is shot up to great heights, rolling outwards in cumulus or cauliflower- like forms (Fig. 230). In the more violent explosions, these columns are projected several miles. In the phenomenal case of Krakatoa, the projection was estimated at seventeen miles. By reason of its great expansion as it rises, and by its contact with the colder air, steam is condensed quickly, and prodigious floods of rain accompany many an eruption. This rain, carrying down a portion of the ash and gathering up much that had previously fallen, gives 238 VULCANISM rise to mud-flows, which in some cases constitute a large part of the final deposit. These mud-flows lodge chiefly on the lower slopes of the cone or adjacent to its base. The common view that lava is melted rock, is hardly the correct one. At any rate, it is at least equally correct to regard it as a solution of mineral matter in mineral matter. A familiar illustration will show what is meant. If ice and salt are mixed at a temperature of 30° F., the two form a liquid, though the tem- perature is too low to melt either. We say the salt is dissolved, but it would be Fig. 230. The eruption cloud of Pelée, December 16, 1902. (Lacroix.) NATURE OF LAVA 239 just as correct to say that the ice is dissolved. The two minerals, ‘ce and salt, are dissolved in each other, and the solution takes place at a temperature below Fig: 231. Relatively smooth lava surface near the Jordan craters, Malheur Co., Ore. (U.S. Geol. Surv.) Fig. 232. Ropy surface of lava, Mauna Loa, flow of 1881. (Calvin.) 240 VULCANISM the melting point of either. Something of the same sort appears to take place when rock becomes liquid. The distinction between such solutions and molten rock is not very sharp, but it is essential to know that the order in which the min- erals crystallize from lavas is not. dependent on their melting temperatures. It appears rather to depend on the order in which the solution becomes saturated with the constituents of each of the several minerals. For example, quartz, which has a very high melting-point, may crystallize out from a lava much later than minerals which have lower melting temperatures. The solutions are exceedingly complex, and include a wide range of chemical substances. Chief among them are silicates of aluminum, potassium, sodium, calcium, magnesium, and iron (Chapter X), with minor ingredients of nearly all knownsubstances. Gases as well as rock materials enter into the composition of the igneous rock. When lava is cooled suddenly, the result is glass, every part of which has essentially the same composition that the liquid had, but even in this case some of the gases of the lava escape. If the cooling is slower, the various substances in the mixture crys- tallize out into minerals in the order in which they severally reach saturation. This involves the principle that solubility is dependent on temperature, and that as the temperature sinks the degree of solubility declines, and the saturation-point for some constituents of the solution is reached earlier than that for others. With sufficiently slow cooling, all the material passes into the solid state by the crys- tallizing of the several minerals in succession. This does not mean that two or more minerals may not be forming at the same time, but it means that some minerals may be crystallized out while the surrounding material is still fluid. In most igneous rocks, nearly perfect crystals of certain minerals are common, while other minerals, crystallizing later, adapt themselves to the space left between older crystals. This conception is supported by the fact that some lavas, while still in the fluid condition, contain well-formed crystals, very much as water in certain conditions may be filled with crystals of ice. Temperature of lava. Accurate determinations of the temperatures of liq- uid lavas have not been made; but it is clear from the white heat of some lavas that their temperatures are appreciably above the melting-point. This is also a necessary inference from the length of time lavas remain fluid, in spite of its contact with cooler rock, through its miles of ascent. From various facts it is probably safe to assume that the original temperatures of lavas as they rise to the surface are in some cases considerably above 2,000° Fahr. (1,093° C.). Even such a temperature must be somewhat below the original temperature of the lava, because some heat must be lost in rising, both by contact with the cooler rocks through which it rises, and by the expansion of the gases within them. Depth of source. Attempts have been made to determine the depth from which lavas rise, by calculations based on the earthquake tremors accompanying eruptions; but such calculations really tell very little concerning the true point of origin of the lava. At most they probably tell merely where the ascending lavas begin to rupture the rock through which they pass, and rupture may not be possible below the zone of fracture,which is probably not more than eleven miles deep.! In the zone of flowage below, where the pressure is too great to permit fracture, the lava not improbably makes its way by some boring or fluxing process, which might not be capable of giving rise to seismic tremors. ‘The tremors perhaps com- 1Adams: Jour. Geol., Vol. xx (1912), pp. 97-118. —~ —a NATURE OF LAVA 241 pel us to place the beginning of movement of lava at least as low as the bottom of the fracture zone, but they probably offer no sufficient ground for limiting the lava’s origin to this or any other specific depth. Volcanic gases. One of the most distinctive features of volcanoes is the explosive action arising from the gases and vapors pent up in the lava. Lavas in the interior, under high pressure, contain much gas, and as they rise and the pressure is relieved, some of these gases escape from the hot liquid. In those cases in which the eruption is quiet, the escape of the gases is but partial while the lava is in the crater, and much gas remains to be given off after the lava has been extruded and is about to congeal. The gases are then given off slowly and quietly. If, however, the lava is surcharged with gases, and if their escape is retarded by the viscosity of the lava, they gather in large vesicles or bubbles in the lava in the throat of the volcano, and on coming to the surface explode, hurling the enveloping lava upwards and outwards. The violence of the explo- sion reduces a portion of the lava to the fineness of dust,— the “ash” and “smoke” of the volcano. The causes of the differences of gas action in different volcanoes are undeter- mined, but the following suggestions may point to a part of the truth: (1) Some lavas contain more gases than others, and hence are predisposed to be more ex- plosive; (2) some are more viscous than others and hence hold the gases more tenaciously until they accumulate and acquire explosive force, while the more liquid lavas allow their gases to escape more freely; (3) probably a main occasion of violent explosions lies in the fact that the lavas have begun to crystallize while yet in the volcano. When crystals form in the magma (lava), they exclude the gases which were in the substance from which they are developed, and this excluded gas overcharges the remainder of the lava. This view is supported by the fact that the pumice and ash of such extraordinary eruptions as those of Krakatoa and Pelée contain many small crystals which had formed before the explosion took place. Incipient crystallization does not, however, appear to be a_ universal accompaniment of explosive action. Igneous rocks contain gases in large quantities. When the lavas lodge under- ground without free communication with the surface, there is reason to think that they retain a larger percentage of their original gases than the lavas which are exposed freely at the surface. At any rate, deep intrusive rocks contain notable quantities of gases. Recent surface lavas also contain gases of similar kinds, but not in equal amount, so far as available analyses show. One of the outstanding problems of geology is to determine (a) how far the material of the gases had the same origin as the material of the lavas, and (b) how far the material for the gases penetrated from the surface. The peculiar propor- tions of the rock-gases, among which hydrogen and carbon dioxide greatly pre- ponderate, seem to imply that they are not derived chiefly from surface waters or the atmosphere; they appear to be original constituents of the rocks in the main, and when given forth they appear to constitute real additions to the atmosphere. THE CAUSE OF VULCANISM The fundamental explanation of volcanic phenomena is wrapped up in the origin of the earth, for the conditions which the earth in- 1 Rollin T. Chamberlin, Gases in Rocks, Carnegie Institution, 1908. 242 VULCANISM herited from its birth are doubtless leading factors in the explana- tion of vulcanism.! The explanation includes (1) the origin of lavas, and (2) the forces by which they are expelled. The current explanations of vulcanism fall into two general classes: (1) those which assume that lavas are residual portions of an original molten mass, and (2) those which assign lavas to the local liquefaction of rock. The first of these views prevailed formerly, but it encounters grave difficulties because of the independent action of adjacent vents. When lava columns vary thousands of feet in height on the same mountain mass, as in the Hawaii volcanoes, even a resort to the hypothesis of local residual reservoirs is un- satisfactory. Another view which has had much currency supposes that surface water and its absorbed gases penetrate to heated rock and are absorbed by it, rendering the whole liquid, and that the lava thus formed is forced to the surface. It does not appear, however, that surface water penetrates below the zone of fracture, and hence is far from reaching highly-heated rocks. Relief of pressure lowers the melting point of rock, and when felt by rocks already hotter than their melting temperatures at lowered pressures, has been held to be a possible cause of vulcanism. The necessary relief of pressure is assigned to faulting and denudation; but many volcanoes are located in the bottom of the ocean, where denudation does not take place, and faulting that would give relief of pressure is not always related to vulcanism in any clear way. Melting by crushing has been suggested, but in the deeper parts, crushing involves increase of pressure, which opposes melting. Sinking to the zone of high tem- perature under the weight of accumulated sediments, is also assigned as a cause of melting, but there is very little sedimentation in the ocean far from land where many volcanoes are situated. If the earth grew up by slow accessions of matter, and if its interior heat is due chiefly to the internal compression resulting from growth, the distribution of internal temperature would be such that, with like conductivity, the flow of heat from the deep interior to a thick outer zone (about 1/5 of the radius) of the earth would be greater than the loss from this zone to the superficial shell. The deeper parts of the outer zone might thus rise in temperature. This zone is, under this view, supposed to be composed of various For fuller statement see the authors’ larger work, Vol. I, pp. 395-607, Vol. IT, Pp. 99-106, 116-118, 120, 130. — : | : CAUSES OF VULCANISM 243 kinds of matter, mixed as they happened to fallin. If its tempera- ture rises, the fusion-points of some of its constituents will be reached sooner than those of others. A fusion or solution of the more soluble portions may thus take place while the rest of the rock remains solid. The gases and volatile constituents in the original material would obviously unite with the liquid part. With continued rise of temperature, the liquefaction would extend itself until adjacent pockets or threads of lava united, and the lighter portions SIS SwS of the fluid would be forced 5 ween Ginny yt oko Tp upward and would work their \4\\% way toward the surface by fus- ing and fluxing. As the lavas rise, the pres- sure on them becomes less, and hence the temperature neces- sary for liquefaction gradually falls, leaving them a constantly renewed margin of temperature available for melting their way through the upper horizons. Thus it is conceived that these fusible and fluxing selections from the middle zone might thread their way up to the zone of fracture, and thence, taking advantage of fissures and Fig. 233. Ideal section of a portion of the early earth, illustrating its assigned modes of vulcanism. C, center; S, sur- face; a-a’, fragmental zone; a’-f, zone of cracks, reach the surface (Fig. 233). It is conceived that such liquefaction and extrusion would carry the excess of tem- perature received by the lower part of the outer zone toward the surface, or even out to it. The outward movement of the continuous rock below melting tempera- ture at the surface; ff-c, interior portion whose temperatures rise from the surface melting-point at f-f to a maximum at C; V, V, threads or tongues of molten rock rising from the interior to various levels, many of these lodging within the frag- mental zone as tongues, batholiths, etc.; PPP, explosion pits formed by volcanic gases derived from tongues of lava below. lava would tend to lower the temperature below, forestalling gen- eral liquefaction, and keeping the zone as a whole, solid. The independence of volcanoes is assigned to the independence of the liquid threads that work their way to the surface. Nothing like a reservoir or molten lake enters into the conception. The prolonged 244 VULCANISM action of volcanoes is attributed to the slow feeding of the liquid threads from the middle zone, which is liquefied in spots only. The frequent pauses in volcanic action are assigned to temporary defi- ciencies of supply, and the renewals to the gathering of new supplies after a sufficient lapse of time. The distribution of volcanoes in essentially all latitudes and longitudes is assigned to the general nature of the cause. The special surface distribution is assumed to be influenced, though not altogether controlled, by the favorable or unfavorable conditions for escape of lava to the surface. The per- sistence of volcanic action in time is attributed to the magnitude of the interior source, to its deep-seated position, and to the slowness of conduction of heat from the earth’s interior. The force of expul- sion is found in the stress-differences in the interior, particularly the periodic tidal stresses, and in the slow pressure brought to bear on the slender threads of liquid by the creep of the adjacent rock. The violent explosions are due to the included gases, of which steam is chief. Little efficiency is assigned to surface-waters, and that little is regarded as secondary and incidental. The true volcanic gases are regarded as coming from the deep interior, and as being, after expulsion, accessions to the atmosphere and hydrosphere. The standing of the lavas in volcanic ducts for hundreds and even thousands of years with only little outflow, as in some of the best- known volcanoes, is regarded as an exhibition of an approximate equilibrium between the hydrostatic pressure of the deep-pene- trating column of lava and the flowage-tendency of the rock-walls, the outflow being also conditioned on the slow supply below, and on the periodic stress-differences of the interior. For the present, volcanic hypotheses must be left to work out their own destiny, serving in the meantime as stimulants of research. All but the last have been long under consideration. The recent discovery of the heating effects of radio-activity has given rise to the hypothesis that the origin of lavas is due to this cause. It seems clear that this must at least be a cooperative agency. It is too early in the new investigation to decide whether it can wisely be regarded as the sole cause or even an essential one. How lava reaches the surface. All views that locate the origin of the lavas deep in the earth must face the difficulty of the passage of lava through rock below the fracture zone. Near the surface, the lavas usually take advantage of bedding-planes, or of fissures already existing, or made by themselves. There is little evidence a CAUSES OF VULCANISM 248 that they bore their way through the zone of fracture. In the denser and warmer zone below, the alternatives seem to be (1) mechanical penetration without fracture, or (2) melting or fluxing. As rocks “‘flow”’ in this zone by differential pressure without rup- ture, an included liquid mass may perhaps be forced to flow through the zone by differential pressure. Lava probably fuses or fluxes its way, under pressure, through the rock below the zone of fracture. In this it may be supposed to be assisted by its gases, by the selective nature of its fluxing, by its exceedingly high temperature if it comes from very great depths, and by the stress-differences which attend tidal strains in the deep interior. In ascending, the lava would be invading regions of lesser pressure and lower melting-point. It would therefore have heat in excess of the local melting temperature, until it reached the cool rock. From that point on, the rising lava must constantly lose temperature by contact with cool rocks. If. its excess of temperature is insufficient to enable it to reach the zone cf fracture, the ascending column is arrested and becomes plutonic rock. If it suffices to reach the zone of fracture, advantage may be taken thereafter of fissures, and the problem of further ascent probably becomes chiefly one of hydrostatic pressure, in which the ascent of the lava-column is favored by its high temperature and its included gases. The hydrostatic contest is here between the lava- column measured to its extreme base, and the adjacent rock-columns measured to the same extreme depth. The result is, therefore, not necessarily dependent on the flowage of the outer rocks, but may be essentially or wholly dependent on the deep-seated flowage of the rock of the lower horizons. The ascending column may reach hydrostatic equilibrium before it reaches the surface, and then form intrusions of various sorts, or it may find equilibrium only by coming to the surface. References. C. E. Dutton, Hawaiian Volcanoes, Fourth Ann. Rept., U. S. Geol. Surv., 1883. Judd, Volcanoes, 1881; J. D. Dana, Characteristics of Vol- canoes, 1890. A. Geikie, Ancient Volcanoes of Great Britain, 1897. I. C. Russell, Volcanoes of North America, 1897. T. G. Bonney, Volcanoes, Their Structure and Significance, 1899. A. Heilprin, Mont Pelée and the Tragedy of Martinique, 1903. Accounts of same volcanoes in the Nat’l. Geog. Mag., Vol. XIII, 1902 (Russell, Hill, Hovey, Diller, and Hildebrand). Map work. See Plates CLV to CLXIV, of Professional Paper 60, U. S. Geol. Surv. and Exercise XVI, in Interpretation of Topographic Maps. CHAPTER X MATERIALS OF THE EARTH AND THEIR ARRANGEMENT The general constitution of the lithosphere has been referred to already (p. 7), but we are now to study in more detail the nature, the arrangement, and the history of the rocks. The igneous rocks will be considered first. IGNEOUS ROCKS Appearanceatthesurface. Thepreceding chapter has acquainted ‘us with the fact that some igneous rocks were extruded either from volcanoes or from fissures, and that extrusive rocks include both lava flows and pyroclastic materials. Under proper conditions, extruded rocks may be buried later beneath sediments, or may be worn away by erosion. It follows that only a part of the igneous rocks extruded in the past, and especially those of relatively recent times, remain at the surface. By removing the overlying rocks, erosion exposes the intruded rocks of dikes, sills, laccoliths, batholiths (p. 228), etc., and a considerable part of all accessible igneous rock is now at the surface because the rocks which overlay it have been worn away. The great areas of granite in Canada, and the long axes of many of our western mountains, are examples. Extruded igneous rock which has been buried, also is subject to subsequent exposure by the wasting away of its cover. Structural features of igneous rocks. The names applied to the principal forms of igneous intrusions imply certain large structural features; but igneous rocks have certain other structural features which distinguish them from other rocks. ‘Thus the rock of lac- coliths, bysmaliths, and batholiths is generally massive. This term means not simply that the rock occurs in large bodies, but that the rock has no distinct cleavage. It is not in beds, and it is not schistose. Sills and some extrusions of lava take on the form of sheets. Where one extrusive sheet of lava overlies another, the succession of sheets has some resemblance to stratified rock; but 246 4 <——_ei ne IGNEOUS ROCKS 247 the rock of the individual sheets shows little indication of arrange- ment in layers. Some extruded rock has a structure developed by the flow of the lava after it had become stiff from cooling. This Fig. 234. Flow structure in volcanic glass. About half natural size. (Photo. by Church.) Fig. 235. Columnar structure in igneous rock. Giant’s Causeway. 248 MATERIALS AND THEIR ARRANGEMENT is known as. flow structure (Fig. 234). On cooling, some lavas develop columnar structure (Fig. 235), the columns being roughly perpendicular to the surface of cooling. The explanation of the columns is probably somewhat as fol- lows: The surface of the lava contracts about equally in all direc- tions on cooling. ‘The contraction may be thought of as centering about equidistant points. About a given point, the least number of cracks which will relieve the tension in all directions is three (a, Fig. 236, A). If these radiate symmetrically from a point, the angle between any two is 120°, the angle of the hexagonal prism. Similar radiating cracks from other centers (0, c, etc.) complete the columns (Fig. 236, B). A five-sided column would arise from the failure of cracks to develop about one of the points. Igneous rocks are affected by cracks or joints, which run through them in various directions, but this is not a feature peculiar to igneous rocks. Pyroclastic rocks have somewhat the structure of ale sedimentary rocks. If the fragmental volcanic matter accumulates on the surface A B of the land, it may lack Fig. 236. Diagrams to illustrate the for- distinct stratification: but if mation of columns of basalt: A, the first : stage in the development of a hexagonal it falls or is washed into column. 8B, the completion of a hexagonal water, it may be assorted ee pai and stratified. In this case it is distinguished from other clastic rock by its constitution. Texturalfeatures. Most igneous rocks are made up of interlock- ing crystals of different sorts. These crystals may be so small that they are not distinguished readily by the eye, or they may be so large as to be seen easily, or some may be large and some small. If theyare large enough to be distinct to the eye even without close scrutiny, the rock is coarsely crystalline. All such rocks may be called phanerites. In. phanerites, the interlocking of the crystals is evident (Fig. 237). lf the crystals are so small as not to be seen readily by the eye, the rock is aphanite. In all igneous rocks, the crystals are of somewhat unequal size; but in some, there are certain crystals, usually of some one mineral, which are so much larger than the others as to be © conspicuous. The rock is then porphyritic (Fig. 238). The smaller IGNEOUS ROCKS 249 crystals of one or two kinds of mineral; the dark parts represent crystals of others. crystals in which the larger ones are set may be so small as not to be readily distinguished (aphanitic), or they may be visible sepa- rately (phaneritic). Some igneous rock is, in reality, volcanic glass. Vol- canic glass (obsid- ian) is one phase of solidified lava. It is formed when the liquid lava solidifies quickly, before the crystals have time to grow. Some ig- neous rock is made up partly of glass and partly of crys- Fig. 238. Porphyritic texture. pp=phenocrysts of feldspar. The smaller crystals are of feldspar, mica, and quartz. (Watts.) tals, and between the rock which is all glass and that which is all crystals there are all gradations. Whether lava becomes glassy or crystalline on hardening, or whether it is partly the one and partly 250 MATERIALS AND THEIR ARRANGEMENT the other, depends on the conditions under which it solidifies. All liquid lava contains the materials out of which crystals may be formed, under proper conditions. | Glassy and partly glassy rock may be compact or porous. Porous rock of the type shown in Fig. 239 is called scorzaceous. Fig. 239. Scoriaceous texture. About 4/5 natural size. (Photo. by Church.) Rock of this sort is really lava froth, solidified. The pores are the spaces occupied by gases when the lava hardened. Some of the bubbles were large and some small. Pumice is porous volcanic glass, the pores being small. Besides these varieties of texture which originate as lava hardens, there are the textures peculiar to pyroclastic rocks. When quanti- ties of volcanic dust, etc. (sometimes called volcanic ash), become coherent, as by cementation, the resulting rock is called tuff (or volcanic tufa). If the constituents are largely coarse, the resulting rock is volcanic agglomerate. Liquid lava (=Jiquid glass). Liquid lava is essentially fluid glass. Itis analogous to common glass, which is a silicate of potash, soda, or other base, except that manufactured glass is relatively free from iron and other coloring substances which abound in lavas, rendering them dark and more or less opaque. Lavas, too, are Fr | IGNEOUS ROCKS 251 usually mixtures of several silicates, while manufactured glasses consist of only one, or at most a few. Furnace slag is essentially an - artificial lava. Solidification and crystallization. When lava is cooled quickly, its components solidify essentially as they were in the liquid; for there is no time for the molecules of one kind to come together in regular systematic order, as is necessary to'form crystals. In a thick viscid liquid, the arrangement of molecules into definite crystal forms takes place slowly. Because of this slowness, the solidification of the lava may catch the process of crystallization at any stage. This is why some igneous rocks are glassy (cooled quickly), some partly glassy (cooled less quickly), and some wholly crystalline. In general, the slower their growth, the larger the crystals. Stages of crystallization. Eruptions take place intermittently, and the lava beneath the surface may be cooling during the inter- vals between eruptions. After a certain stage of partial crystalliza- tion has been reached during such time of quiet, a new eruption may shift the whole mass of lava into new surroundings, anda second phase of solidification may be added to the first. The rock may then show two phases of crystallization: (1) large crystals of the kind or kinds developed during the first stage of slow subterranean cooling; and (2) small crystals or glass developed during the more rapid cooling of the second stage. The result is large crystals set in a matrix of small crystals or of glass. This is perhaps one way in which por- phyry is formed. Composition of Igneous Rocks Nearly all the chemical elements are found in igneous rocks, though but few of them are abundant. These few are regarded as the essential constituents, while the rarer substances are regarded as incidental. The relative amounts of the more abundant elements in the crust of the earth, as nearly as now known, are shown in the following table: Per cent in the Element Symbol Solid Crust Oxygen U5 AEA AE ee ae Ue ake AE: 0a Sa 47.02 Silicon (oh) oie nny NPE RGIS ewe k 1S eo Aloe et in nr ee 28.06 PRISCA)... io PE A ae. ERCP Ct 8.16 Tron Be) eh et PR Ny Ait: SAR I SE NS 5. AG it ag 04 Calcium “fe UR At ON ae its Dok aise |? a RNa he Mees 3.50 - Sodium SC oe GE id Sag Bei Wt UAE | ant ase) ae £r4:63 252 MATERIALS AND THEIR ARRANGEMENT Potassium) (K)..cei9 4. Sei e.45 Re Ws a ot 2.32 Titanium «., (Ti) Aid. ga ee si 9 os Siew sh pan 41 Hydrogen © (H) ooo. ie. ye tte kd © oale eee 17 Carbon (Ol Beret te Cae RN .12 Phosphorus (P).. fid5. ete tee. urbe > et 09 Manganese’ (Mn) #2.) Su. n.000. UL Se .O7 Sulphur (S) + sid debe al ive es eae eee "ae. ae .07 It will be seen that only eight of the elements enter into the earth’s crust to the extent of one per cent, and no other one reaches half of one percent. Many elements that are of great importance in the affairs of men occur in quantities too small to be estimated in percentages. The precious metals, such as platinum, gold, and silver, and even some of the more common ones, as lead, zinc, and copper, are of little importance quantitatively. Union of elements. For present purposes we may neglect all but the first eight of the elements mentioned above. Out of these elements come various chemical combinations when the lava solidifies; out of these combinations come the various minerals; and from combinations of minerals come various kinds of rocks. The union of oxygen with the other seven elements may be taken as a funda- mental step in this series of combinations. The result is the following oxides: Silica (SiO2), alumina (Al,O3), the ferrous, ferric, and magnetic oxides (FeO, Fe.O03, and Fe.O3), magnesia (MgO), calcium oxide (lime) (CaO), soda (Na20), and potash (K2O). The oxygen sometimes unites in proportions different from those here given, but exceptions may be neglected here. Of these nine oxides, silica acts as an acid, or more strictly as an acid anhydride. All the rest except the magnetic oxide of iron, and sometimes the oxide of alumi- num, act as basic oxides. The proportion of silica in igneous rocks is so significant that all such rocks are sometimes grouped into three classes, as follows: those with more than 65% of silica are acidic; those containing 55 to 65%, intermediate; and those containing less than 55%, basic. The union of silica (SiO2) and lime (CaO) forms calcium silicate, CaO,SiOs, or CaSiO3. The union of silica and magnesia forms magnesium silicate, MgO,SiO», or MgSiO;. Corresponding unions of silica and the other oxides named, give rise to other silicates. Formation of minerals. Since but one of the leading oxides (silica) that abound in the average lava plays the part of an acid, a very simple conception of the general nature of igneous rocks may be reached by noting that they are com- posed mostly of silicates of the eight leading basic oxide—those of alumina, potash, soda, lime, magnesia, and iron. This general idea represents a most important truth; but in its use we must not forget that there are many exceptions. Sulphur, phosphorus, chlorine, and other elements unite with the bases to form sulphates, sulphides, phosphates, phosphides, chlorides, etc. So also there are many minor bases that form silicates; and these minor bases unite with minor acids to form many of the rarer minerals. Again, there are native metals in some igneous rocks; but altogether these minor compounds hardly reach more than one or two per cent of the whole, his aa...) - MINERALS OF IGNEOUS ROCKS 253 There are two exceptions of more importance. In the liquid lava the acid and basic elements are not always evenly matched. When there is an excess of silica, a portion remains free and takes the form of quartz (SiO2). If there is an excess of the basic oxides, the weakest one is usually left out of the combination. This is commonly an iron oxide (FesO4), called magnetite. It is a singular fact that quartz forms in some cases where there is no excess of silica, and magnetite where there is no excess of base. Quartz (free acid anhydride) and magnetite (free basic oxide) may occur in the same rock. The explanation of this is yet to be found. The oxides of silicon and iron form rather important exceptions to the general state- ment that igneous rocks are made up mostly of silicates; but, thus qualified, the statement expresses the essential truth. But here simplicity ends, and the sources of complexity are several. In the first place, silica unites with the bases in different ratios, and thus gives rise to uni-silicates or ortho-silicates (ratio of oxygen of base to oxygen of silica, 1:1), sub-silicates (the above ratio more than 1), bisilicates (ratio 1:2), tri-silicates or poly-silicates (ratio 1:3 or higher), etc. All the bases are not known to combine in all these ways, but many do in more than one. If the silica united with each of the bases one by one, the results would still remain comparatively simple; but instead, it may unite with two or more at the same time. Thus we may have an aluminum-calcium silicate. Not only this, but the different silicates may crystallize together in the same mineral, so that a crystal may be made up of alternating layers of different silicates. As such alternations are not governed by any known mathematical law, there is no deter- minate limit to the number of combinations that may arise. As a result of all this fertility of combination, the total number of siliceous minerals in igneous rocks is large. Geology deals with these minerals as constit- uents of the earth, but only a few of them are so abundant as to require special notice here. It may be remarked also that, as they occur in the rocks, only a few of them can be identified by simple inspection, partly because some of them look much alike, and partly because many of the crystals are minute. Summary of salient facts. The salient facts are, (1) that out of the 70-odd chemical elements now known in the earth, eight form the chief part of it; (2) that one of these elements uniting with the rest forms nine leading oxides; (3) that one of these oxides acts as an acid and the rest as bases; (4) that by their combination they form a series of silicates of which a few are easily chief; (5) that these silicates crystallize into a multitude of minerals of which again a few are chief; and (6) that these minerals are aggregated in various ways to form rocks. Possessed of these leading ideas, we are pre- pared to turn to the consideration of some of the conditions under which these combinations take place in the formation of rocks from liquid magmas. Principal minerals of igneous rocks. A few minerals make up the mass of igneous rocks. ‘These few are quariz, the feldspars, the ferro-magnesian minerals (amphiboles, pyroxenes, micas), and the 254 MATERIALS AND THEIR ARRANGEMENT iron oxides. ‘These minerals are described briefly below. Some of them occur in sedimentary and metamorphic works, as well as in igneous rocks. Quartz. SiOe; H. 7; Sp. gr. 2.65. A mineral of very widespread occurrence. It is found not only in igneous rocks, but in veins and cavities in other sorts of rock, as nodules and concretions-in limestones, and is the most abundant constituent of sands, sandstones, and quartzites. Quartz is a very hard mineral; that is, it cannot be scratched with steel and it will scratch glass. It is said to have conchoidal fracture; that is, it breaks like glass, without any distinct tendency to break along parallel planes. In igneous rocks, quartz usually looks rather dark and glassy by contrast with the lighter colored, less transparent minerals with which it is commonly associated. Some quartz has a sort of greasy or oily look because of its comparatively high luster. In veins and cavity fillings it may occur, (1) as 6-sided crystals capped by pyra- mid-like forms. Thecrystals may be so closely spaced that only the pyramid-like forms can be seen; (2) as a sort of hummocky crust with a waxy luster (chalcedony), or (3) as a series of bands of variegated color (agate). As concretions in limestone it may have a variety of colors, but is usually between white and dark grey, in some cases nearly black. Concretions are usually irregular in form, and contain a considerable proportion of impurities, but can be recognized by the hardness of a freshly broken surface. Some of the less common varieties are used as semi-precious stones; for ex- ample, amethyst, cairngorm, rose quartz, jasper, prase, cat’s-eye, and agate. True onyx is also a variety of quartz. The feldspars. H. 6; Sp. gr. 2.5-2.6. Orthoclase, Potassium aluminum silicate : Sodium aluminum silicate Plagioclase ie lei ; sae alcium aluminum silicate Feldspars are abundant in igneous rocks and their metamorphic products, but are’not found abundantly in other rocks. Feldspars are not so hard as quartz, but cannot be scratched by any but the very hardest steel. They have good cleavage; that is, they have a strong tendency to break along parallel plane sur- faces. This can be detected by holding a freshly broken surface to the light so that a reflection is seen. If the whole surface of a crystal seems to reflect the light when the fragment is held in a given position, it is usually due to the cleavage of the mineral. Feldspars are commonly the dominant light colored constituents of igneous rocks, but they range in color through white, buff, pink, red, and grey, and a comparatively rare variety is green. It isnot always easy to distinguish orthoclase, KAISi3;Os, from plagioclase, a mix- ture of NaAISi30g and CaAl,Si.0g; but the cleavage faces of some plagioclase crys- tals show distinct parallel striations, almost as true as if made by a ruling engine. These are never present in orthoclase. The dark, dull or waxy looking feldspars are more likely to be plagioclase, while buff or pink feldspars are more likely to be orthoclase; but such distinctions are too uncertain to be used with great assur- ance. Some of the rarer feldspars are used as semi-precious stones. Among these are Amazonstone, sunstone, moonstone, peristerite and labradorite. . 2 — MINERALS OF IGNEOUS ROCKS 255 Amphiboles and pyroxenes. Complex silicates, usually containing iron, lime, and magnesia. H. 5-6; Sp. gr. 2.8-3.6. The amphiboles and pyroxenes occur chiefly in igneous and metamorphic rocks, in some of which they are the most abundant dark colored constituents. They are hard minerals, that is, can be scratched by steel with difficulty. The commoner ones are black, greenish black or brown. MHornblende is the most important of the amphiboles, and azgite of the pyroxenes. These minerals re- semble eachother rather closely, and in very small crystals it is in some cases diffi- cult to distinguish them. The most notable difference is in the cleavage. Horn- blende cleaves in two directions at an angle of 124° (and 56°) from each other, while in augite the two cleavage directions are nearly perpendicular to eachother. Most hornblende has a jet-like luster, while augite is more likely to be dull, and is likely to be coated with rust on weathered surfaces. Another amphibole of importance is actinolite, which occurs only in metamorphic rocks and is easily recognized by its long, slender, needle-like crystals, which have a diamond-shaped cross-section and are usually bright green in color. With the exception of bronzite, which is distinguished by its brown color, the other important pyroxenes cannot be distinguished readily from augite. The micas. Complex hydrated silicates. H. 2; Sp. gr. 2.76-3. The common micas occur in igneous and metamorphic rocks and to a small extent as minute flakes in some sandstones and shales. The commonest micas are muscovite, which is white, greenish or yellowish brown, and biotite, which is dark brown or black. They are very soft, that is they can be scratched with the thumb-nail, and have a conspicuously good cleavage — so good that they may be split into sheets thinner than the thinnest paper. These thin sheets or flakes are very elastic and tough. In metamorphic rocks the plates are roughly parallel, and this results in the char- acteristic schistose appearance of many such rocks. Microscopic flakes of mica, arranged parallel to one another are responsible for the cleavage of slates. The only simple means of distinguishing between muscovite and biotite is the color. | Olivine. (FeMg)2SiOu; H. 6.5—7; Sp. gr. 3.3-3.5. Olivine occurs in the more basic igneous rocks, that is, those that are comparatively low in silica, especially those that are rich in ferromagnesian minerals. Olivine also occurs in meta- morphic rocks, especially metamorphosed dolomitic limestones. It is a very hard mineral, and has conchoidal fracture. In color it is yellowish green, varying somewhat according to the amount of iron present. It is usually transparent, and has a vitreous luster. It commonly occurs in granular aggregates which contain some pyroxene, but is found also in crystals in some dark colored igneous rocks. A variety used as a gem is called peridot. Chrysolite is another name applied to olivine. | The iron oxides. Hematite, FexO3; H. 5.5-6.5; Sp. gr. 4.8-5.4. Limonite, 2Fe203,3H2O; H. 5-5.5; Sp. gr. 3.6-4. Magnetite, Fe3;04; H. 6; Sp. gr. 5.18. Hematite + is the most important of the iron ores. It occurs (1) in sedimentary rocks, in some cases being the cementing material in sandstones, as in many of the red sandstones; (2) in igneous rocks as the result of weathering of the iron minerals; (3) in veins, and (4) in contact metamorphic deposits. It is mostly hard, but 1 Turgite is here included with hematite. _ 256 MATERIALS AND THEIR ARRANGEMENT shows some variation in hardness, as some specimens can be scratched by steel while others cannot. Hematite is red, dull steel blue, or black. Its most char- acteristic feature is the color of the streak, that is, the color of the fine powder left behind when the specimen is drawn across an unglazed porcelain plate, or powdered very finely inany way. ‘The color of the streak or fine powder is dark brownish red. The pigment called Venetian red is merely very finely ground hematite, contain- ing a small amount of clay. Limonite occurs in the same sorts of situations as hematite and as an alteration product of many ore deposits containing iron sulphides. It is deposited in some bogs in sufficient quantity to be valuable as an iron ore. Chemically it is the same as iron rust. Limonite has a very wide range of hardness, from soft earthy material, to compact material which cannot be scratched with steel. In color it ranges from light yellowish brown to very dark brown — in some cases almost black. The streak is characteristically yellowish brown, no matter what color the specimen has in mass. The common pigment yellow ocher owes its color to limonite. Magnetite. Magnetite occurs in igneous and metamorphic rocks, in contact metamorphic deposits, and to a limited extent in veins. It is very hard, and characteristically black in color, except on weathered surfaces, where it is usually coated with rust. The streak is black. Magnetite is most easily recognized by its magnetic properties. It is strongly attracted by a magnet, and may be mag- -netized. Naturally magnetized magnetite is called lodestone, and it is from this substance that our term magnet is derived. Magnetite is of comparatively little importance in America as an iron ore, but in some parts of Europe it is a very important ore mineral. Other important rock-forming minerals. A few other common rock-making minerals are mentioned here, though most of them do not occur in igneous rocks, except as secondary minerals introduced subsequent to the hardening of the lava. Several of them occur in metamorphic rocks only. Calcite and dolomite occur abundantly in certain sedimentary rocks, but are secondary in igneous rocks. Calcite. Calcium carbonate, CaCO3. H. 3; Sp. gr. 2.72. Calcite is a mineral of very widespread occurrence. It is the chief constituent of limestones and marbles (metamorphosed limestones), and occurs as cavity fillings in many kinds of rocks. It is a very common vein mineral and occurs as an alteration product of lime silicates in many weathered igneous rocks. Calcite is rather soft; that is, while it cannot be scratched with the thumb-nail, it is scratched easily with steel. It has a very good cleavage in three directions which may give rise to rhombohe- drons, that is figures like cubes, which have been compressed along one diagonal. It is recognized most readily by its behavior towards acids, which act upon it rapidly, causing an effervescence of carbon dioxide. Some other minerals effer- vesce similarly, but none of the very common ones show such rapid action as calcite. Transparent pieces of calcite show double refraction; that is, if a piece of transparent calcite is placed over a dot on a piece of paper, two dots may be seen distinctly. Most calcite is white or colorless, but some is colored brown, yellow, green or pink by impurities of various sorts. Chlorite. A complex, hydrated silicate containing Fe and Mg. H. 2; Sp. gr 2.65-2.75. Chlorite is a secondary mica; that is, it is a mica formed by the action of weathering on certain silicates containing iron and magnesium. It occurs in metamorphic rocks, and as an alteration product in igneous rocks, It is very MINERALS 254 soft, usually dark green or greyish green in mass, and gives a grey or greenish grey streak. Chlorite has micaceous cleavage, and the plates are mostly flexible and inelastic. The greenish color of many igneous rocks is due to the presence of chlorite formed by the alteration of pyroxenes, amphiboles, etc. Dolomite. CaMg(COs3)2. H. 3.5-4; Sp. gr. 2.8. A mineral closely resembling calcite. Dolomite occurs in some of the older limestones, in some marbles, and to a small extent in veins. It is a rather soft mineral, showing a cleavage like calcite when crystals of sufficient size are seen. Some dolomitic rocks are so fine grained that the individual crystals cannot be detected without the aid of the microscope, and in such specimens no cleavage is apparent. Some dolomite crystals exhibit peculiarly curved faces, which are rare in calcite. Most dolomite is milky white, brownish, or pink, but it varies greatly in color with impurities of various sorts. It is most easily distinguished from calcite by its behavior with dilute acids. Calcite effervesces vigorously even in quite dilute acids, while dolomite is only very feebly attacked by such acids. Garnet. A complex silicate, different varieties having different composition. 16-5-7.5;9D. gr. 3.15—4.3. Garnet is rarely found outside of metamorphic rocks, in which it is in some instances so abundant as to be the chief constituent. It is a very hard mineral — about as hard as quartz; but when it has been exposed to weathering for a long time it may in some cases be scratched with steel. Garnet has no cleavage, but breaks with conchoidal fracture like glass or quartz. In color most garnets are red or brown, but other colors, as pink and green, are known. Garnet has rather high specific gravity compared with the rest of the common silicates. Garnet is used in large amounts as an abrasive, and to a small extent as a gem stone. Graphite (carbon). H. 1-2. Sp. gr. 2.1. Graphite is one of the crystalline modifications of carbon. It occurs in meta- morphic rocks — mostly in metamorphosed sedimentary rocks. It is very soft and has a marked greasy feel, like that of talc. It is black, and will mark paper with a black streak, a property of which use is made in common lead pencils. Graphite also is used in the manufacture of crucibles, as a lubricant, and as an adulterant. Gypsum. Hydrated calcium sulphate; CaSO4,2H2O; H. 2; Sp. gr. 2.3. Gypsum occurs in beds among other sedimentary rocks, in some places as a residue from the evaporation of saline lakes, and in crystals scattered through shales. It occurs sparingly in veins. It is a very soft mineral and commonly has a very good cleavage, resembling mica, except that the thin leaves are not elastic nor strong like those of mica. It has not the greasy feel characteristic of talc. A number of varieties of gypsum are recognized. Selenite, the common variety of gypsum, has been described above. Satin spar is a variety that has a fibrous structure, and a bright satin-like luster. Alabaster is white massive gypsum; that is, gypsum which is made up of minute crystals which cannot be readily dis- tinguished as individual crystals. Gypsum is characteristically white, but may be reddish, brownish or gray when it is impure. Gypsum is largely used in the manufacture of various sorts of plaster. Alabaster of high grade is sometimes used for ornamental vases, etc. Kaolin. A hydrous aluminum silicate. H. 2-2%; Sp. gr. 2.6. Kaolin occurs in igneous rocks as an alteration produce of feldspars, and in sedimentary rocks. Shales and clays are made up largely of kaolin, and it is present in varying 258 MATERIALS AND THEIR ARRANGEMENT quantities in other sedimentary rocks. It is a very soft mineral, and when free from grit usually has a soapy feel, but differs from talc in that it becomes plastic if ground up and moistened with water. Kaolin is earthy in appearance, and breaks like an earthy substance. No cleavage is apparent, because the mineral is not commonly crystallized, and when crystallized the individual crystals are too small to show cleavage without the aid of the microscope. Its color is nearly white when pure, but more commonly is brown, or bluish gray, according to the impurities it contains. Pyrite. FeSy. H. 6-6.5; Sp. gr. 5.0. Pyrite occurs in minute crystals in igneous rocks, in large masses in some veins and in metamorphic rocks, and is not uncommon in limestones, sandstones and shales. It is abundant in some coal beds and is the source of the sulphurous odor of coal smoke. Pyrite is very hard, has a bright metallic luster resembling that of light colored brass, but its streak is black. Crystals of pyrite may be cubes, octahedrons, or more complex forms; slightly deformed cubes with striated faces are the most common. When exposed to weathering, pyrite rusts — that is changes to limonite. If the original form of the pyrite crystal is retained by the limonite, it is called a pseudomorph. Pyrite is used in the manufacture of sulphuric acid, and to a very small extent as a source of low-grade iron. Serpentine. Hydrated silicate of magnesium. H. 4; Sp. gr. 2.5-2.6. Ser- pentine occurs chiefly in metamorphic rocks, and as an alteration product in basic igneous rocks. Serpentine has a greasy feel, but less marked than that of talc. Most of it is yellowish green or yellow in color, but may be stained brown by iron oxides. One variety of serpentine consists of fine, closely packed, flexible fibers, called asbestos. Most asbestos occurs in vein-like masses in massive serpentine, the fibers running across the vein. Serpentine is used as a building stone and for interior decoration; asbestos is used in fire-proofing and in the thermal- insulation of material of various sorts. Talc. A hydrous magnesium silicate. H. 1; Sp. gr. 2.75. Talc is an alteration product of magnesium silicates, especially those free from alumina, and abounds in their metamorphic products. It may occur about the mineral grains in weathered igneous rock, but occurs more abundantly in metamorphic rocks, such as soap- stone and talc schist. It is very soft and has a peculiar greasy feel that usually serves as a valuable aid in identification. Some specimens show very good cleavage, resembling mica except for the fact that the thin plates are not elastic. In translucent specimens most talc is light green in color; in opaque specimens, it varies from nearly white to dark gray. Some varieties resemble kaolin (pure clay), but may be distinguished readily from it by moisten- ing some of the finely powdered mineral; kaolin becomes plastic while talc does not. Classification of Igneous Rocks Several features are involved in the classification of igneous rocks. Some of them have been noted already, but may be recapitulated here. All fragmental igneous rocks are pyroclastic, and pyroclastic rocks may be tuffs, agglomerates, etc. (p. 263). Rock formed from lava without the development of crystals, is obsidian, if not porous. If porous (hardened rock-froth), the rock is pumice, scoriaceous glass, etc. If the rock is largely glass, but partly of small crystals, it is sometimes called pitchstone, because its freshly fractured surface looks like pitch or resin. When the cavities of scoriaceous rock become filled by minerals deposited from ~ ee CLASSIFICATION OF IGNEOUS ROCKS 259 Fig. 240. Graphic granitic (or pegmatitic) texture. Nearly natural size. (Photo. by Church.) solution, the rock becomes an amygdaloid. Porphyry, phanerite and aphanite have been defined already (p. 248). All these names are based on texture, rather than on mineralogical or chemical composition, Most igneous rocks are wholly crystalline, and are classified on the basis of their composition. Their chemical composition determines their mineral com- position, and the rocks are named according to the minerals they contain. The number of varieties of igneous rock is very large, but only a few of the more im- portant need be’ mentioned here. The granites. The name granite was originally used to designate a granular rock (a phanerite, p. 248), and it is still popularly and properly so used. In scientific treatises it usually has been confined to a rock composed chiefly of crystals of quartz, feldspar (especially orthoclase) (p. 254), and mica. Recently it has been proposed to give it again a more general application by including under it all phanerites composed chiefly of quartz and feldspar of any kind, with mica, hornblende, or other minerals in subordinate amount. In normal granite, the crystals are distinct and in some cases large (Fig. 237), and more or less intimately interlocked. Granites are among the most common and easily recognized of the phanerites. Their color is determined largely by the feldspar, the red and pink varieties of the mineral giving rise to red and pink granite, and the whitish varie- ties to gray granite. Granites vary widely from their type by the addition and substitution of other minerals. Whenever one of these replacing or accessory 260 MATERIALS AND THEIR ARRANGEMENT minerals, is abundant, its name is often prefixed, as hornblende-granite. Granite grades insensibly into other types of igneous rock, as syenite, diorite, etc. Varia- tions also arise from the absence of one. of the leading minerals. Granites were formed from lavas rich in silica (normally 68-70%), alumina, potash, and soda, but generally poor in lime, iron, and magnesia. Granite is generally an intrusive massive rock. When rock of the composition of granite is banded, it is gneiss. Graphic granite, composed chiefly of inter-grown crystals of quartz and feld- spar, has a peculiar texture (Fig. 240). Pegmatite is a variety of coarsely cry- stalline granite composed chiefly of quartz, feldspar, and muscovite (p. 255). It occurs principally in dikes and veins associated with granitic and other similar rocks. Rock of similar texture may have the composition of syenite (syenite pegmatite), diorite (diorite pegmatite) etc. The syenites. The term syenite (from Syene on the Nile, where this sort of rock occurs) is now applied to rock consisting essentially of feldspar and horn- blende, with or without mica; but there is a complete gradation from granites to syenites. Syenites are richer in iron and magnesium than granites, and poorer in silica (about 58-60%). Syenites also grade into other classes of rock as do granites, and special varieties are named by similar prefixes, as augite-syenite, etc. Syenites are red or gray according to the color of the feldspar, and most of them are rather darker than granite, which they resemble. The texture of syenite is like that of granite, and its mode of occurrence the same. The diorites. Diorites are rocks which crystallized from lavas having about the same amount of silica as the lavas of the syenites, but poorer in the alkalies, and richer in the earthy bases. In current usage, diorite is defined as a rock composed of an intimate mixture of crystals of hornblende and a plagioclase feld- spar. It differs from syenite in having plagioclase feldspar (p. 254) instead of orthoclase. By substitutions and the addition of accessory minerals, the diorites grade toward the granites and syenites on the one hand, and toward the gabbros on the other. In color most diorites are rather darker than the gray granites. The gabbros. The gabbros embrace a large group of rocks whose principal minerals are plagioclase (normally labradorite) and pyroxene (normally diallage), with magnetite or ilmenite (titanium iron oxide). Most gabbros are dark colored and rather heavy. The pearly luster of the cleavage faces of the diallage gives a peculiar sheen to a fresh surface of the rock, in many cases. Silica constitutes about 46 to 55 per cent of gabbros. The peridotites. Peridotites were formed from a magma in which silica was low (39-45%), as were also alumina, lime, and the alkalies, but in which magnesia was high (35-48%). The rock consists largely of the mineral olivine (p. 255.) asso- ciated with pyroxene, magnetite, and other basic minerals. Little or no feldspar is present. Peridotites are much less abundant than the preceding rocks. Closely allied with the peridotites are rocks made up largely of a single basic mineral, as augitite, pyroxenite, hornblendite, rocks essentially formed of the min- erals augite, pyroxene, and hornblende, respectively. The basalts. The term basalt is used in a somewhat comprehensive way for dark, compact, igneous rocks the crystals of which are in most cases so minute as not to be distinguished readily by the eye. The leading minerals are a plagioclase feldspar and pyroxene (usually augite), with olivine, and magnetite or ilmenite usually present. There is a considerable range in chemical composition, but the CLASSIFICATION OF IGNEOUS ROCKS 261 basalts are relatively poor in silica (46-55%), and most of them in potash and sada, but rich in lime, magnesia, and iron. Basalts are classed as basic, and some are highly so. The lavas of many basaltic flows were very fluid, and spread out.in thin sheets when poured‘ out upon the surface. In cooling, basalt is prone to take on a columnar structure (p. 248). The columns of Giant’s Causeway and Fin- gal’s Cave are familiar examples. Basalts graduate insensibly into dolerites, which may be regarded as basalts of coarse crystallization. Diabase is a rock of similar composition and ophitic texture; that is the pyroxene crystals are separated into thin plates by inter-growths of plagioclase. General names. The difficulty of distinguishing many of the foregoing rocks from each other by any means available in the field, owing to the minuteness of the crystals, and to the gradation of one type of rock into another, makes it desir- able to employ certain general names which will correctly express the leading character of the rock without implying a knowledge of its precise mineral com- position. A convenient term of this kind is greenstone, which merely indicates that the ferro-magnesian minerals are prominent, and give a greenish or greenish black cast to the rock. The greenstones embrace the dolerites and basalts, and some of the gabbros and diorites, and may even extend to the peridotites and per- haps to others. Another convenient name is trap, which may be used for any dark, heavy igneous rock, such as basalt. The term basalt is sometimes used in much the same way. Varieties of rock dependent upon conditions. From what has preceded, it is clear that the chemical nature of the liquid magma determines the mineralogical composition of the rock, if it is crystalline; but it may now be pointed out that the same lava which made a plutonic granite, might have made a porphyry, an obsidian, a pumice, or a tuff, under other conditions of solidification. The same is true of diorites, gabbros, etc. A New System of Classification and Nomenclature of Igneous Rocks The current systems of classifying and naming rocks, if indeed they can be called systems, have grown up gradually out of earlier and cruder methods, many of which were inherited from popular usage. Most of the names and definitions came into use before modern methods of study were adopted. These systems, therefore, retain many crudities and inconsistencies, and lack adaptation to present needs and knowledge. A more adaptive and consistent classification is needed, and in response to this need, a new system of classification of igneous rocks has been offered by a group of leading American petrologists.1 To some extent this proposed system may be extended to metamorphic crystalline rocks. The classi- fication and nomenclature of the'sedimentary rocks probably must always remain plastic, to express the various points of view which it is desirable to consider. The proposed system includes two parts, a field system and a quantitative system, the one applicable to rocks on casual inspection, and the other only after detailed study. The field system only is here outlined. The proposed field system. ‘The proposed field names are based largely on texture and color. Mineral constituents are used for subdivisions when they can be determined easily; otherwise they are neglected. 1 Cross, Iddings, Pirsson, and Washington. Quantitative Classification of Igneous Rocks. See also Johannsen, Jour. Geol. Vol. XIX (1912), p. 317. 262 MATERIALS AND THEIR ARRANGEMENT Classifying chiefly on the basis of texture and crystallinity, there are three groups: Phanerites, in which all the leading mineral constituents can be seen with- out a lens; aphanites, in which at least an appreciable part of the minerals cannot be distinguished by the unaided eye; and glasses, in which the material is wholly or largely vitreous. I. The phanerites are classified further as follows: 1. Granites, consisting largely of quartz and feldspar of any kind, with or without mica, hornblende, pyroxene, or other minerals. This differs from the present common use of the term granite, in not regarding mica as an essential constituent, and in not distinguishing between alkali feldspars and calcic feldspars. The term therefore includes more than formerly. 2. Syenites, consisting predominantly of feldspar of any kind, with subordinate amounts of hornblende, mica, or pyroxene, but with little or no quartz. This differs from the common usage in giving hornblende a subordinate place, and in embracing rocks with calcic feldspars. 3. Diorites, consisting predominantly of hornblende and subordinately of feldspar of any kind, with which there may be mica, pyroxene, or other minerals. This is nearly the present use, except that any kind of feldspar may be the sub- ordinate mineral. 4. Gabbros, consisting predominantly of pyroxene and subordinately of feld- spar of any kind, with or without other minerals. This nearly coincides with one of the various present uses of the term, except that the range of the feldspar is increased. 5. Dolerites,: consisting predominantly of any ferromagnesian mineral not distinguishable as hornblende or pyroxene, with subordinate amounts of feldspar of any kind, and with or without other accessory minerals. In other words, the dolerites (deceptive) embrace diorites and gabbros when they cannot be distin- guished by the eye. 6. Peridotites, consisting predominantly of olivine and ferromagnesian minerals without feldspar, or with very little. 7. Pyroxenites, consisting essentially of pyroxene. 8. Hornblendites, consisting essentially of hornblende. Lh. bhe aphanites may be non-porphyritic or porphyritic. (a) Non-porphyritic aphanites, if light-colored, may be classed as felsites: when dark-colored, as basalts. (b) The porphyritic aphanites or porphyries, if light-colored, are /eucophyres, when dark-colored, melaphyres. They may be classified further, according to the kind of phenocryst (distinct crystal) imbedded in the aphanitic groundmass, as Quartz-por phyries, or quartzophyres; Feldspar-por phyries, or felspaphyres (not felsophyres) ; Hornblende-por phyries, or hornblendophyres; and so on. These may be subclassed by color, as Quartz-leuco phyres, light-colored quartz-porphyries; Quartz-melaphyres, dark-colored quartz-porphyries; Feldspar-leucophyres; Feldspar-melaphyres; and so on. III. The glasses are classified, according to color and luster, into obsidians 1 Added by the authors of this work. DISRUPTION OF IGNEOUS ROCKS ele or pitchstones when dark and lustrous; perlites when a spheroidal fracture gives them a pearly appearance; and pumice when greatly inflated by included gases. IV. Pyroclastic rocks are Tuff, if composed of finely comminuted pyroclastic material; Volcanic breccia if composed of coarse angular pyroclastic materials, Agglom- erate is a term much used for volcanic breccia, and for similar rock whose con- stituents are but little rounded. If the constituents are well rounded, the rock becomes volcanic conglomerate. In general discussions, it is serviceable to use the term granitoids in a broad generic sense, to include all crystalline rocks of the general granitoid type, including the granites, syenites, etc. In a similar broad way, gabbroids may be used to include the dark crystalline rocks in which the ferromagnesian minerals predomi- nate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and comprehensive way, two contrasted groups of igneous rocks may be designated. As the granitoids are usually acidic and the gabbroids basic, the grouping repre- sents a broad fact of importance. The Disruption of Igneous Rocks At the surface, igneous rocks are subject to mechanical dis- ruption, and to chemical change which results in decay. Mechanical disruption. One great agent of mechanical disrup- tion at the surface is change of temperature. This has been dis- cussed in Chapter II and other phases of mechanical disruption are discussed in Chapters IV and V. All mechanical disruption of igneous rock leaves the fragments essentially like the original rock in composition. Chemical disintegration. Most of the silicate minerals which make up the larger part of all igneous rock are complex, chemi- cally. Not a few of them contain as many as three or four basic elements, in union with oxygen and silicon. Substances which are complex chemically, are, as a rule, less stable than those of simple constitution. Complex silicates, such as the feldspars, micas, amphiboles, and pyroxenes tend to break up into simpler sub- stances. Chemical changes are helped along by the oxygen, carbon dioxide (COz), and water vapor of the air, and by water after it is precipitated. Some of the simpler changes may be noted. Oxygen may enter into combination with the iron of a silicate mineral containing iron. ‘The iron is thus taken out of its silicate combination, and in union with the oxygen forms iron oxide, a simple and stable chemical compound. This process is oxidation. Oxidation affects other elements also. Similarly, carbonic dioxide from the air may enter into com- bination with the base of a silicate mineral. Thus it enters into 264 MATERIALS AND THEIR ARRANGEMENT combination with the calcium of a mineral which contains calcium, taking the latter out of its union with silica. The union of the calcium and the carbon dioxide gives rise to calcium carbonate. Magnesium and iron may be taken out in the same way, forming. magnesium carbonate and iron carbonate, respectively. This proc- ess is called carbonation, and the carbonates thus formed are simple Fig. 241. Exfoliation of granite. Wichita Mountains, Okla. and stable in composition. ‘The carbonates are more soluble than most other common mineral substances. Water may enter into combination with mineral matter, and the union is hydration. ‘Thus when iron rusts (oxidizes), it is not merely oxygen which enters into combination with the iron, but water also. Iron rustis a hydrated oxide of iron (see limonite, p. 256). Oxidation, carbonation, and hydration, involving respectively the addition of oxygen, carbon dioxide, and water, increase the volume of the mineral matter. The result is that the rock affected crumbles. Thus the iron rust formed on a knife blade crumbles off. So the iron rust formed when oxygen and water unite with the iron in the rock, causes the rock in which the change takes place to crumble, partly because of the expansion involved. Again, some of the simple compounds, especially the carbonates, formed when the rock decays, are somewhat soluble and may be dissolved and taken away. ‘This tends to make the rock less com- pact by taking away one of its ingredients. Oxidation, carbonation, and hydration therefore. not only SEDIMENTARY ROCKS 265 change the chemical nature of the rock, but they change its volume, allow some of its material to be carried off in solution, and in many cases cause it to fall to pieces. The result is decayed rock — or one variety of rock waste. It is to be observed that the rock waste which arises from decay is unlike the original rock in composition. Fig. 242. Exfoliation on the slope of a granite mountain near Royal Arch Lake, Yosemite Quadrangle. (Turner, U.S. Geol. Surv.) Some things have been added, and others taken away. In this respect, the waste arising from decay is unlike that arising from rock breaking. The products of decay may remain where formed, or may be taken away. If they remain where formed for long periods of time, they may come to make a thick mantle of residual earth. Decayed rock is scores of feet in depth in many places, and hundreds of feet in some. Chemical decomposition is greatest in warm regions, and products of decay are least readily removed where there are forests. The products of decay are therefore likely to be deepest in warm, forested regions. ‘They are very deep, for example, in some parts of Brazil. SEDIMENTATION AND SEDIMENTARY ROCKS Removal of decayed rock. The breaking-up of igneous rock prepares the way for other processes. The loosened material may be blown away by the wind, washed away by running water, or 266 MATERIALS AND THEIR ARRANGEMENT moved by any agency which shifts materials about on the surface of the earth. If the products of rock disintegration are coarse, they may become gravel after being rounded by streams or waves. If the material is finer, say of the size of small grains, it is sand; if still finer, it is mud when wet, and.dust when dry. Deposition of sediment. When carried by any transporting agency, such as wind or water, rock waste becomes sediment, and sooner or later is deposited. Some of the material picked up and transported by running water is left at the bases of the slopes of mountains and hills from which it is washed, and some of it is left on the flats through which streams flow; but much of it is carried to the sea and left there. The coarser part of the sediment carried to the sea is left near the shore, and the finer parts are taken farther out. Thus along many coasts the gravel of the shore-line grades out into sand, and this into mud as distance from the water’s edge increases. The coarser materials are thus separated more or less perfectly from the finer. When the disintegration of the parent rock results from decay, the rock-waste is unlike the parent rock in composition, because some of the original material has been dissolved and carried away in solution. Not only this, but the fine products of decay may differ from the coarser in composition. Thus the quartz grains of granite are generally large enough to be readily seen individually; and as the granite decays, this mineral, already a simple compound, undergoes little change, and the grains remain in the rock waste. By moving water, they are rounded into the sand grains with which we are familiar. On the other hand, the crystals of feldspar, which have a complex composition, decompose into very fine particles of kaolin (p. 257) or clay, unlike the feldspar in composition, and containing but a few of the elements of feldspar. Thus it happens that the coarser products of decay, such as quartz, are chemically unlike the finer, such as clay, and the two are partly separated when they are deposited. In this case, the composition of the rocks formed from the sediments may be very different from that of the rock from which the sediments were derived. On the other hand, when rock-waste resulting from the mechanical breaking of rock is depos- ited, the sediment has about the same composition as the rock from which it came. Sediment which contains feldspar derived from granitic rock is called arkose. Arkose represents incomplete decom- position of the parent rock. SEDIMENTARY ROCKS 267 Cementation of sediment into solid rock. After gravel, sand, mud, etc., are deposited in the sea or elsewhere, they may be cement- ed into solid rock by the deposition of mineral matter held in solu- tion in water. This cement binds the pebbles, the grains, and the smaller particles together, much as lime binds sand in mortar. The cemented gravel makes conglomerate, or if the pieces of rock are angular, breccia; the cemented sand makes sandstone; and the ce- mented mud makes shale. These are common sorts of sedimentary rock. The cementation may take place while sedimentation is in progress, or at a later time. Conglomerate, sandstone, and shale, made up chiefly of particles derived directly from other rock, are clastic rocks. Limestone may be broken up, and its particles redeposited and cemented again into solid rock. Such limestone is clastic, and limestone made of broken shells, coral, etc., is in some sense clastic. In contrast with igneous rocks, clastic rocks are made up of particles of other rock, particles which were once separate and distinct, bound together by some sort of cement. The particles touch one another, but do not interlock like crystals of igneous rock. When sand, mud, etc., are deposited in the sea, shells of sea _ animals may be imbedded in them. If the shells or their forms are preserved, they record the kinds of life that lived when the sediment was being laid down. If the sediments are deposited in lakes or on land, the shells or other relics of freshwater or land life may be buried in them. All distinct relics of past life are fossils. Non-clastic sediments. Not all sedimentary rocks are clastic. It has already been noted that some of the compounds formed when rock decays are soluble. A part of the materials dissolved are carried in solution to the sea, where some of them are extracted by animals and made into shells or other hard parts. When the animals die, their shells and other secretions are left behind. If these are of calcium carbonate, they make limestone when cemented together. Much, if not most, limestone is composed of the secre- ‘tions of organisms. The shells, coral, etc., may or may not have been broken up before cementation. Limestone has many varieties, one of which is chalk. Magnesium may replace the calcium in various propor- tions, and if there is any considerable amount of magnesium, the rock is dolomite. ‘The dolomization of some limestone (the con- version of CaCO; into CaMg(COs)2) appears to have taken place long after the limestone was formed, while in other (perhaps in 268 MATERIALS AND THEIR ARRANGEMENT most) cases it appears to have taken place while the material of the limestone was being deposited. Siliceous deposits. In the decomposition of igneous rocks, a little of the silica, as well as of the bases, is dissolved and carried away in solution. Certain organisms extract this silica from the water for their tests, shells, etc., just as others extract calcium car- bonate. Siliceous secretions may form siliceous rocks. Dziatom and radiolarian oozes of the deep sea are examples. Familiar examples Fig. 243. Globigerina ooze, similar to chalk in composition. Magnified 20 times. (Murray and Renard.) of indurated rock formed in this way are certain flints and cherts that occur in limestone, both as nodules and in distinct beds. Some of these are developed about fossil sponges. Precipitation from solution. Some sedimentary rock is formed by direct precipitation from water which is saturated. Thus limestone might be formed by direct precipitation from water if it became saturated with CaCQ3, and some limestone has been formed in this way. Rock-salt has been deposited in thick beds at various. times and places, as it is being deposited now about Salt Lake in Utah. The sodium of the salt (NaCl) doubtless came from decay- ing rock, for many igneous rocks contain a little sodium in some complex combination. In the decay of the rock, the sodium is taken out of its complex combination, and made into some soluble compound, and then taken to the sea or to a lake. Its union with chlorine makes common salt. Gypsum (CaSQu,) is another form of SEDIMENTARY ROCKS 269 rock deposited in a similar way. Jron ore occurs in large bodies, and some of them were formed by precipitation. Salt, gypsum, limestone, and iron ore are peculiar among rocks in that but one mineral enters into their composition when they are pure. Coal is a sort of rock formed from accumulations of vegetable matter. Some other sedimentary rocks, as noted above, are formed organically, though they can hardly be said to be organic. The principal classes of sedimentary rocks are given below: ( Conglomeratic rocks,— gravel, conglomerate, breccia, etc. Mechanically formed | Arenaceous rocks,— sand, sandstone, some arkose, etc. Clastic Argillaceous rocks,— clay, shale, etc. A few limestones. { Some carbonate rocks, e. g., travertine, siderite. Chemically formed Chloride rocks,— especially rock-salt. Non-clastic Sulphate rocks,— especially gypsum. Some siliceous rocks,— some cherts, etc. Organically formed / Calcareous rocks,— most limestones. Non-clastic ~ Siliceous rocks,— siliceous oozes, sinter, etc. Carbonaceous,— coal, etc. Distinctive Features of Sedimentary Rocks Stratification. Most sedimentary rocks are arranged in more or less distinct layers; that is, they are stratified (Fig. 2). Stratifi- cation consists primarily in the superposition of layers one on an- other. Layers of like constitution or compactness may be sep- arated by films of different material which cause the partings. The bedded arrangement is due to various causes, but primarily to the varying agitation of the waters in which the sediments were laid down. Where depositing waters are agitated vigorously to the bottom, coarse sediment only is deposited. Where waters are quiet at the bottom, fine sediment is the rule. Since the agitation of waters is subject to frequent change, coarser sediment succeeds finer, and vice versa, in the same place. Hence arise beds, layers, and Jamine. ‘The terms layer and bed generally are used as syno- nyms, while /amine are thinner divisions of the same sort. The term stratum is sometimes applied to one layer, and sometimes to all the consecutive layers of the same sort of rock. For the latter meaning the term formation is often used. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates. In many places the bedding of limestone is caused by films of clayey matter between the layers, 270 MATERIALS AND THEIR ARRANGEMENT the films causing natural partings. Bedding arises also from varia- tions in the physical condition of the calcareous sediment itself. Lamination is not, as a rule, conspicuous in pure limestone, though it may be in the shaly phases of this rock. Shales are normally laminated as well as bedded, and the lamination may be more notable than the thicker bedding. Bedding in shale may arise from the introduction of sandy lamine, or by changes in the texture Fig. 244. Cross-bedded sandstone. Maol Donn, Arran. The layers are horizontal, some of the laminz, diagonal. (H. M. Geol. Surv.) of the mud, etc., of which the shale was made. Some sandstones are divided into beds by shaly or clayey partings, or by variations in the coarseness or coherence of the sand itself. Sandstones may be thick or thin-bedded, and their bedding pe insensibly into lamination. Sand is deposited normally in relatively shallow’ water where it is subject to much shifting before it finds permanent lodgment. SEDIMENTARY ROCKS 271 In the shifting, bars or reets are formed, most of which have a rather steep face in the direction in which the sand is shifted. The sand carried over the top of the bar finds lodgment on the sloping face beyond. The inclined laminz thus formed constitute a kind of bedding, but its planes do not conform to the general horizontal attitude of the formation as a whole. The structure is called cross-bedding, or, more accurately, cross-lamination (Fig. 244). The same structure is developed on delta fronts, and generally in water shallow enough to be subject to frequent agitation at the bottom. Sandstone is cross-bedded more commonly than shale or limestone. The bedding of conglomerate is due chiefly to varia- tions in coarseness. Lamine or beds of sand occur between the layers of coarser material in many places. The beds of conglomer- ate are likely to be thick, and in conglomerate cross-bedding is common. Lateral gradation. When the varying nature of the agitation of the sea at different depths and along the different parts of a coast is considered, it will be understood that deposits of one kind may grade into others horizontally. Thus a bed of conglomerate (gravel) may grade laterally into sandstone (sand), and this into shale (mud) or limestone. It is indeed more remarkable that sedi- mentary strata are as regular and persistent as they are, than that they grade into one another in some places. Position of strata. At the time of deposition, beds of sediment conform in a general way to the slope of the bottom where they are laid down. Since the slope of the sea bottom near shore is very gentle, as a rule, beds of sediment are, in most cases, nearly hori- zontal when deposited. Their slope is rarely so much as 20°, and commonly less than 5°. Special markings. The rhythmical action of waves gives rise to ripple-marks (Fig. 196), which are also made by streams, stream- like currents, and wind (Fig. 13). They are usually only a few inches from crest to crest, but in rare instances they attain much greater size. Under proper circumstances, ripple-marks are pre- served indefinitely. Some sediments are exposed between tides, or under other cir- cumstances, for periods long enough to permit drying and crack- ing at the surface. On the return of the waters, the cracks may be filled and permanently preserved. (Figs. 200 and 201) Such rec- ords of sun cracks affect shales chiefly, but are seen occasionally in 202 MATERIALS AND THEIR ARRANGEMENT limestones and fine-grained sandstqnes. During the exposure of the sediments, a shower may pass, and raindrop impressions (Fig. 245) be made which are subsequently filed by fine sediment and preserved. Unconformities. Figs. 246, 247, and 248, show, in each case, one set of beds out of harmony with another set. This relation is one of Fig. 245. Raindrop impressions. (Brigham.) Fig. 2460. Unconformity between the Devonian (the thick-bedded rock) below, and the Coal Measures (thin-bedded and broken) above. Iowa. (Udden.) STRUCTURAL FEATURES 273 unconformity. Most unconformities are developed by the erosion, the deformation, or both, of the older and lower set of beds, before the deposition of the younger and*upper set. The interval of time Fig. 247. Unconformity in Bighorn Basin, Wyoming. The lower (Laramie) beds dip notably to the left, and the upper horizontal (Wasatch) strata rest upon their cut-off edges. (Fisher, U. S. Geol. Surv.) between the deposition of the unconformable sets of beds may be very long — when the unconformity is great, or short — when the unconformity may be slight. Unconformities are of great signifi- cance in the interpretation of geological history. Unconformities Fig. 248. One phase of unconformity... The beds at the right were tilted to their present position before the deposition of the beds at the left. exist between stratified rock and igneous rock, and between strati- fied rock and metamorphic rock, as well as between different series of bedded rocks. Structural Features Arising from Disturbance Inclination and folding of strata. The original attitude of beds, whether formed by water or by lava-flows, commonly departs but little from horizontality. Locally, however, both kinds of deposits are made on considerable slopes. ——~ 274 MATERIALS AND THEIR ARRANGEMENT Fig. 249. Open anticlinal fold, near Hancock, Md. (U. S. Geol. Surv.) Fig. 250. Closed anticlinal fold, near Levis Station, Quebec. (U. S. Geol. Surv.) STRUCTURAL FEATURES 275 Many sedimentary rocks and many lava flows have lost their original position through crustal movements, so that beds which were once horizontal now dip; that is, they depart from horizontality. The beds of a given region may all dip in one direction, or the dip may change from point to point. They may be folded, and the folds may be open (Fig. 249) or closed (Fig. 250). The beds of sedimentary rock may even be on edge (Fig. 251), having a ; dip of 90°. These diverse positions a piiaes Seen (Van Hise, in which strata are found are the result of disturbance subsequent to their deposition. Modifications of the original attitude result from earth move- ments, and the measurement of these modifications is an important part of field study. The position of beds is recorded in terms of dip and strike. The dip is the inclination of beds referred to a horizontal plane (Fig. 252) and is usually measured by a clino- meter. In measuring the dip, the maximum angle of slope is always taken, and its direc- tion as well as its amount is recorded. Thus dip 4o°, S. 20° W., gives the full record of the position of the bed of rock under con- sideration. The strike | is the direction of the horizontal edge of dip- ping beds, or more gen- erally, the direction of a horizontal line along the outcropping edge of a anne 3 dipping bed, as illus-_ - pee S trated in Fig. 252. Since Fig. 253. Recumbent anticline. (Van Hise, the strike is always at U- 8 Geol. Surv.) right angles to dip, strike need not be recorded if the direction of the dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 70° W. Fig. 252. Diagram illustrating dip and strike. 276 MATERIALS AND THEIR ARRANGEMENT When beds incline in a single direction, they form a monocline. When they are arched up as in a fold, they form an anticline (Figs. 249 and 250). ‘The anticline may depart from its simple form, as Fig. 254. Syncline, C. and O. canal, 3 miles west of Hancock, Md. Shale and sandstone, near base of the Silurian. (Walcott, U. S. Geol. Surv.) shown in Fig. 253. The downfold corresponding to an anticline is a syncline (Fig 254.) When beds assume the position shown in Fig. 251, the folds are zsoclinal. When considerable tracts‘are bent so as to form great arches or great troughs with many minor undula- / 4 i, ‘ Ft * ; ; , A m 1 hiro. aN . - P, Ws GE GG MILES) I.E ie eee SAN | MY tA aes iY WRI WMD oe ANTE WRU SELIG SES Ney SSS) SEW WRG pay i Fig. 255. Generalized fan fold of the central massif of the Alps. (Heim.) STRUCTURAL FEATURES 277 tions on the flanks of the larger, they are called geanticlines, or anticlinoria (Figs. 219 and 255), and geosynclines or synclinoria (Fig. 256). Folding may be accompan- ied by the develop- ment of slaty cleav- age (p. 293). As found in the field, most folds are much eroded, and in many cases completely truncated (Fig. 255). The structure is then determined by a careful record of dips and strikes. On the field ie e/ ee Fig. 256. Synclinori U.S. Geol. Surv.) 5 , Mt. Greylock, Mass. (Dale, map, the record may be made as Fig. 258 | shown in Figs. 257 and 259, where the _ AN. free ends of the | -- lines with but one Fig. 257 Fig. 260 Fig. 259 ree end, Bore i Fig. 257. Map record of dip and strike, showing the direction of dip, synclinal structure. Fig. 258. Diagram showing the structure correspond- ing with Fig. 257, as seen in cross-section. Fig. 259. Map record of dip and strike showing anticlinal structure. Fig. 260. The structure of the area shown in Fig. 250, in cross-section. sents a syncline, and that in Fig. 259 an anticline. while the other lines represent the direc- tions of strike. Ap- plying this method, the structure shown in Fig. 257 repre- In cross-section, the structure represented by Fig. 257 is shown in Fig. 258; that of Fig. 250, in Fig. 260. Fig. 261 shows a doubly pitching anticline; that is, an anticline the axis of which dips down at either end. Fig. 262 shows a com- bination of synclines and anticlines, and Fig. 263 a cross-section along the line ab of Fig. 262. The out- PA Y A - Fig. 261. anticline. - F k r+tb$sett4setdT* Map record of dip and strike showing plunging (dipping down at ends) crops of rock where the dip and strike can be determined may be few and far between, but when they are sufficiently near one 278 MATERIALS AND THEIR ARRANGEMENT another, the structure of the rock, as shown in Figs. 262 and 263, may be worked out, even though the surface is flat. Much the larger portion of the earth’s surface is occupied by beds that depart but little from their original horizontal attitude, Se but in many moun- tainous regions the beds have suffered bending, folding, crumpling, and crush- ing, in various degrees. Distortion is on the whole most consider- able in the most an- cient rocks. Distortion is assigned chiefly to lateral thrust arising . from the shrinkage of . Fig. 262. Map record of dip and strike showing the earth, as explained complex structure. : ? i b in Chapter VIII. Complicated struc- I NSE’ / NN tures may be very dif- ficult of interpreta- tion. Thus overturned folds reverse the order of the strata in the under limb of the fold (Fig. 253). After such folds have been greatly eroded, so that their outer form is lost and their relations have become obscure, the beds are likely to be in- terpreted as though they lay in natu order. Thus Fig. 264 might ae a cS simple monoclinal structure, or any one of Fig. 264. Diagram rep- the complex structures tae in Figs. 265, resenting either isoclinal 266, or 267, so far as dip and strike show. or movocinaljsteaceare Joints. The surface rocks of the earth are almost universally traversed by deep cracks called joints (Figs. 2 and 268). In most regions there are at least two systems of joints, the members of each system being roughly parallel, while those of the two systems, where there are two, are approximately at right angles. In regions of great disturbance, the number of sets of joints may be three, four, or even more. The joints of each set may be many yards apart, or in exceptional cases, inches, or even a fraction of one inch. € a o + rhe ith Pols Leta ae: On > nae Fig. 263. Cross-section of Fig. 262, along the line ab. STRUCTURAL FEATURES 279 In horizontal rocks the joints approach verticality, but where the rocks have been deformed notably, the joint planes may have any position. In igneous and metamorphic rocks they may simu- 2+ oe wer eae ; ae my é o ‘ ‘ et Pes. * =e na > ‘ * ., *. Yo Pe Cal : FY : * % wee i Meee NT ie : \ ; L \ \ *, - AS H ee SAY} $ : 5 ea \ . wey : ‘ay aye et, a ; : H f a \ \ = * H Se ‘ 5 % oak 4 . Ex ae : 4 \ 4 ‘ H ‘ Sy a \ oe ates | \ con MEW RR ‘ani Viale, ‘lal as a, ‘ WO 3 * 2 ees iz. ; 9 ee j Fig. 265 Fig. 266 Fig. 267 ‘ Fig. 265. A possible interpretation of Fig. 264. (Dana.) Fig. 266. A possible interpretation of Fig! 264. (Dana.) Fig. 267. A possible interpretation of Fig. 264. (Dana.) late bedding planes (Fig. 269). Joints do not ordinarily show them- selves at the surface in regions where there is much mantle rock, but they are readily seen in the faces of cliffs, in quarries, and, in general, wherever rock is exposed. Though some of them extend to greater depths than rock has ever been penetrated, they are be- lieved to be relatively superficial phenomena. They must be limited to the zone of fracture, and most of them are probably much more narrowly limited. pes, joints end at the plane of contact of two Fig. 268. Jointed rocks, Cayuga Lake, N.Y. (Hall.) sorts of rock. Thus a joint extending down through limestone may end where shale is reached. Joints may be offset at the contact of layers or formations, and a single joint may give place to many smaller ones. All these phenomena may be explained by the vary- ing elasticity of various sorts of rock. Generally speaking, rigid rock is more readily jointed than that which is more yielding 280 MATERIALS AND THEIR ARRANGEMENT Fig. 269. Tabular joints in granite. Summit of Goatfell, Arran. (H. M Geol. Surv.) : Fig. 270. A surface of sandstone marked by numerous joints, chiefly in tw rectangular sets. Near Kinghorn, Fife. (H.M. Geol. Surv.) : FAULTS 281 Joints may remain closed, or may gape. They may be widened by solution, weathering, etc., and they may be filled by detritus from above, or by mineral matter deposited from solution (veins, p. 286). Many rich ore-veins are developed along joint-planes. (p. 45). Joints have been referred to various causes, among which tension, torsion, earthquakes, and shearing are the most important. Most of them may probably be referred to the tension or compres- sion connected with crustal movements.' In the formation of a simple fold, for example, tension-joints parallel with the fold will be developed if tension goes beyond the limit of elasticity of the rock involved. If the axis of a fold is not horizontal, that is, if it pitches, as it commonly does, a second set of tension-joints roughly perpendicular to the first may be developed. If the uplift is dome- shaped and sufficient to develop joints, they will radiate from the center. It is true that joints affect regions where the rocks have not been folded, and where they have been deformed but little, but deformation to a slight extent is well-nigh universal. Shrinkage is a cause of certain minor tension-jointing. The columnar structure of Javas and sun cracks are examples. These causes, however, are not believed to affect rock to great depths. Exceptionally, open joints are filled “y the intrusion of sedi- mentary material from beneath. Thus have arisen the remarkable sandstone dikes? of the West, especially of California. Some such dikes are several miles (nine at least) in length. The sand of these dikes was forced up from beneath a either by earthquake movements or by hydrostatic pressure. Faults. The beds on one side of a joint-plane or fissure may be raised or sunk relative 3 to those on the opposite side. ~ : Such a displacement is one type pan a ofia fault (Figs. 271 and 272). Fig. 271. A normal fault. Fault-planes vary from verticality to horizontality. The angle by 1 Van Hise. Principles of North American Pre-Cambrian Geology; 16th Ann. Rept., U. S. Geol. Surv., Pt. I, pp. 668-672. 2 Diller. Bull. Geol. Soc. Am., Vol. I, pp. 441-442. Ibid., Hay, Vol. III, pp. 50-55; and Newsom, ibid., Vol. XIV, pp. 227-268. 3 Various articles in Economic Geology, Vols. I and Il; Chamberlin, Fairchild, Jaggar, Ransome, Reid, Spurr, and Willis. i. 6 5 1 282 MATERIALS AND THEIR ARRANGEMENT which the fault-plane departs from the vertical is the hade (bac, Fig. 271). The vertical displacement (ac) is the throw, and the hori- zontal displacement (bc) the heave. The displacement is the amount of movement along the fault-plane, ab. The cliff above the edge of the downthrow side is a fault-scarp. In many cases the scarp has been destroyed by erosion; but a few fault-scarps of mountain- SO eee aon ous heights are known, as along Se a a some of the basin ranges of : Utah and Nevada. Most fault- peop mane reo? scarps which persist are much ae ES RIE: modified by erosion. 4 Faults involving vertical dis- placement along joints are of ‘two general classes, normal (or Fig. 272. A thrust-fault. The dotted gravity) and reversed (or thrust). lines at the left show the portion which ‘ has been removed by erosion. The pres- In the normal fault (Fig. 271) ent surface is shown by the line to the left the overhanging side is the of a. downthrow side, i. e., the down- throw is on the side towards which the fault-plane declines. Nor- mal faults, as a rule, indicate an extension of strata, this being necessary to permit one of the dissevered blocks to settle. In the thrust fault (Fig. 272), the overhanging beds appear to have moved up the slope of the fault-plane, as though the displacement took Fig. 273. Diagrams showing relations of faults and folds. place under lateral pressure. This is clearly shown to be the case where an overfold passes into a thrust fault. Another type of thrust-fault is shown in Fig. 273. In thrust-faults, the heave may be great. The eastern face of the Rocky Mountains near the boundary-line between the United States and Canada has been pushed over the strata of the bordering FAULTS 283 plains to a distance of at least seven miles.1. Overthrusts of com- parable displacement have been detected in Scotland? and else- where. Some faults branch, and in some cases the faulting is along a series of parallel planes near one another, instead of being along a single plane. Such a fault is distributive (Fig. 274). Faults are found to die out when traced horizontally, in some cases by passing into mono- clinal folds (Fig. 275), and in some cases without connection with folding. In depth they robably die out in various Fig. 274. A branching fault. (Powell, ee (Fig. 276). A fault of eepeaa et) thousands, or even hundreds of feet is probably the sum of numer- ous slight slippings distributed through long intervals of time. The faulting along one plane may be the cause of many earthquakes. Fig. 276. The fault above grades into a fold below. Thick- ening and thinning of layers next the fault-plane is evident. Based Fig. 275. Diagram of a fault pass- on experiments of Willis. (13th ing into a monoclinal fold. Ann. Rept., U. S. Geol. Surv.) The rock on either side of a fault-plane may be smoothed as the result of the friction of movement. Such smoothed surfaces are slickensides. The significance of gravity and thrust faults.’ Faults afford an indication of the conditions of stress and tension to which a region has been subjected, but some caution must be exercised in 1 Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 331-336, and McConneli, Canada Geol. and Nat. Hist. Surv., 1886, Pt. II. 2 Geikie, Text-book of Geology. * Van Hise, Sixteenth Ann, Rept., U.S. Geol. Surv., Pt. I, pp. 672-678. 284 MATERIALS AND THEIR ARRANGEMENT their interpretation. Most gravity faults indicate an extension of the surface sufficient to permit the fault-blocks to settle down unequally. Thrust faults, as a rule, signify a compression of the surface which required the blocks to overlap one another. In other words, normal faulting usually implies tensional stress, and reversed faulting compressional stress. Exceptional cases aside, the infer- ence from gravity faults is that the regions where they occur have undergone stretching, while the inference from thrust-faults is that the surface when they occur has undergone compression. In view of the current opinion that the crust of the earth has been subjected to great lateral thrust as a result of shrinking, it is well to make especial note of the fact that the faults which imply stretching are called normal because they are the more numerous; and that the faults which imply thrust are the less common. The testimony of normal faults in favor of tension is supported by the prevalence of gaping crevices, and veins. All these phenomena seem to testify to a stretched condition of the larger part of the surface of the continents. Faulting may bring about numerous complications in the out- crops of rock formations. These are difficult of detection in some cases, especially after erosion has destroyed the fault-scarps.1 Faults of horizontal displacement. Horizontal displacement may take place along a joint-plane, with no vertical displacement. This also is faulting. Horizontal displacement accompanies verti- cal displacement, in many cases, and the former is as much a part of the faulting as the latter. The tendency of recent study, whether based on theory or on field observation, is to emphasize the importance of the horizontal movement in faulting. In many mines, for example, where the walls of shafts and tunnels afford excellent opportunity for observation, horizontal movement is more in evidence than vertical. There are various displacements of rock bodies not mentioned above which are akin to faulting, if not to be regarded as such. Thus when strata are folded there is some slipping of layer on layer. In places there is displacement of layer on layer, even when the beds are not folded. Such a case with a well developed ‘“‘slickenside”’ contact is known in Ohio, between beds which are nearly horizontal. The recognition of such movements as faults opens a wide door. The great variety of displacements along joints or other partings in 1 See authors’ Geologic Processes, pp. 522-524. ALTERATIONS OF ROCKS 285 the rock, shows the difficulty of defining faults sharply. Many movements of displacement, which can hardly be separated from faults logically, are not usually called faults. Map work. The sections of the Structure Section Sheets of the folios of the U. S. Geol. Surv. furnish abundant illustrations of a variety of structural features, such as folding and faulting, and the relations of sedimentary, metamorphic, and igneous rocks. The sections of various Bulletins, Professional Papers, etc., of the same Survey afford other illustrations. See also Exercise XVII in Interpretation of Topographic Maps. INTERNAL CHANGES IN IGNEOUS AND SEDIMENTARY ROCKS; METAMORPHISM We have seen already that igneous rocks undergo physical and chemical changes, whereby they are disintegrated, giving rise to what has been called rock waste: Similarly, sedimentary rocks may be decomposed and converted into waste. The waste from one generation of rock is the raw material for rock of a new generation. It is ‘‘rock waste’’ in somewhat the same sense that lumber is forest waste. Properly speaking, all changes which rocks undergo after being formed are metamorphic changes. According to this view, de- cayed rock is a phase of metamorphic rock; but it has been cus- tomary in the past to limit the term ‘‘metamorphic”’ to rocks which are made more compact, more complex in constitution, or more crystalline. Both sedimentary and igneous rocks may be meta- morphosed. Induration of sediments. The first step in the alteration of sediments is their induration, through the aid of cement, pressure, etc. Sandstone and shale are not commonly called metamorphic rocks, but they are metamorphosed sand and mud, respectively. The cementing material of sediment, as already noted, is mineral matter deposited from solution in water. Thus mineral matter dissolved at and near the surface may be carried down by descending water, and deposited between the grains of sediment, binding them together. The cementation may be slight, or it may go so far that all the spaces between the grains of sediment are filled. When the . spaces between sand grains are filled with silica, the rock becomes quartzite. Between loose sand at the one extreme, and quartzite at the other, there are all gradations. Quartzite is classed as meta- morphic rock, but it is formed by a continuation of the process which converts sand into sandstone. Important changes in rock are 286 MATERIALS AND THEIR ARRANGEMENT brought about by the solution and re-deposition of mineral matter by the water in the rocks. This process may be called aqueous metamorphism, because of the important part played by water. Since water is present in almost all rocks down to considerable depths, the changes which it produces are nearly universal down to the depths to which it penetrates. Cavity filling. Cavities in rocks larger than the spaces between grains also receive deposits, if the waters entering them carry min- Fig. 277. Veins of calcite in volcanic tuff. Shore west of Kincraig Point, Elie, Fife. (H. M. Geol. Surv.) eral matter in solution. Thus joints or cracks may be filled with mineral matter, making veins (Fig. 277). The agates developed in some cavities afford another illustration of cavity filling. Here the successive layers are commonlyof quartz, differing from one another in color and texture. Geodes are cavities partly filled with crystals (Fig. 278), mostly of quartz or calcite. Replacements. In both sedimentary and igneous rocks there are replace- ments. “Giie through the dissolv- ing and depositing Fig. 278. Geode. (Bassler, U. S. Geol. Surv.) action of water the ALTERATIONS OF ROCKS 287 calcium carbonate of corals, shells, etc., may be replaced by silica. The substitution may take place so that even the minutest details of structure are preserved. Woody matter is, under proper condi- tions, replaced by silica, forming petrified wood. The material of one crystal may be replaced by different mate- rial, as the molecules of calcite by zinc carbonate. This gives a pseudomorph of zinc carbonate after calcite, the zinc carbonate taking the form of calcite, instead of the form which it would take if crystallizing under other circumstances. This sort of change may affect the crystals in any sort of rock. Concretions. Another phase of the internal reconstruction of sedimentary rocks is the assembling of matter of the same kind. For instance, silica that was deposited in the form of siliceous shells Fig. 279. Deposits of calcite (travertine, stalactites, and stalagmites) in Wyandotte Cave, Ind. (Hains.) and spicules of plants and animals, and disseminated through the sediments as they were deposited, may be aggregated later into nodules or concretions of chert or flint (Fig. 280). Similarly, con- cretions of calcium carbonate or iron carbonate grow in silts or muds. In many other cases, too, kind comes to kind. ; 288 MATERIALS AND THEIR ARRANGEMENT Fig. 280. Nodule of chert, about half natural size. (Photo. by Church.) In general, concretions are made by the deposition of mineral matter which was in solution, about a nucleus. The nucleus Fig. 281. Irregular calcareous concretions. Ryegate, Vt. (Photo. by Church ) may be a leaf, a shell, or some bit of organic or inorganic matter. The material of the concretion probably comes, in most cases, from the immediately surrounding rock. Concretions are generally ALTERATIONS OF ROCKS . 289 of matter unlike that of the rock in which they form. Thus con- cretions of calcium carbonate (Fig. 281) are common in clay, con- cretions of chert (silica) (Fig. 289) in limestone, and concretions of iron oxide in sandstone. Many concretions develop after the enclosing sediment was deposited. This is shown, in some cases, by the fact that bedding planes run through the concretions. Concretions also form in sediments during their deposition, and exceptionally, rock is made up chiefly of them. The chemical precipitates from the concen- trated waters of certain enclosed lakes may take the form of minute spherules. From a fancied re- semblance of these concretions to the roe of fish, the resulting rock was called odlite (Fig. 282). Odlite is now forming about some coral reefs, presumably from the precipitation of lime carbonate temporarily in solu- tion. Some considerable beds of limestone are odlitic. The calcium carbonate of such rock may be replaced by silica, leaving the odlitic structure in siliceous rock. Some beds of iron ore are concretionary. Thus there are widespread beds Te 52- Outi texture, About nat of “‘flax-seed” iron ore made up of concretions of iron oxide which, individually, resemble the seed which has given the ore its name. Some concretions develop cracks within themselves, and the cracks may be filled with mineral matter differing in composition or color from that of the original concretion (Fig. 283). Such concretions are called septaria. In size, concretions vary from microscopic dimensions to huge masses, 10 or more feet in diameter. The variations in shape are also great, conditions of growth having much to do with the form. A concretion which starts as a sphere may find growth easier in one plane than another, when it becomes discoid. ‘Two or more concretions may grow together, giving rise to complicated forms. None of the changes thus far mentioned (p. 285 e¢ seq) consti- 290 MATERIALS AND THEIR ARRANGEMENT tute metamorphism, in the generally accepted meaning of the term, but all are metamorphic changes, if that term be given its largest meaning. Surface vs. deep-seated changes. Near the surface, the action of water commonly tends to the decomposition of rock; but below a few hundred, or at most a few thousand feet, its general effect is Fig. 283. Section of a septarian nodule (clay ironstone). About 34 natural size. (Geikie.) to solidify the rock, for at these depths deposition exceeds solu- tion, and oxidation, carbonation, etc., go on much more slowly than near the surface, or not at all. Oxidized and hydrated sediments may be buried to great depths, and under the pressure and perhaps the high temperature of these depths, deoxidation and dehydration may take place, with resulting diminution of volume. These changes at considerable depth are one phase of metamorphism, even according to the older use of the term. Incipient crystallization: A common metamorphic change in sedimentary rock is incipient crystallization. Some limestones and dolomites are made up largely of small crystals, though the mass was originally a calcareous mud or ooze. New crystals also are METAMORPHISM 291 developed in shales and other sedimentary rocks out of materials already present, or with such additions as ground-water may make. Such changes take place even under ordinary conditions of heat and pressure, through the help of ground-water. _ Change in composition. Besides simple deposition in pores and cracks, mineral matter in solution may enter into combination with other mineral matter, giving rise to new and in some cases to more complex and more compact mineral substances. The changes effected in this way go on slowly, but in the long course of time, they may go so far that none of the original rock material remains in its original condition —all having entered into new combinations. Soapstone or steatite, serpentine, chloritic and talcose rocks, all of which occur in large bodies, were developed primarily through the chemi- cal rearrangement of the mineral matter of some older rock, with the addition of some mineral matter brought in by ground-water, and with the subtraction of some soluble matter. Their metamorphism is largely chemical. Other conditions favoring metamorphism. Besides water, heat and pressure favor the metamorphism of rocks. ‘Their action gives rise to three general cases, but these three blend indefinitely: (1) great heat without exceptional pressure, (2) exceptional pressure without great heat, and (3) great heat and great pressure acting to- gether. Exceptional heat arises especially from the intrusion of lavas, and from pressure. Exceptional pressure arises chiefly from the weight of overlying rocks, and from lateral thrust due to shrink- age of the earth. Thrust generates heat as well as_ pressure. The water in the rocks greatly facilitates the chemical and mineral- ogical changes favored by heat and pressure. Metamorphism by heat. When lava is poured out on the sur- face, it bakes the mantle-rock which it overflows. The extent of the baking depends on the mass and temperature of the lava. The nature of the effect is much the same as in the baking of brick. It consists in the dehydration of the material, its induration by welding due to partial fusion, and the development of new compounds. The time involved is short, the pressure slight, and the water action limited. If the heat were great enough, the loose material over which lava flows would be fused; but complete fusion does not usu- ally take place when lava spreads out on the surface. Intrusions of lava (p. 228) heat the surface above as well as that below. The heat of the lava can escape only through the neigh- 292 MATERIALS AND THEIR ARRANGEMENT boring rock, and the changes effected by a given mass of lava are more considerable. Furthermore, the time during which the adjacent rock is hot, and therefore the time during which thermal waters are operative, is usually longer than in the case of extruded lavas, and the chemical and crystalline changes are greater. ‘The changes are greater the greater the mass of the lava and the higher its temperature. | In limestones and sandstones the changes are simple, and in shales more complex. In pure limestones and dolomites little chemi- cal change takes place, but the molecules are rearranged into larger crystals, making marble. The coarseness of the crystals is a rough sort of measure of the length of time during which the heat acts, and of its intensity; but much depends on the freedom of the attend- ant water circulation. If impurities, as silica, alumina, iron, etc., were present in the limestone, various silicate minerals may be formed in the marble. In pure quartzose sandstones, the effect is to bring about more complete cementation, converting the sandstone into quartzite (p. 285). Here, asin marbles, impurities form adventi- tious crystals. In shales, the material to be acted upon is more complex, for, while the main mass is composed of hydrous aluminum silicate, there is usually much free quartz, and in many cases some potash, soda, iron, compounds of calcium, magnesium, etc., for many muds from which shales arise contain not only the fully decom- posed matter of the original crystalline rocks, but some fine matter worn from them by wind and water without decomposition. When this mixed matter is acted upon by high heat and moisture, it tends to return to its original crystalline state, so far as its changed com- position permits. The result is the development of complex sili- cates, similar to those of igneous rocks, such as feldspar, mica, hornblende, etc. Mica schists are common products of the meta- morphism of shales by contact with bodies of lava. Mica schists also are formed in other ways, and other schists, dependent on the composition of the shales, are formed about intrusions of igneous rock. In all such cases pressure probably attends the heat, and is a factor in the development of the schists. When the change in- duced by the heat is less considerable, the shale is baked, with incipient re-crystallization, and may take the form of argillite, a compact, massive sort of shale. Beds of hydrous iron oxide (limonite) or of iron carbonate (sider- ite) may be converted by heat into hematite or magnetite (p. 255). METAMORPHISM 293 Beds of peat, lignite, and bituminous coal are converted into anthra- cite by the driving off of the volatile hydrocarbons. If the process goes to the extreme, graphite is the result. Metamorphism by pressure. When rocks made up of clastic particles are compressed in one direction, and are relatively free to Fig. 284. Figure showing the elongation of pebbles under pressure. Carbon- iferous formation, Newport, R.I. (Walcott, U.S. Geol. Surv.) expand in others, the particles which are already elongated tend to turn so that their longer axes are at right angles to the direction of pressure, and all particles, whether elongate or not, are more or less flattened at right angles to the direction of stress. This is readily seen where the particles are large (Fig. 284). Asa result of the turn- ing (or orientation) and flattening of their particles, rocks so affected split more readily between the elongate and flattened particles than across them. In other words, the rocks cleave along planes normal to the direction of compression. ‘The structure thus induced is known as slaty structure (Fig. 285), and is illustrated by roofing- slate, which was originally a mud, later a shale, and finally assumed the slaty condition under strong compression. In some cases the original bedding may still be seen running across the cleavage planes 204 MATERIALS AND THEIR ARRANGEMENT Fig. 285. Pre-Cambrian fossiliferous slate. Deep Creek Canyon, 16 miles southeast of Townsend, Mont. (Walcott, U. S. Geol. Surv.) developed by pressure (Fig. 286). As the original mud beds were horizontal or nearly so, and as thrust is most commonly horizontal or nearly so, the induced cleavage commonly crosses the bedding planes at a high angle. If the beds are tilted or bent before the development of the slaty cleavage, the angle between the bedding planes and the slaty cleavage may be small. Limestones, sandstones, and conglomerates are not so easily compressed as mudstones, and they commonly take on only an im- perfect cleavage normal to the direction of pressure. Foliation, schistosity. Extreme pressure in a given direction is capable of breaking down and deforming the most resistant rock. This must necessarily be attended with the evolution of heat, and thermal effects are combined with pressure effects. The first effect. METAMORPHISM 295 Fig. 286. Slaty structure and its relation to bedding planes. Two miles south of Walland, Tenn. (Keith, U.S. Geol. Surv.) of the compression of such a rock as granite may be to crush it. It then becomes granular or fragmental, and is really a peculiar species of clastic rock (autoclastic). By further compression, the fragmented material may be pressed into layers or leaves, much as in the develop- ment of slaty cleavage; but as a result of the nature of the material, the cleavage is less perfect. These changes may be attended by more or less shearing of the material upon itself. The result is a foliated or schistose structure (Fig. 4), the most distinctive feature of highly metamorphic rock. A foliated structure may be developed even in the most massive rocks. ‘Thus granite may be transformed into gneiss — which is like a granite in composition, but has a foliated structure, and basalt may be converted into schist, a common term for foliated crystalline rocks. The kind of schist produced by metamorphism depends on the constitution of the rock metamorphosed. Basic rocks give rise to basic schists, and acidic rocks to acidic schists. It is obvious that 296 MATERIALS AND THEIR ARRANGEMENT ordinary shales cannot become basic schists, because in the produc- tion of the muds from which shales are made, the bases were mostly removed; but shales which are highly calcareous and magne- sian may be changed basic schists (say hornblende schists) by meta- morphism. Schists are commonly named for the abundant cleav- Fig. 287. Porphyry rendered schistose by pressure. Near Green Park, Cald- well Co., N. C.. (Keith, U. S. Geol. Surv.) able mineral constituent, as mica schist (chiefly of quartz and mica), talc schist, chlorite schist, etc. The crystallizing processes of metamorphism are fundamentally similar to the processes by which rocks crystallize from lavas; but in metamorphism, the work is done chiefly by the aid of an agueous solution, while in the solidification of lavas the crystallization is from a mutual solution of the constituents in one another, where water was but an incident. Metamorphic rocks are of course subject to deformation and faulting, the same as sedimentary and igneous rocks. They are also subject to alteration through decay, or through the reorganiza- tion of their materials into new forms, METAMORPHISM 297 VARIOUS CLASSIFICATIONS AND NOMENCLATURES From the foregoing sketch of the processes of rock-making it will be understood that the varieties of rocks are many, and that they may be defined, named, and classified on many different bases; for example: (1) If the mode of origin is chiefly in mind, rocks may be classed as igneous (lavas, tufis, etc.); metamorphic (schists, gneisses, an- thracite, etc.); and sedimentary. The last includes (a) aqueous (water-laid sediments, travertine, etc.); (6) eolian (wind-blown sand and dust); (c) glacial (deposits by glaciers); (d) organic (peat, coal, etc., and indirectly, limestone, infusorial earth, etc.). (2) On the basis of textural character, rocks are designated ve- sicular (pumice, scoria, etc.); glassy (obsidian); porphyritic (p. 248); granitic or phaneritic (pp. 248 and 262); compact, porous, earthy, arenaceous (sandy), schistose, etc. (3) If chemical composition is to be emphasized, they may be classed as siliceous, calcareous, carbonaceous, ferruginous, etc.; or, _in case of igneous rocks, as acidic, basic, or neutral. (4) If crystallinity is made the basis, igneous rocks are desig- nated phanerites (crystals distinct), aphanites (crystals very small), porphyries, glasses, etc. (5) On the basis of mineral composition, rocks are quartzose, micaceous, chloritic, pyritiferous, garnetiferous, etc. (6) Regarded as mineral aggregates, some rocks are simple and some complex. If simple, they are named from the dominant minerals, as dolomite, hornblendite, etc.; if complex, they take spe- cial names, as syenite, gabbro (pp. 260, 262), etc. (7) On the basis of structure of the mass, rocks are classed as massive, stratified, shaly, laminated, slaty, foliated, etc. (8) When physical state and genesis are considered, they are clastic, fragmental, or detrital, (conglomeratic, brecciated p. 263, arenaceous, argillaceous (clayey), etc.); or pyroclastic (tufaceous, agglomeratic, p. 263), etc. As one of these characteristics is most important in a given rock or in a given study, and another in another, no one classification is satisfactory in all cases. ee | ak el Ee ae “a re 4 ¢ ~~) , PF i ey" u ; . ‘ x hee} -_ + . . ‘ ‘ ' ' . \ Peer tL HISTORICAL GEOLOGY CHAPTER XI THE ORIGIN OF THE EARTH The bedded rocks of the earth’s shell reveal its history far back into the past with great fidelity; but the record of the earlier ages is indistinct, and if we attempt to go back to the earth’s beginning, the indistinctness merges into obscurity. The rocks below the well- bedded strata are so broken and altered, and so cut up by intrusions, that their history is read with great difficulty. Still lower lies the inaccessible interior of the earth whose nature is more a matter of inference than knowledge. Some suggestions as to the origin of the earth are found in its relations to the other bodies of the solar system, and certain features of this system give pointed hints concerning its early history. The interpretation of these outside relations of the earth and of the secrets of its hidden interior is yet far from clear, and our only recourse is to hypotheses; but it is important that we study these hypotheses, and note the ways in which they enter into interpretations of the earth’s phenomena, for not a few of the leading doctrines of geology hang on some hypothesis of the earth’s beginning, and have no greater strength than the hypothesis on which they depend. HYPOTHESES It is the nearly unanimous conviction of astronomers that the solar system was evolved in some way from a nebula of some form. Until recently, astronomers so generally accepted the view of La- place that it came to be known as ‘*The Nebular Hypothesis’’; but the advance of knowledge makes it necessary to consider other hypotheses which postulate that the solar system arose from a nebula whose constitution and mode of evolution differed from that 299 300 ORIGIN OF THE EARTH assumed by Laplace. The leading hypotheses of the earth’s origin fall into three groups: 1. The gaseous hypotheses, in which the parent nebula is assumed to have been formed of gas collected into a spheroid by gravity, and to have been evolved into the present solar system by loss of heat, and the separation of the outer parts into planets. The type of the class is the Laplacian hypothesis. 2. The meteoritic hypotheses, in which the parent nebula is assumed to have been a swarm of meteorites, the members of which moved in diverse directions. Frequent collisions gave rise to heat, light, and vaporization. The swarm of meteorites is thought to have behaved essentially as a coarse gas, and the evolution of the system to have been dynamically like the preceding. 3. The planetesimal hypothesis, in which the original constitu- ents of the nebula are assumed to have been small bodies, molecules or aggregates, moving in orbits about a common center and form- ing a disk-like system. ‘The evolution consisted in the gathering of these small bodies (planetesimals) into planets and satellites. Dy- namically, this hypothesis differs more from the other two than they do from each other. 1. The Laplacian hypothesis. During the last century the Laplacian hypothesis was generally accepted, and geological theories as to the early states of the earth, and as to many later events in its history, were built upon it. The hypothesis is so well known that a few sentences will recall its essential features. It holds that the sun, the planets, and the satellites were once parts of a glowing, rotating, spheroidal, gaseous nebula, which was expanded enough to occupy the whole space of the solar system. The nebula was assumed to have cooled by radiation of heat, and in cooling to have shrunk. The shrinkage accelerated the rate of rotation, and this increased the equatorial bulge which rotation developed. The progressive increase of cooling, rotation, and bulging finally led to the separation of an equatorial ring. As this ring cooled and contracted, it was disrupted and its substance gathered into a planet whose orbit lay in the plane the ring had occupied. A series of rings, separated in this way, gave rise to the several planets in turn, while the central mass formed the sun. ‘The orbit of any planet bounds approximately the space assigned to the nebula at the birth of that planet. At the time of origin, the several planets were thought to be hot, gaseous, and rotating. Cooling and shrinkage NEBULAR HYPOTHESIS 301 increased the rate of their rotation, and this caused equatorial bulging, till some of them, following the example of their parent body, shed rings which became satellites. In support of this theory many harmonies in the motions of the members of the solar system were cited, and in the early days of the hypothesis, existing nebulz were thought to give it support, for among them, as then known, there seemed to be nebulous aggre- gations in various stages of development, from diffuse nebulous masses to forms almost as concentrated as suns; but the best photo- graphs now taken fail to show that any follow the lines of this hypothesis. Grave difficulties arise from the dynamics of the theory, but without some knowledge of celestial mechanics, it is not possible to appreciate the full force of the arguments against it. Some of them may be stated briefly. 1. In the evolution of a gaseous nebula, it is highly improbable that rings would be formed, for the molecules of gas would separate from the parent nebula one by one. 2. Even if rings were formed, there are grave difficulties in their development into spheroids as set forth by this hypothesis. 3. In the intensely hot condition of the assumed ring which was to form the earth and moon, its gravity could hardly have held its gases together. Even now the earth does not appear to hold permanently very light gases, though it holds the heavier ones. 4. Itis probable that the material of a ring, such as the supposed earth-moon ring, would have cooled to solid particles long before it could collect into a spheroid. In this case no secondary ring to form a moon would be developed. | 5. Theinner satellite of Mars (Phobos) revolves about that planet three times while the planet rotates once. According to theory, these motions must have corresponded at the time of separation, and since that time the planet should have increased its rotation by cooling. Its period of rotation should therefore be shorter than the period of the satellite’s revolution. Explanations have been suggested for this difficulty, but they do not meet the case. The small bodies that make up the inner edge of the inner ring of Saturn also revolve in about half the time that planet rotates. 6. If the solar system were converted into a gaseous spheroid, with its matter distributed according to the laws of gases, and expanded to Neptune’s orbit, and if this nebula had the total momentum (technically, the moment of momentum) of the solar 302 ORIGIN OF THE EARTH system, it would not have acquired a rate of rotation rapid enough to detach matter from its equator until it had contracted well within the orbit of the innermost planet. 7. If the nebula were a spheroid of gas whose density followed the law of gases, and if it had a rotation rapid enough to shed rings from its equator as the theory supposes, its moment of momentum would need to have been very much greater than the system now possesses. This is at variance with the established law that the moment of momentum of such a system must remain constant. To separate Neptune, the moment of momentum would need to have been 200 times what it is; to separate Jupiter, 140 times; to separate the earth, 1,800 times. These are enormous discrepancies and they are not consistent with one another. 8. Comparing the masses of the planets with the moments of momenta they carried off from the parent nebula, strange dis- crepancies appear. The matter in the ring supposed to have formed Jupiter and his moons had a mass less than '/1ooo that of the nebula at the time of separation; but Jupiter and his moons have about g5 per cent of the total moment of momentum which the nebula then had. ‘The Laplacian hypothesis asks us 'to believe that an equatorial ring, having a mass less than 1/1000 that of the parent body, carried off 95 per cent of the total moment of momentum when it separated. The supposed separation of other rings involves similar incredible ratios. g. Under the Laplacian hypothesis, the satellites should all revolve about their planets in the direction in which the planets rotate on their axes; but the sixth satellite of Jupiter and the ninth satellite of Saturn revolve in the opposite direction. to. Our knowledge of nebule has been extended greatly in recent years, but nebule with such rings as the Laplacian hypothe- sis calls for have not been found. The force of these objections appears to be such as to make the hypothesis untenable. 2. The meteoritic hypotheses. It was long ago noted that shooting stars enter the upper atmosphere in great numbers, and that occasional fragments of stony and metallic matter fall to the earth. Out of this grew the notion that the earth may have been built up in this way, save that the process was more rapid in the early days of the earth’s history. This notion, however simple and natural, may be dismissed without serious consideration, for METEORITIC HYPOTHESES 303 the different directions of motion and the various velocities of meteorites are such as to forbid the belief that the solar system, with its symmetrical discoidal form and its harmonious motions, could have been formed in this way. A more logical meteoritic hypothesis is based on the conception that meteorites may be aggregated into swarms and constitute nebule. This hypothesis _ is, therefore, nebulo-meteoritic. Sir George Darwin came to the conclusion that such a swarm of meteorites would act very much like a gas, and that the laws of gases could be applied in determining its mechanics. If the meteorites of such a nebula move in various direc- tions, this hypothesis, as ap- plied to the origin of the earth, is practically identical with the gaseous hypothesis; and as applied to the solar system, it is subject to the criticisms al- ready urged against that hy- © pothesis. The term meteoritic hypothesis is used commonly in the above sense. It was ap- plied by its authors (Lockyer and Darwin) chiefly to the earlier and more scattered con- ditions of the nebule, and has not been applied specifically to the formation of a planet. If, Fig. 288. A spiral nebula in Canes Venatici, Messier 51. The exposure was long and has given relative exaggeration to the fainter parts. The nucleus is apparently dense and relatively massive; the coiling is pronounced and rather sym- metrical in the inner parts, but departs from symmetry in the outer parts. A notable feature is the comet-like streamers of some of the knots and denser portions. If these are true streamers, curved by motion, they imply an active rotation, and strengthen the inference drawn from the coiled condition. (Photo. by Ritchey, Yerkes Observatory.) on the other hand, the meteorites were so assembled as to have con- centric orbits and form a disk-like system, the system, to all intents and purposes, falls into class 3. 3. The planetesimal hypothesis. When the shortcomings of the Laplacian hypothesis were seen to be so serious that there was no apparent way of escape from them, an alternative better in accord with the facts was sought. It has been shown by photography that there are a multitude of 304. ORIGIN OF THE EARTH nebule,— at least ten times as many as were known a few years ago,— and that in this multitude there is one dominant form, the spiral nebula (Fig. 288). The spiral nebula has a central nucleus, from which two arms or sets of arms project on opposite sides, and curve spirally outward. The arms of some nebule branch, and are much interrupted and knotted, and between them there is much scattered hazy matter. The prevalence of this form of nebula implies that it is due to some process which has been common. The numerous nebulous knots on the arms, and in some cases more or less outside them, are significant features. Clearly the matter of the nebula is very unequally distributed, and does not conform to the laws of gaseous distribution. Recent advances in spectroscopy throw much light on the con- stitution of nebula. As inferred from their forms, the spiral nebula seem to be composed of solid or liquid particles, though gases may be present particularly in their nuclei and knots. These tiny bodies are believed to revolve about the center of gravity of the nebula, like little planets (planetesi- mals), but this has not yet been proved. ‘The planetesimal hy- pothesis is based on a spiral nebula of this supposed organi- zation.! The planetesimal hypothe- sis starts with a spiral nebula Fig. 289. A typical spiral nebula in consisting of the following ele- Piscium, Messier 74, with very symmetri- : cal arms, pronounced nucleus and knots, ments: (1) The main knots to and a relatively limited amount of nebu- serve as nuclei for the planets, lous haze. (Photo. from Lick Observa- (2) small scattered knots as the se nuclei of asteroids, (3) other small knots near to the large ones and controlled by them, as the nuclei of satellites, and (4) scattered matter or nebulous haze to be gathered into these nuclei to give them their mature sizes, and (5) the great central mass of the nebula, forming the nucleus of 1 The manner in which it may have arisen is discussed in the authors’ larger work on Geology, Vol. II. PLANETESIMAL HYPOTHESIS 3053 the sun. The gathering of the scattered planetesimals into the knots to form the planets, planetoids, and satellites is assigned to the coming together of these bodies as they pursued their slightly different orbits, not as the result of falling directly together under the control of gravity. It is assumed that the planetesimals had rather highly elliptical orbits arranged in disk-like form. Such orbits would be favorable for the meeting and union of the bodies following them. It can be shown mathematically that under such conditions the addition of planet- esimals to the nuclei would give them more and more circular orbits as the nuclei grew, and it is significant that most of the planetoids (asteroids), which presumably have grown little, have the most eccentric orbits, that Mercury and Mars, the smallest of the planets, have more eccentric orbits than the others, while the orbits of the larger planets approach circularity more closely. ‘The photographs of spiral nebulz show large knots with small ones near them, which appear quite capable of evolution into planets and satellites. They also show small scattered knots susceptible of forming planetoids (asteroids). The earth-moon system is assumed to have been derived from companion nuclei of very unequal sizes. The knots might have had a rotary motion at the outset, arising from inequalities of projection at the time of their formation; but in part, the rotations of the planets are assigned to the impacts of the planetesimals as they joined the nuclei to form the planets. There would be no fixed relation between the time of rotation of a planet and. the time of revolution of its satellites; the period of the latter might be longer or shorter than that of theformer. Evenif the revolu- tion-period of a satellite-nucleus was originally the same as the rota- tion-period of the planetary-nucleus, the growth of the planet might draw the satellite nearer to itself and shorten the time of its revo- jution. Thus the difficulty of Phobos and of the innermost part of the ring of Saturn is obviated.—The mode of accretion assigned - might give rise to forward rotation or to retrograde rotation of the planets and satellites; the forward rotation should be the rule and retrograde rotation the exception, as is the case. In a spiral nebula formed in the way assigned, the outer parts of the arms should be composed of lighter materials than the inner parts, and since the planets were formed from these arms, the inner ones should have higher specific gravities than the outer ones, as is the fact. Other peculiarities of the solar system seem to find a fitting explanation 306 ORIGIN OF THE EARTH Fig. 290. Theoretical restoration of the parent nebula of the solar system. The nuclei of the several planets may be identified by their distances from the center. The dimensions of the inner parts are made disproportionately large. in the planetesimal hypothesis, but most of these must be passed without mention here. The assumed meetings and unions of planetesimals and nuclei at the crossings of their orbits imply a relatively slow evolution of the nebula into the solar system. The planetesimal hypothesis therefore implies a slow growth of the earth. With such a mode of growth, the stages of the earth’s early history depart widely from those postulated by the Laplacian and the meteoritic hypotheses. ‘CHAPTER XII STAGES OF THE EARTH’S HISTORY PRIOR TO THE KNOWN ERAS The conception of the history of the earth prior to the earliest stage which can be read from the strata must depend upon the view which is entertained as to its origin. The course of its early history according to each hypothesis of its origin, will be sketched sepa- rately. Though these sketches are necessarily hypothetical, their study is important, for the great features of the earth and of the earth-shaping processes were inherited from these early stages. I. STAGES UNDER THE LAPLACIAN HYPOTHESIS The hypothetical stages of the earth’s early history, according to the Laplacian view have been stated as follows,! and they must have been essentially the same for any view of primitive conditions that involves a molten globe. I. ‘The Astral zon, or that of the fluid globe having a heavy vaporous envelope containing the future water of the globe or its dissociated elements, and other heavy vapors or gases. II. The Azoic eon. Without life. 1. The Lithic Era: Commencing with the earth a solid globe, or at least solid at the surface; the temperature at the beginning above 2,500° F.; the atmosphere still containing all the water of the globe (estimated at 200 atmos- pheres), all the carbonic acid now in limestone and that corresponding to the carbon now in carbonaceous and organic substances (probably 50 atmos- pheres), all the oxygen since shut up in the rocks by oxidation, as well as that of the atmosphere and of organic tissues. ‘The time when lateral pressure for crustal disturbance and orographic work was begun; when “‘statical meta- morphism,” or that dependent on heat of a statical source — the earth’s mass and the vapors about it,— began. 2. The Oceanic Era: Commencing with the waters condensed into an ocean over the earth, or in an oceanic depression, with finally some emerging lands, the temperature perhaps about 500° F., if the atmospheric pressure was still 50 atmospheres. ‘The first of tides and the beginning of the retardation of the earth’s rotation. Oceanic waves and currents and embryo rivers begin work about the emerged and emerging lands; the large excess of carbonic acid and oxygen in the air and water a source of rock-destruction; before the 1 Dana, Manual of Geology. 307 308 EARLY STAGES OF EARTH’S HISTORY close of the era, the formation of limestones and iron-carbonate by chemical methods, removing carbonic acid from the air and so commencing its purifi- cation; the accumulation of sediments without immediate crystallization or metamorphism, and thereby the beginning of the earth’s supercrust. III. The Archeozoic won. Life in its lowest forms in existence. 1. The Era of the First Plants: Algz, and later of aquatic Fungi (Bacteria), commencing with the mean temperature of the ocean at possibiy 150° F., since plants mow live in waters up to and even above 180° F. Limestones formed from vegetable secretions, and silica deposits from silica secretions; iron-carbonate, and perhaps iron-oxides formed through the aid of the carbonic acid of the atmosphere and water; large sedimentary accumulation, where conditions favored, thickening the supercrust. 2. The Era of the First Animal Life: Mean temperature at the beginning probably about 115° F., and at the end go° F., or lower; limestones and silica deposits formed from animal secretions; deposits of iron-carbonate and iron- oxides continued; large sedimentary accumulations.” Difficulties Quite apart from the objections to the Laplacian hypothesis, stated in the last chapter, two serious questions exist relative to the stages sketched above. The one grows out of the failure to find any great formation beneath all others having the distinctive char- acteristics of an original crust; and the other from doubt as to the possibility of the prodigious atmosphere postulated. Relative to an original crust. The theory of a molten earth carries the presumption that the liquid substance of the earth was arranged so that the heaviest matter was at the center and the lightest on the outside. As the granitoids are the lightest of the large classes of igneous rocks, the granite-like magmas should have formed the outer zone of the molten earth. The solid crust should | have been light (for rock) and homogeneous, and it should have formed a stratum over the whole earth. Except at the very surface, it should have been completely crystallized, for the cooling must have been very slow, a condition favorable for the growth of crystals. No very large amount of fragmental volcanic material can be as- sumed to have covered the original crust if the atmosphere contained all the water of the future hydrosphere, for that would allow no steam in the molten globe to produce abundant volcanic fragments. Pyroclastic material of later times can hardly be supposed to have concealed the original crust permanently, for many thousands of feet of rock have been eroded from the surface of the oldest known areas. It is equally improbable that the original crust has been concealed everywhere beneath sediments derived from itself. UNDER LAPLACIAN HYPOTHESIS 309 Until recently, the great granitoid areas of the Archean system (the oldest known rocks) were thought to possess these obvious characteristics of an original crust; but it has been found that most of them were zntruded into rocks which had previously been formed on an older surface by (1) lava outflows, (2) volcanic explosions, and (3) sedimentation. This reduces to an unknown, and apparently to a vanishing quantity the rocks that can be referred plausibly to a supposed original crust. If further investigation shall finally exclude all accessible rocks from an original crust, the molten theory will have lost its observational support. Relative to the primitive atmosphere. Under the Laplacian hypothesis, the primitive atmosphere has been held to have been vast, hot, and heavy, and to have contained (1) all the water of the globe, (2) all the carbon dioxide now in carbonated rocks, (3) that portion of the oxygen which has been added to the rocks by oxida- tion, as well as (4) that portion of all these constituents which is now found in the atmosphere and in organic tissues. ‘The assump- tion back of this seems to be that heat always promotes the expulsion of gases from rock; if so, the exclusion of the gases from the rock should have been most complete in the white-hot primitive globe. The conception that the rocks after cooling re-absorb the atmospheric gases is expressed in the view, once prevalent, that the former atmosphere and hydrosphere of the moon have been absorbed into it, and in the familiar prophecies of a similar doom for the atmo- sphere and hydrosphere of the earth. Adverse evidence. So great an atmosphere with so much carbon dioxide and water-vapor should have given the earth a warm and equable climate. Such climates indeed seem to have prevailed at certain times during the earlier parts of the earth’s history, as during the later; but the studies of the past two decades have shown that there was extensive glaciation on the very borders of the tropics, as early as the close of the Paleozoic, and that there was glaciation in northwestern Europe, in China in Lat. 31°, in Australia, and perhaps in South Africa, at the very beginning of the Paleozoic. It is even claimed that there was glaciation in the early part of the Proterozoic long before the Paleozoic, and this claim seems likely to be made good. ‘There seem to have been, even in very early times, much the same alternations of uniform with diversified climates that have marked later eras. The air-breathing animals of early ages, and the devices that protected the leaves of plants against too intense sun- 310 EARLY STAGES OF EARTH’S HISTORY light and too rapid evaporation, seem irreconcilable with a vast cloudy atmosphere overcharged with carbon dioxide and water- vapor. The hypothesis of an enormous original atmosphere, suffer- ing gradual depletion, finds, therefore, scant support in a critical study of either the biological or the physical history of the earth. Modifications of the Laplacian hypothesis (known commonly as the Nebular hypothesis) have been suggested,’ with a view to obviating the objections to the current form of the hypothesis as applied to the earth. But the suggested changes do not seem very satisfactory, and there is reason for thinking that all hypotheses of the earth’s origin involving a molten condition of the globe, will soon be abandoned by geologists. II. STAGES OF GROWTH UNDER THE PLANETESIMAL HYPOTHESIS It is possible to suppose that the earth grew up by accessions in some other mode than that of planetesimal evolution, but the latter furnishes the basis for the following sketch: 1. Nuclear stage. A knot of the nebula was the nucleus of earth growth. The knot may have been gaseous, or planetesimal, or both. It caught planetesimals from the nebular haze as it crossed their paths, and thus grew in mass while it was being con- densed into the beginning of the earth-body. ‘This stage lasted until the knot was condensed into a solid mass. This mass then served as the nucleus for further growth by captured plane- tesimals. 2. Initialatmospheric stage. There may possibly have been an early stage when the mass of the earth was too small to hold perma- nently the lighter free molecules such as form our present atmos- phere. In this case the nucleus must have been made up mainly of heavy molecules such as form the stony and metallic parts of the earth; but such a stage is not probable. If the mass of the nucleus were one-tenth of that of the present earth, it would hold some atmosphere, but it would be thin and composed mainly of the heavier gases. This early thin atmosphere grew as the earth grew, by capturing molecules from the nebulous mass. The stony and metallic planetesimals also contained atmospheric material in combination or occluded,? and some of this, set free when the planetesimals were heated by plunging into the air or when they 1 Vol. II of the author’s three-volume Geology. 2 The Gases in Rocks, R. T. Chamberlin, Carnegie Institution, 1908. UNDER PLANETESIMAL HYPOTHESIS SDL struck the earth, added to the atmosphere. Thus the atmosphere grew as the earth-body itself grew. Volcanoes, when they came into action, also added to the atmosphere, for they discharge much gas. ‘This picture of the early atmosphere is very different from the vast hot vaporous atmosphere of the supposed molten earth. 3. Initial volcanic stage. As the earth grew and its gravity increased, its interior became more and more compressed and therefore more and more heated. Radio-active matter was no doubt gathered in with the other matter, and this developed heat. When the heat from these two sources became sufficient to liquefy the most fusible portions of the earth matter in particular spots, the fluid parts began to work their way toward the surface by fluxing. Other fusible matter was picked up on the way, and the radio-active matter in particular joined the rising threads of lava. When this rising lava reached the surface, volcanic action was inaugurated. According to this view, volcanoes do not originate from a ‘‘ molten interior,” or from ‘‘reservoirs” of molten matter left over from a general molten state, but the lava is generated from time to time by the continued action of radio-active substances, conjoined with the effects of compression and molecular rearrangement within the earth. ‘The heat of the interior of the earth is thus carried outward about as fast as it liquefies the more fusible parts within its reach. Thus the interior of the earth only reaches the temperature neces- sary to melt the more fusible parts, leaving the earth as a whole solid all the time. 4. Initial hydrospheric stage. Water in the form of vapor is light and active, and probably was not the first gas to be held by the growing earth; but when the earth became large enough, water- vapor was held in the atmosphere, and when at length saturation was reached, it condensed into water and initiated the hydrosphere. The source of water, according to the hypothesis, was the same as that of atmospheric gases. It may be added that the hypothesis gives a simple explanation of ocean basins and continental protuberances. Because of unequal growth, the surface of the earth may never have been perfectly spheroidal, so that when water began to accumulate on its surface, it gathered in depressions. The planetesimal material which after- wards fell into the water was protected from weathering, while that which fell on the higher land was exposed to weathering, with its attendant lessening of specific gravity. ‘Thus the depressed areas 312 EARLY STAGES OF EARTH’S HISTORY tended toward higher specific gravities, and hence toward still further depression when deforming stresses were brought to bear on them, while the elevated areas tended to grow relatively lighter, and to suffer relative elevation, under the stress of deformative movements. ‘Thus the differentiation of the oceanic basins from the continental masses began as soon as the hydrosphere began, that is, long before the earth reached its present size, and has continued to the present time. } 5. Initial life stage. Suitable conditions for life seem to have existed after an atmosphere and hydrosphere had developed to the proper extent, but it seems possible that life began long before the earth was full-grown. Under the planetesimal hypothesis, therefore, the time during which life may have existed on the earth is very much longer than the time assumed under the older hypotheses. 6. Climax of volcanic action. While volcanic action may have begun early, it probably had to await (1) sufficient growth to give the requisite internal heat by compression, and (2) sufficient time for the heat so developed to creep out to zones of less pressure, where it would suffice to liquefy the more fusible (soluble) parts of the rock. Vulcanism was probably hastened by radio-activity. Once begun, it is believed to have increased in importance, reaching its climax some time after the more rapid growth of the earth had ceased. For obvious reasons, the climax of vulcanism was attended by deformations of exceptional intensity. The transfer of so much material from below to the surface required readjustment within, and the intrusion of the enormous granitic batholiths, such as are found in the early formations, was in itself a cause of deformation. Diastrophism probably had its climax with the climax of vulcanism, and both came, by hypothesis, about the time of the opening chapter of the well-recorded history of the earth. The formations of the period when volcanic action was at its height, including some con- temporaneous sedimentary deposits, are regarded as constituting the oldest accessible rocks of the earth (the Archean), though prob- ably only the later part of the great volcanic series is represented by the known Archean. It is for each student to judge whether the assigned antecedents lead felicitously or otherwise to the condi- tions which the oldest known rocks reveal. The value of a hypothe- sis, when its truth cannot be demonstrated, lies mainly in its work- ing qualities. UNDER PLANETESIMAL HYPOTHESIS 313 7. Gradational stage. To complete the survey of stages, it is necessary to note that after the growth of the earth had nearly ceased, and volcanic action had passed its climax, the surface was no longer subject to universal burial, but was exposed, age after age, to the action of air and water. The material removed by these agents from the higher parts was deposited in the basins. Through- out all the remaining part of this stage, the dominant geologic processes were gradational. Vulcanism and diastrophism continued to be important, but not dominant. This stage embraces the Pro- terozoic and later eras. These stages of the earth’s history may be grouped as follows: III. Eon of Dominant Gradational | Cenozoic Era Processes (the well known Mesozoic Era eras) Paleozoic Era Proterozoic Era II. Eon of Dominant Extrusive Processes (transitional from the hypothetical to earliest known era) Archeozoic Era b) The known portion i a) The buried portion Initial life stage ; Initial hydrospheric stage I. Eon of Dominant Formational | Initial volcanic stage Processes (hypothetical) Initial atmospheric stage Gee stage (Early nuclear growth) Nebular stage CHAPTER XIII THE ARCHEOZOIC ERA Though the preceding sketches of the early stages of the earth’s history are but hypothetical, they afford a helpful introduction to the study of that part of the earth’s history recorded in the rocks. Figs. 291-293 represent diagrammatic radial sections which illus- trate the different conceptions of the constitution of the earth. The following summary should make the figures clear: 1. According to the Laplacian hypothesis, there should be pre- sedimentary igneous or meta-igneous rock everywhere below the prevailing sedimentary rocks of the surface. The plane of demark- ation between these two sorts of rock should, as a rule, be distinct. A modification of the Laplacian hypothesis, so far as applied to earth, pos- tulates that much gas and vapor were occluded in the molten earth, instead of being all in the atmosphere. On this assumption, it is conceived that there might have been a period of great vulcanism after the formation of a crust, and that the original crust was covered deeply with extruded rock (Fig. 292). If this were the case, the original crust might not be accessible. On the meteoritic hypothesis of the earth’s. origin, the conditions would have been much as on the planetismal hypothesis so far as concerns the oldest rocks accessible. 2. According to the planetesimal theory, (1) the core of the earth (Fig. 293) is made up of planetesimal matter. After aggrega- tion, this matter was probably re-crystallized under the influence of the heat and pressure which the aggregation involved, the result- ing rock being essentially igneous in its nature. Outside the central core there should be (2) a thick zone made up largely of planet- esimal matter, but partly of igneous rock erupted from below, and partly of sedimentary rocks formed at the surface at all stages of the earth’s growth after the hydrosphere came into existence. The planetesimal matter is assumed to predominate in the lower and major part of this zone. Igneous rock is assumed to have a some- what irregular distribution through it, while sedimentary rock increases in importance above, but remains throughout a subordinate constituent. This zone records the growth of the earth from the beginning of volcanic and atmospheric processes, until it reached 314 COMPOSITION OF EARTH 315 LR AR Re Primitive igneous Primitive igneous rock. k rock. Planetesimal matter predominant; igneous rock abundant; sedi- mentary rock a minor constituent. increasing toward the surface. Planetesimal] matter with more or less igneous rock. Fig 293 Fig. 291. A diagrammatic sector of the earth illustrating its structure accord- ing to the Laplacian hypothesis. The great body of the earth is made up of the original igneous rock. Sedimentary rocks, together with some extrusive rocks, make but a thin coating, represented in the diagram by black, outside the great igneous interior. The original igneous rock is represented as appearing at the sur- face insome places (AR). This, according to one view, might represent the Archean rock. Fig. 292. Diagram illustrating the composition of the earth on the modified form of the Laplacian hypothesis. The great body of the earth is the original igneous rock. Outside this original igneous mass, there is a zone (zone 2) of ex- trusive material, with perhaps some sedimentary rock interbedded. The material of this zone is represented as coming to the surface at some points (A). Outside this zone there is a third, made up primarily of sedimentary, but subordinately, of extrusive rocks. The material of the second zone might constitute the Archean rock. Fig. 293. Diagram representing the structure of the earth according to the planetesimal hypothesis. The material of zones 1 and 2 is indicated in the diagram. Zone 3 of this figure corresponds to zone 2 of Fig. 292, and zone 4 of this figure corre- sponds to the outermost zone of Figs. 291 and 292. 316 ARCHEOZOIC ERA nearly its present size. The central core and this thick.zone about it represent the Formative Eon (p. 313). (3) The next zone, rela- tively thin, is assumed to be made up largely of extrusive igineous rocks, with subordinate amounts of sediment, and matter gathered from space. This zone represents the Extrusive Eon (p. 313). (4) On the outside lies the superficial zone in which sedimentary rocks predominate, though associated with not a little rock of igne- ous origin. This fails to cover the globe completely. The oldest rocks. The deepest excavations yet made in the earth are little more than a mile deep. Because of deformation and erosion, rocks once at much greater depths are now exposed; but the maximum thickness of rocks open to observation is no more than afew miles. Definite knowledge of rock formations and structures is therefore limited to some such thickness. (1) According to the gaseo-molten hypothesis, we might hope to reach the originai crust; for it is not to be supposed that this original crust is everywhere covered so deeply by material derived from it as to be inaccessible. (2) According to the modified form of this hypothesis (Fig. 292), the oldest accessible rock should be in the zone of mingled extrusive and sedimentary rocks between the original crust and the domi- nantly sedimentary formations above. (3) On the planetesimal theory, the oldest rocks which we might hope to reach would be those referred to the Extrusive Eon (p. 313, zone 3, Fig. 293), during which more or less sedimentary rock was mingled with the volcanic. On this hypothesis, as on the preceding, the line of demarkation between dominantly sedimentary rocks above, and dominantly non-sedimentary rocks below, would not be sharp. The rock-formations now most widely exposed at the surface are | sedimentary, and were formed during the Gradational Eon (p. 313). In many places, however, diverse formations which are predomi- nantly extrusive (igneous or meta-igneous) are found, either beneath the prevailing sedimentary rocks, or projecting up through them in such relations as to show their greater age (Fig. 302). In many cases these lower and older rocks were thoroughly metamorphosed, and in essentially their present condition, before the deposition of the overlying beds. These dominantly igneous and meta-igneous formations, older than the oldest known dominantly sedimentary rocks, are the oldest formations known, and the era during which they were formed is the first era of hich there is definite record i in the accessible formations of the earth. THE ARCHEAN ROCKS 317 This lowest and oldest group of rocks is very complex, embracing lava flows, volcanic tuffs, igneous intrusions and sedimentary rocks, all more or less metamorphosed and deformed. Distinct fossils have not been found in them, but the presence locally of (1) carbo- naceous slates similar to younger slates containing carbon of organic origin, and (2) occasional formations of limestone and chert, are thought to imply the existence of life, and to warrant placing the era when these rocks were formed in the zoic group of eras (p. 313). The time during which, or during the later part of which, this oldest system of accessible rocks was made, is the Archeozoic era. Under the planetesimal hypothesis, the oldest known rocks may be referred confidently to the Archeozoic era, for, according to this hypothesis, rocks of organic origin and rocks containing organic products were not only mingled with all series that are accessible, but with great thicknesses of rock below, since life is supposed to have originated long before the earth acquired its present size. The oldest formations known also may be Archeozoic under the modified form of the nebular hypothesis (Fig. 292); but under the original form of the hypothesis, the original crust cannot be Archeo- zoic, since it antedated life. The term Archean (Archean system, Archean complex) is applied to the formations here referred to the Archeozoic era. ‘This term is applied to the oldest group of accessi- ble rocks, whatever their origin, and whether contemporaneous with life or antedating it. Delimitations of the Archean. The bottom of the Archean sys- tem is assumed to be inaccessible. Its upper limit has been fixed differently by different investigators. As first defined, the Archean (very old) included all rocks below the Cambrian (p. 323); but later it became clear that the rocks below the Cambrian should be differentiated into two great groups, the upper of which consists of several great systems of dominantly sedimentary or meta-sedi- mentary rocks, unconformable with one another, while the lower is dominantly igneous and meta-igneous. The term Archean is now generally restricted to the latter. The upper limit of the Archean is therefore the base of the oldest dominantly sedimentary system. GENERAL CHARACTERISTICS OF THE ARCHEAN! As now defined, the Archean includes two great classes of forma- tions, (1) a great schist series, and (2) a great granitoid series. ‘Van Hise and Leith, Mono, LII. U.S. G, S, Chapter XX, and 16th Ann. Rept. U.S, G.S., Pt. I. pp. 744-759. 318 ARCHEOZOIC ERA (1) The schist series is made up chiefly of the metamorphosed products of lava flows and volcanic tuffs. In composition they vary greatly, but the dominant types are hornblende schists, other green- stone schists, and mica schists. Associated with the metamorphosed surface lavas and pyroclastic formations, there are some massive igneous rocks, and occasional beds of metamorphosed conglomerate, sandstone, shale and limestone, and beds of iron ore, all of which imply the contemporaneous activity of water. (2) The granitoid series. One of the conspicuous features of the Archean system, in its present eroded condition, is the great masses of granite and gneiss that protrude through the schists. Formerly, these granites and gneisses were regarded as the oldest rocks, and were styled ‘‘primitive” or ‘‘fundamental”’; but it is now known that many of them, at least, are intrusions into the schists, and therefore younger than the latter. The gneisses are regarded as metamorphosed granites. In the formation both of the surface flows and the intrusions, the ascending lavas must have occupied numerous fissures or con- duits connected with the interior; hence there are numerous dikes and other intrusions, traversing the older parts of the Archean. It is to be borne in mind also that all younger intrusions and extru- sions of lava must have passed through the Archean, leaving in- trusions of diverse sorts (p. 228). These later intrusions are not strictly a part of the Archean, but they are not always separable, and their presence adds to the complexity of the Archean as a whole. Diastrophism and metamorphism. The most satisfactory ex- planation of the prevalent foliated structure of the Archean (Fig. 294) is that which refers it to the movements of the outer part of the earth in Archeozoic and later time. Intrusions of igneous rock probably aided metamorphism (1) by furnishing heat, and (2) by de- veloping pressure. ‘The pressure was developed in two ways, (a) by the intrusion itself, which developed pressure when it was intruded, and (0) the shifting of so much lava from below upward must have caused the outer parts of the earth to settle down to take the place of the material transferred upward. That the rocks should have been much metamorphosed under these conditions is natural. By crushing and shearing, massive igneous rocks were given a foliated or schistose structure, and it is in the rocks of this era especially that metamorphism of this type prevails. It is now believed that the larger part of existing gneisses, THE ARCHEAN ROCKS 319 Fig. 294. Metamorphic rock, showing foliation distinctly; bank of the Ottawa River. (Ells.) as well as a considerable part of existing schists, got their foliated structure in this way; but it is to be understood that some of the schists and perhaps some of the gneisses arose from sedimentary formations in other ways. It is not to be understood that the metamorphism of the Archean rocks was completed during the Archean era. ‘The metamorphosing processes of subsequent times have affected them. It would be difficult to obtain an exaggerated idea of the com- plexity of the rocks which has caused this system to be called a ‘“complex.”” It consists in some places of rocks which are mainly massive (igneous intrusions); in other places, of rocks which are mainly gneissic (chiefly meta-igneous); and in still others, of rocks (largely meta-igneous and subordinately meta-sedimentary) in which a schistose structure predominates. Furthermore, the rocks of each of these structural types have a wide range in composition, from acid on the one hand to basic on the other. Rocks of all these classes are intimately associated locally, and any one may pre- dominate over the others. In places the rocks of the several struc- oor ARCHEOZOIC ERA tural types graduate into one another so completely as to leave no line of separation, while in others their definition is sharp. Thus massive rock appears in distinct dikes in gneisses and schists in some places, while in others schists are in dike-like sheets in rocks which are more massive. Furthermore, the relations of these sev- eral sorts of rock have been complicated greatly by the distortion to which they have been subject. The structure and relations of the several sorts of rock in the system indicate that it was (1) by successive intrusions, large and small, of rocks of different chemical composition into (2) still older rocks which were originally (a) chiefly extrusive-igneous and of varying chemical composition, but (b) subordinately sedimentary; and (3) by successive dynamic movements resulting in various degrees of metamorphism and de- formation of the various parts, that the intricate structure and composition of the Archean complex was attained. Though the variations in the rocks of the Archean system are great, there is yet a certain homogeneity in the heterogeneity of the whole. No large part of the system is very different from any other large part, and no definite and orderly relationship between the different parts has been made out over any considerable area. There appears to be no traceable succession of beds, and no definite stratigraphic sequence, such as can be made out in great series of younger meta-sedimentary rocks. Earlier views concerning the Archean. In explanation of the Archean system, many hypotheses have been suggested at one time and another, most of them starting with the Laplacian hypothesis as a beginning. One of them is that the Archean rocks are wholly of metamorphosed sediments, a second, that they are igneous rocks produced by the fusion of sediments, and a third, that they are igneous rocks intruded beneath the oldest sedimentary rocks after the deposition of the latter. These hypotheses have historic interest, but are not now generally held by geologists.! DISTRIBUTION In speaking of the distribution of a formation, its distribution at the surface generally is meant, and in speaking of its surface distribution, the mantle rock (glacial drift, etc.) which overlies and conceals it is usually ignored unless it is so thick as to make the underlying formation indeterminable. When the surface dis- tribution of the formation is given, therefore, it is not to be under- stood that the formation is literally at the surface everywhere within the area specified, but rather that it is exposed here and there within 1 See the authors’ Earth History, Vol. II, THE ARCHEAN ROCKS 225 that area, and that between the points of exposure it is the upper- most formation beneath the mantle rock. In this sense, the Archean rocks are estimated to appear at the surface over about one-fifth of the area of the land; but since great areas in some continents have been reconnoitered only, geologically speaking, this figure is only a rough estimate. Concerning the real, as distinct from the surface distribution, the Archean is the one accessible rock system which, theoretically, envelops the globe completely. No later system does this, for wherever the Archean comes to the surface, later formations are necessarily absent. In North America,‘ by far the largest area of Archean rock is in Canada (Fig. 295). Formations of the Proterozoic and later eraS occupy numerous small tracts within the area shown on the map, though the Archean underlies them at no great depth. Lying rudely parallel to the great Canadian area on the southeast is an interrupted series of probably-Archean tracts, extending from Newfoundland to Alabama. Similarly, on the southwest, there is a belt of detached areas stretching from Mexico to Alaska. In few places within these belts have the ancient rocks been studied in detail. Lesser areas of Archean rock appear in northern Michigan, Wisconsin, and Minnesota, and in the Adirondack region, but in some of these places, Archeozoic rocks have not been carefully separated from Proterozoic. The vicinity of Lake Superior in Canada, Michigan, Wisconsin, and Minnesota, the area north of Lake Huron, and the Ottawa region in Ontario, are the areas where the system is best known. The Archean system contains some iron ore (p. 332) and some ores of other metals, but not as a rule of great richness. Gold especially is widespread ”, but in few places is it known to occur in workable quantities. In other countries, the general characters and relations of the Archean of North America seem to be duplicated. A corresponding system of rocks, made up primarily of meta-igneous rocks, but subordinately of meta-sedimentary rocks inextricably involved with them, is known in all continents. The general characteristics and relations of the Archean therefore appear to be world-wide. 1Van Hise, Pt. II, 16th Ann. Rept., U. S. Geol. Surv., pp. 744-843, and Van Hise & Leith, Monograph LII. 2 Op. Cit., p. 295. 322 ARCHEOZOIC ERA Fig. 295. The white areas north of Mexico represent exposures of Archean; those of Mexico represent lack of knowledge. The black areas represent exposures of Pro- terozoic, and the lined areas represent Archean beneath later formations. The light shading about the borders of the land represents the continental shelves, or, what is the same thing, the area of the epicontinental seas for this continent. TIME DIVISIONS 323 GENERAL TABLE OF GEOLOGIC TIME DIVISIONS! aed Present : Pleistocene Cenozoic ¢ Pliocene | Miocene Oligocene Eocene Transition ( Cretaceous (Upper Cretaceous) Mesozoic Comanchean, or Shastan (Lower Cretaceous) Jurassic Triassic Permian Pennsylvanian (Coal Measures) Wide-spread unconformity Mississippian (Subcarboniferous) Paleozoic Devonian Silurian Wide-spread unconformity Ordovician Cambrian Great unconformity [ Keweenawan Unconformity Upper Huronian (Animikean) Proterozoic Unconformity Middle-Huronian Unconformity | Lower Huronian Great unconformity Great Granitoid Series (Intrusive in the main; Laurentian) Archeozoic ) Archean Complex + Great Schist Series (Mona, Kitchi, Keewatin, ’ Quinnissee; Lower Huronian of some authors) THEORETICAL CONSIDERATIONS Bearing on theories of the earth’s origin. With the essential facts concerning the constitution and structure of the Archean in mind, it is in order to inquire to what hypothesis of the earth’s 1 There are many unconformities not suggested in the table, where only those which appear to be extensive are noted. Those between the systems of the Pro- terozoic are known to be general for the Lake Superior region only. 324 - ARCHEOZOIC ERA origin they best adjust themselves. The constitution of the system makes it clear that it does not represent the original crust of the earth or its downward extension. Jt cannot be affirmed, however, that no part of what is now classed as Archean is referable to an original crust; that is, it cannot be affirmed that no part of the Archean is referable to an azoic or pre-zoic period, strong as the evidence against such reference may seem. On the other hand, all the facts now known concerning the Archean adjust themselves to the planetesimal hypothesis, or to the modified form of the gaseo-molten hypothesis. They cannot, however, be said to establish either, or to preclude other hypotheses of the origin of the earth. Life. The presence in the Archean system of carbonaceous material and of limestones, seems to imply the presence of life during the era of its formation. Since no fossils have been found, nothing is known of the character of the life, and little, except by inference, of its abundance. Duration of the era. Of the duration of the Archeozoic era nothing can be said beyond the general statement that it was very great, a conclusion which is independent of any theory of the earth’s origin. If the planetesimal hypothesis is correct, there is no readily assignable lower limit to the Archean system, and the duration of the Archeozoic era may exceed that of all subsequent time. Climate. Nothing is known of the climate of the era except that :t seems to have been such as to permit the existence of life, and the ordinary phases of sedimentation. CHAPTER XIV THE PROTEROZOIC ERA! FORMATIONS AND PHYSICAL HISTORY? The time between the Archeozoic era and the deposition of the oldest system (the Cambrian) of rocks containing abundant fossils constitutes the Proterozoic era. It was during this era that sedi- mentation first became the leading process in the formation of the geological record. The composition of the sediments, now indura- ted, implies mature weathering, and their extent and thickness imply the prolonged deposition on low lands or in the sea, of the ‘ sediments which were the products of mature weathering. During the era several great systems of sedimentary formations were formed. With the sedimentary formations there is much igneous rock, some of which is intrusive and some extrusive. Stratigraphic relations of the Proterozoic rocks. Great uncon- formities separate the Proterozoic rocks from the Archean below and the Paleozoic above. Great unconformities usually involve three elements: (1) a change in the attitude of the lower formation, as the result of which it is subject to erosion; (2) a long period dur- ing which its surface is eroded; and (3) the deposition of the over- lying rocks on the eroded surface. A sequence of events which might have given rise to. the uncon- formable relations of the Archean and Proterozoic is illustrated by Figs. 296 and 297. Fig. 296 represents an area of land composed of Archean rock, subject to erosion. The sediments derived from it are deposited in the sea (ata). In Fig. 297, the land is represented as having sunk so as.to be mostly submerged. -. Sediments (Al) washed down from the remaining land are being deposited uncon- formably on the eroded surface of ®. Though widespread, the 1 Proterozoic, as here used, is a synonym for Algonktan as used by the U. S. Geol. Surv. 2 A review of the pre-Cambrian geology of North America, by Van Hise and Leith, is found in Bull. 360, U.S. Geol. Surv., 1909. This Bulletin suggests probable correlations of the pre-Cambrian of different regions, so far as now warranted. 325 326 PROTEROZOIC ERA unconformity between the Archean and the Proterozoic is probably not universal, for there are doubtless places where the surface of the Archean did not suffer notable erosion before the deposition of Proterozoic sediments upon it. Fig. 296. Diagram showing Archean land (A) with sedimentation, a, along its vorders. (Compare Fig. 297.) Fig. 297. Diagram representing the same region as Fig. 296, after subsidence: The a of this figure corresponds to a of Fig. 296. Subdivisions. No existing classification of the Proterozoic formations has general application, but in the Lake Superior region, where these rocks are best known, four great unconformable systems are referred to this era. In some other regions the number is three (Fig. 298), in others two, and in still others but one. In most places each system is thousands of feet thick. These thick systems of Fig. 298. Diagram showing Proterozoic where it is composed of three systems of rock in the Lake Superior region. H,Huronian; 4, Animikean; K, Keweenawan. The diagram also shows the relation of these Proterozoic systems to the Archean (€R) below and to the Cambrian (€) above. The cross-pattern represents igneous rock. The lines, dots, etc., above the Archean represent sedimentary beds. HURONIAN SEDIMENTATION 327 rock and the unconformities between them record the history of the era for this region. Proterozoic sedimentation. The surface of the land on which the Proterozoic sediments were deposited was probably comparable to existing land surfaces of crystalline rock which have been long exposed to weathering and other phases of erosion. The topog- _ raphy was doubtless more or less uneven, and the surface mantled by soil and residual earths and rock debris (mantle rock) which had arisen from the decay of the underlying rock. The general nature of the clastic sediments laid down on such a surface when it became an area of deposition are readily inferred. They were made up chiefly of (1) the disintegrated products already on the surface, (2) the materials worn from the rocks by waves, if the surface was covered by the sea, and (3) river detritus. 1. One of the first effects of the Proterozoic seas, as they slowly transgressed the land — for it is presumed that this transgression was slow — was to work over, assort, and re-deposit the loose material on the surface. The coarse sediments were left in the shallow waters, while the fine materials were carrjed farther from shore, and left in the more quiet waters beyond. Deposits of gravel, sand, and mud were doubtless being made at the same time in different places, and changes in the position of the shore line, and in the depth of water, brought about, in time, the deposition of fine sediment on coarse, and of coarse sediment on fine. Thus the sedi- Sea Leve/ Fig. 299. Diagrammatic section showing relations which are conceived to have existed around Archean lands early in the early Proterozoic. Huronian sediments (A/) are in process of deposition. They are affected by intrusions and extrusions of lava, d1, do, d3, etc. mentary deposits came to be arranged in beds of different sorts, coarser and finer alternating in vertical section, and grading into each other laterally. At the base of the Proterozoic there is a widespread formation of conglomerate (Fig. 299) which appears to be composed of the 328 PROTEROZOIC ERA coarse parts of the mantle rock which were on the surface when the Proterozoic seas transgressed the lands of Archean rock. Such a formation is known as a basal conglomerate, and is one of the best indices of an unconformity. 2. Besides working over the decayed rock, the waves doubtless attacked the solid rock wherever exposures were favorable. The sediments thus acquired resembled the parent formation in average composition, and are thus distinguished from those of the preceding class, which were the products of rock decay. 3. Streams descending from the land must have brought down gravel, sand, and mud. The larger part of the river-borne detritus was probably decomposed rock, but a smaller part was doubtless derived by the mechanical action of running water on undecayed rock. Once in the sea, these several sorts of detritus were mingled. Since some of the constituents (especially alkalies and alkaline earths) of the Archean rock dissolved during the processes of de- composition probably remained in solution in the sea-water, it is thought that the clastic sediments were more siliceous than the rock from which they were derived. The sorting power of moving water takes account of the physical characteristics of the material handled, and not of their chemical constitution; but in the decomposition of Archean rock, the quartz remaining in the residual mantle was generally in larger particles than the clayey matter derived from the silicates, and under the sorting influence of the waves the quartz grains (sand) were more or less completely separated from the clayey parts (mud). Thus materials which were unlike chemically were separated from one another because they were unlike physically. If the Proterozoic seas had abundant life which secreted calcium carbonate, or if their waters anywhere became overcharged with calcium carbonate, lime- stone might have been formed. Extent. Sediments accumulated in the Proterozoic era are known in limited areas only, but doubtless they were very wide- spread. Water-borne and wind-blown sediment must have reached all parts of the sea, and the life of the salt water probably made deposits over the whole of the ocean bottom. Some sediments, too, must have been left on land, as at all other stages of the earth’s history since sedimentation began. The exposed formations. ‘The sedimentary beds of the Protero- zoic consist of conglomerates, sandstones, shales, and limestones, HURONIAN FORMATIONS 320 or their metamorphic’ equivalents. Before being cemented or otherwise solidified into firm rock, their materials were gravel, sand, mud, etc. The manner in which such materials are derived from older formations and transported to places of deposition, has been explained in earlier chapters. Basal conglomerate is of common occurrence at the bases of the several systems of the Proterozoic. There are also conglomerate Fig. 300. Section of the Proterozoic at a point in northern Michigan. (2), Archean granite. The other formations are Proterozoic. Length of section, 3 miles. (U.S. Geol. Surv.) Fig. 301. Section showing the complex structure of the Archean and Proterozoic formations at one point in the Marquette (N. Mich.) region. A gr, Archean gran- ite. The other formations are Proterozoic. Length of section, 2 miles. (U. S. Geol. Surv.) beds which are not basal, and they point to changes in the condi- tions of sedimentation even where unconformities were not de- veloped. Quaritzite, composed chiefly of grains of quartz firmly cemented, occurs in thick and extensive beds. The quartz grains probably came from granitic rocks, and their separation from the other materials indicates the thorough decomposition of the rock, and ample opportunity for the rolling and rounding of the grains before they came to rest. As the quartzites of the Proterozoic are thousands of feet thick in some places, great bodies of rock must have been decomposed to furnish so much sand. There are also great beds of shales, or their metamorphic equivalents, which are interpreted as the clayey products of the decomposition which set the quartz free. Limestone is present, from which it is inferred that the sea had become calcareous by processes similar to those now in operation, and that a portion of the calcareous content of the waters was extracted and deposited. The inference that these ancient sediments were deposited in the same manner as sediments of modern times is supported by 330 PROTEROZOIC ERA the ripple- and other shallow-water marks on the surfaces of the layers, and by their lamination and stratification, all of which are similar to those of sediments now being deposited. Geographic relations of exposed Proterozoic and Archean. Proterozoic rocks appear at the surface in many parts of North America, but they have been clearly separated from the Archean in Fig. 302. Diagram showing a common surface relationship between Archean (AR), Proterozoic (Al), and Cambrian (€). The Proterozoic formations appear at the surface between younger and older formations. few regions. Fig. 295 shows the area where rocks of known Proter-. ozoic age lie at the surface, together with areas where they have not been differentiated from the Archean. In many places, the Protero- zoic rocks at the surface are near areas of exposed Archean. That the Proterozoic formations should be exposed most com- monly about the borders of the Archean is made clear by Fig. 302, which shows, in section, the general relations of the Prote- rozoic systems (A/) to the Archean (&) below, and to younger formations (€) above. The same relations are shown in ground-plan in Fig. 303. While the relations shown in these diagrams are common, there are areas of Archean not surrounded or bordered by ex- posed Proterozoic formations, and areas of the latter not as- sociated with exposed Archean. Various relations of the two are illustrated by Figs. 304 and 305. Fig. 303. Map of the formations shown in section in Fig. 302. It is to be borne in mind that the map (Fig. 295) shows only the exposed areas (as now known) of Archean and Proterozoic. The Archean is presumably uni- versal, beneath other formations. The Proterozoic is not universal, but its extent is much greater than the area where it appears at the surface, Thus the Proterozoic THE HURONIAN SYSTEMS 331 of Wisconsin is probably continuous beneath younger formations with the Pro- terozoic of southwestern Minnesota, the Black Hills, and the Rocky Mountains on the west, and with that of Missouri and Texas on the south. Fig. 304. Diagram showing how Proterozoic rock (Al) may fail to outcrop about Archean (4). Fig. 305. Diagram showing how Proterozoic rock (A/) may outcrop on one side of an area of Archean (4) and not on the other. THE PROTEROZOIC OF THE LAKE SUPERIOR REGION ! The Proterozoic formations have been most carefully studied and their relations are best understood in the region about Lake Superior, and the formations of this region have become, in some measure, the standard of comparison for the Proterozoic group as a whole. The four great unconformable systems, their relations to one another, to the Archean below, and to the Cambrian above, are as follows: Earliest Paleozoic Cambrian Unconformity 4. Keweenawan Unconformity 3. Upper Huronian (or Animikean) Proterozoic Unconformity 2. Middle Huronian | Unconformity [ 1. Lower Huronian Unconformity Archeozoic Archean The Huronian Systems The first three systems of the Proterozoic group have much in common. All are dominantly sedimentary, and each includes formations of the common sorts of clastic rock or their metamor- phosed equivalents, together with limestone and beds of iron ore. 1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. 2 Jour. Geol. XIII, p. 161. 332 PROTEROZOIC ERA Since none of the limestones are known to contain fossils, their organic origin cannot be affirmed. Each of the three periods of sedimentation was long, though their duration is unmeasured. Each system contains much igneous rock, some of which was extruded while sedimentation was in progress, and some intruded later. Locally, igneous rock is more abundant than sedimentary. The unconformable relations of the three Huronian systems, and the unconformity of the third below the Keweenawan, show that after the deposition of the first, second, and third systems respective- ly, geographic changes occurred, resulting in erosion where sedi- mentation had been in progress. The material for the sedimentary part of the first of these systems doubtless came from the exposed part of the Archean, while the sedimentary parts of the second and third systems came from the exposed parts of all older formations. : In places, the sedimentary rocks still remain in the condition of conglomerate, sandstone, and shale, though more commonly the sandstone has been changed to quartzite or quartz schist, and the shale to slate or schist. Some of the igneous rock is massive, while some of it has been changed to schist. The rocks which are least altered are, as a rule, those which have been least deformed, and in places they are still nearly horizontal, as when first deposited. The oldest system is, on the average, most metamorphosed, and the youngest least. Carbonaceous slates. One of the significant formations of this region is black shale or slate, whose color is due to carbon. The carbon is thought to imply the existence of life when the sediments were deposited. Where the rocks are highly metamorphic, the black shale has been changed to graphitic schist. Iron ore. Another important formation is iron ore. Here belong the iron ores of the Mesabi (Minn.), Penokee-Gogebic (Wis. and Mich.), Menominee (chiefly Mich.) and other regions (Fig. 306). The ore is in the form of ferric oxide (chiefly hematite, Fe.O3), but in this form it represents an alteration from an iron-bearing forma- tion, originally deposited as chemical sediments, composed largely of iron carbonate and iron silicate, with some ferric oxides. These materials are believed to have been derived, directly or indirectly, from basic igneous rocks, extruded into the sea.!. The alteration to ore was brought about at a later time, by ground-water circulat- ing through the rocks. 1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. HURONIAN IRON ORES 333 The region about Lake Superior yields more iron ore than any other area of equal size in the world. In 1913 the aggregate pro- duction of this region was about 50,000,000 long tons,! which was about 83 per cent of all that was produced in the United States that a as u. ene Fig. 306. Map showing (in black) the position of the iron-producing areas in the Lake Superior region. 1, Michipicoten district; 2, Kaministikwia and Matawin district; 3, Steep Rock Lake and Attikokan district; 4, Vermilion district; 5, Mesabi district; 6, Penokee-Gogebic district; 7, 8, and 9, Marquette, Crystal Falls, and Me- nominee districts. year; of this, the Mesabi region produced nearly 34,000,000 tons. The ores of the Lake Superior region are partly in the Archean (about Vermilion, Minn.), partly in the older divisions of the Hur- onian group (about Marquette, Mich.), but most largely in the Animikean. ‘The following table! gives the production in tons for the principal areas for certain years preceding 1911: 1890 1895 Ig00 1905 IQIo Marquette...... 2,863,848 1,982,080 | 3,945,068 | 3,772,645 | 4,631,427 Menominee..... 2,274,192 | 1,794,970 | 3,680,738 | 4,472,630 | 4,983,729 PiOeevigge..... - 2,914,081 | 2,625,475 | 3,104,033 | 3,344,551 | 4,746,818 MermnOniws =. . 891,910 | 1,027,103 1,675,049 1,578,626 | 1,390,360 COO Oe ee 2,839,350 | 8,148,450 | 20,156,566 | 30,576,400 POROUS bec). 8,944,031 ! 10,268,978 ! 20,564,238 | 33,325,018 | 46,328,743 1 Mineral Resources of the United States, 334 PROTEROZOIC ERA Other ores.! Silver, nickel, and cobalt occur in workable quan- tities in the Huronian rocks at various points, especially in Canada. Rich ores of silver and cobalt (largely Lower Huronian) are found at Cobalt, Ontario, and ores of nickel at Sudbury. Thickness. ‘The thickness of the Huronian systems is hard to measure, because of their deformation; but if the maximum thick- ness of the individual formations of different localities is taken, their aggregate is several miles. The following section from the Marquette region may be regard- ed as fairly typical for the region: Michigamme slate and schist. Several thousand feet (maximum) in thickness. Ishpeming formation, largely quartzite. 1,500 feet (max- Upper Huronian - | imum) thick. Negaunee formation or series (slate, schist, Jaspilite, iron ore, etc.). 1,500 feet (maximum) thick. Middle Huronian ~ Siamo slate. 1,200 feet (maximum) thick. Ajibik quartzite (in places schistose). Nearly 1,000 feet | (maximum) thick. than 1,000 feet (maximum) thick. Kona dolomite (some clastic beds). More than 1,300 feet (maximum) thick. Mesnard quartzite. Several hundred feet (maximum) thick. | Wewe slate (including some other sorts of rock). More Lower Huronian | The Keweenawan System Constitution and thickness. In some parts of the Lake Superior region a fourth system of pre-Cambrian rocks, the Keweenawan, overlies the Upper Huronian. Unlike the Huronian systems, it is composed more largely of lava-flows than of sedimentary strata. The lava beds of the Keweenawan constitute its lower and larger part. The earlier flows of lava seem to have occurred on land, and to have followed one another at short intervals, for the surface of one flow was not eroded much before the next overspread it. Later, the intervals between flows appear to have been longer, and thin beds of sediment were deposited between successive sheets of igneous rock. The sedimentary beds increase in importance upward until, in the upper part of the system, lava beds fail alto- gether. In the valley of the St. Croix River, in northwestern Wis- consin and the adjacent parts of Minnesota, there are said to be 65 1Van Hise and Leith. Monograph LII, U. S. G. S., pp. 591-6. THE KEWEENAWAN SYSTEM 335 lava-flows and 5 conglomerate beds in succession, with neither top nor bottom of the system exposed. The igneous rocks of the system consist principally of gabbros, diabases, and porphyries; but other varieties are also present. The sedimentary rocks, chiefly sandstone and conglomerate, were de- rived largely from the igneous, and their character is such as to indicate that they accumulated rapidly. The thickness of the sedimentary beds has been estimated at some 15,000 feet; but there is reason for questioning the interpretation of such figures. The total thickness of the Keweenawan system has been placed as high as 50,000 feet. Interpreted in the simplest way, this would seem to mean either that beds of rock were piled up nearly 1o miles high on land, or that they filled a basin some to miles deep. Since the upper part of the system is sedimentary, and sediments do not Fig. 307. Diagram of a series of beds formed on the abysmal slope of a conti- nent, or in some similar situation, showing that the thickness, as usually measured, ef, is not dependent on the depth of the basin, cd, and that a thick series does not necessarily imply subsidence, even when the exposed portions of it show evidences of shallow-water deposition at various horizons. accumulate in quantity in high places, the first of these suggestions cannot be entertained, and it is extremely unlikely that there ever was a basin ro miles deep. The thickness of great bodies of stratified rock is commonly measured as suggested by Fig. 307. The dip of the rock (p. 275) and its extent at the surface are measured, and the depth is then calculated on the principle that the thickness of the whole is equal to the thickness of all its parts. The thicknesses of the several beds, added together, is shown in the diagram by the line ef, whereas the actual thickness, from top to bottom, is shown by the line cd. The point may be illustrated in another way. On the outer slopes of continental shelves, and in deltas, sediments are laid down with a considerable angle of slope. If the Amazon were to build a delta out 200 miles, the present ocean bottom remaining at an 336 PROTEROZOIC ERA average depth of four miles below the surface, and if the angle of deposition were 2°, the computed thickness of the deposits, accord- ing to the common methods of measurement, would be about 7 miles. If the delta were built out 1,000 miles, the computed depth would be 35 miles, though the basin was but four miles deep. Ifa delta were built half-way across a lake basin too miles wide and 1,000 feet deep, the angle of deposition being 3°, the thickness of the series, measured by the above method, would be 13,800 feet, though the basin was but 1,000 feet deep. With these points in mind it is clear that caution must be used in interpreting the great thicknesses sometimes assigned to sedimentary systems. The sedimentary part of the Keweenawan system has commonly been assumed to imply marine submergence; but so far as now known the sediments may have been accumulated in an interior basin, or may be partly subaérial. Fig. 308. Diagram illustrating the development of the Lake Superior syncline. AR, Archean; H and A, Huronian and Animikean; K, Keweenawan. (Irving, U. S. Geol. Surv.) Deformative movements. About the close of the Keweenawan period, the rocks of the system were somewhat deformed, and the deformation in the Lake Superior region was perhaps contempo- raneous with deformation in other parts of the continent. ‘These changes are regarded as marking the beginning of the end of the Proterozoic era. As a result of these deformations, some parts of the area where Keweenawan sediments had been deposited were brought into such an attitude as to be eroded, but the changes did not, as a rule, involve great folding or faulting of the strata. In keeping with their structure, the rocks are not greatly metamor- phosed. After the warping which followed the deposition of the Kewee- nawan system, the exposed surfaces of this and older systems suf- fered protracted erosion. Ultimately the land about Lake Superior sank again, and when the sea came back, a new series of sedimentary beds was deposited unconformably on the eroded surface of the older. The waters of the returning sea teemed with life, for the formation then made contains abundant fossils. This abundantly THE LAKE SUPERIOR PROTEROZOIC 337 fossiliferous formation is a part of the Cambrian system, the oldest system of the Paleozoic group. Copper. The Keweenawan system contains the most extensive deposits of native copperknown. The metal occursin pores and cracks of the igneous rock, and between the pebbles and grains of some parts of the sedimentary beds. In the conglomerate at some of the richer mines, the copper is so abundant as to be an important cementing material of the rock. The copper is believed to have been deposited by magmatic waters (7. e., waters of lavas), and toa lesser extent by thermal ground waters which had dissolved the metal from the igneous and sedimentary rocks." In 1875 the Keweenawan formation of northern Michigan yielded 16,089 tons of copper, about 90 per cent of all that was produced in the United States. In 1911 the same area yielded 109,093 tons, but this was only about 20 per cent of the copper produced in the country that year. The ores of silver, cobalt and nickel in the Huronian formations are to a large extent at least associated with basic igneous rocks, perhaps of Keweenawan age, intruded into Huronian rocks. General Considerations Concerning the Lake Superior Proterozoic Duration of time. It is difficult to conceive of the great lapse of time involved in the history of the Proterozoic era. The esti- mates give an aggregate thickness of more than 30,000 feet for the sedimentary rocks of the Proterozoic systems. The accumulation of so much sediment would in itself mean a vast lapse of time, and wher it is remembered that the four systems are separated from one another by unconformities, each of which may represent as much time as that involved in the accumulation of a system, it will be seen that the duration of the Proterozoic era was exceedingly long, possibly comparable to all succeeding time. It would appear that it should be spoken of in terms of tens (at least) of millions of years, rather than in terms of a lesser denomination. Destruction of rock implied. Thick beds of sediment mean the destruction of a still larger volume of older rock, for much of the more soluble part of the rock destroyed does not appear in the sedimentary formations. Had the Archean lands in the vicinity of 1Van Hise and Leith, Mono. LII, U.S. Geol. Surv. For earlier discussions, see Irving and Van Hise, Mono. V, U. S. Geol. Surv., Chamberlin, Vol. I, Geol- ogy of Wisconsin. 338 PROTEROZOIC ERA Lake Superior been high enough at any one time to furnish the thick sediments of the Proterozoic, their height would perhaps have sur- passed any existing elevation; but it is not probable that such ele- vations existed at any time. It is more probable that as erosion proceeded, the land reacted by rising slowly, or that the sea bottom sank, drawing off the waters and leaving the land relatively higher. In this way, degradation and elevation may have been in progress at the same time, and the one process may never have got far ahead of the other. The doctrine that the surface of the lithosphere sinks and rises under increase and decrease of load is one phase of the general theory of tsostasy. Succession of events. Reviewing the succession of events in the Lake Superior region, we find (1) that land composed of Archean rocks suffered prolonged erosion, but that the sites of the earliest post- Archean sedimentation areunknown. (2) Theland then sank or wasso eroded or deformed as to permit the deposition of the Lower Huro- nian sediments on parts of its eroded surface. (3) Areas including Archean and Lower Huronian rocks then came into such an attitude, presumably by crustal warping, that they were subject toa long period of erosion, with contemporaneous sedimentation elsewhere. Dur- ing the deformation, the rocks involved were somewhat meta- morphosed. (4) Again the land seems to have sunk, allowing the sea (conditions for deposition) to cover a large part of the area which had been subject to erosion just before, and to deposit upon its eroded surface the sediments of the Middle Huronian system. (5) After this long period of sedimentation, certain tracts seem to have emerged, exposing the landward border of the Middle Huronian system, and the older rocks not covered by it, to erosion. This emergence of areas of Middle Huronian sedimentary formations was accompanied by some deformation and metamorphism. (6) This period of erosion was followed by another period of submergence, when sediments (the Animikean) were laid down again in the Lake Superior region, this time on the eroded surface of the Middle Huronian or some older system. (7) Deformation, accompanied by emergence and followed by erosion, succeeded this third period of Proterozoic sedimentation. (8) Flows of lava of great magnitude were then poured out upon the surface of the land over consider- able areas, and intruded into older terranes. Before they ceased, sedimentation began again in the region, and soon predominated, the lavas and sediments making the Keweenawan system. THE LAKE SUPERIOR PROTEROZOIC 339 (9) After the deposition of this system, much of it was exposed to erosion. This succession of events implies repeated changes of relative level of land and sea in the Lake Superior region during the era. We shall see that such changes are confined neither to this time nor to this region. Changes in the relations of sea and land are among the notable events of the earth’s history, even to the present time. Since many other changes are dependent on them, they are believed to furnish the best basis for the subdivision of geological history. It is not now possible to determine the extent of the crustal oscilla- tions which took place during this era; but enough is known of the extent of land in North America at the close of the Proterozoic to make its representation on maps instructive (the white areas north of Mexico, Fig. 295). Metamorphism. The lower rocks of the Proterozoic are, on the whole, more highly metamorphosed than those above, but the Animikean beds are locally as highly metamorphic as the Lower Huronian, indicating intense dynamic action, at least locally, after the deposition of the third great system. Since different sorts of rock behave differently under dynamic action, it follows that some beds are much more highly metamorphic than others associated with them, even though subjected to the same forces. There is scarcely a phase of metamorphism which the Protero- zoic rocks do not show. The schists, slates, and gneisses are espe- cially the product of dynamic metamorphism; the quartzites are the products of extreme consolidation by cementation; the iron ore is the product of aqueous metamorphism, effected by ground-waters, while other phases of metamorphism are due to the heat of intruded rock. It is not to be understood that the metamorphism of any considerable body of rock is effected by any one process alone. Dynamic action, which seems on the whole the most important factor in metamorphism, always generates heat, and high temper- ature, especially in the presence of water, facilitates chemical and mineralogical change. So, too, in the case of igneous intrusions, there may be great dynamic action as well as great heat, and water, an agent of chemical change, is always present. Events elsewhere. A series of events consonant but not neces- sarily identical with those of the Lake Superior region was probably in progress about every other area of Archean rock, during the Proterozoic era; but it does not follow that about every other 340 PROTEROZOIC ERA Archean land area four great systems of rocks were laid down dur- ing this long era. About some such areas there may well have been one, two, or three systems of Proterozoic rocks instead of four, while about others, continuous sedimentation may have been in progress from the first of the Huronian periods to the end of the Keweenawan. PROTEROZOIC ROCKS IN OTHER REGIONS Pre-Cambrian sedimentary formations occur in many other parts of North America, in relations to the Archean similar to those already described. On the whole, they resemble the rocks of the Proterozoic systems about Lake Superior as closely as could be ex- pected under the general principles set forth. Some of the more important occurrences of Proterozoic rocks outside the Lake Superior region are the following: (1) in an exten- sive area north of the Great Lakes; (2) in the eastern provinces of Canada; (3) in the Adirondacks; (4) in isolated patches in the Mis- sissippi basin, in Wisconsin, northwestern Iowa and adjacent parts of Minnesota and South Dakota, in the Black Hills of South Dakota, in southeastern Missouri, and in Oklahoma; (5) in Texas; (6) in the Piedmont belt of the eastern part of the United States; and (7) at various points in the Cordilleras (Fig. 295). In some of these localities, the rocks are chiefly sedimentary or meta-sedimentary, while in others they are partly or even largely igneous. ‘Thus in the Black Hills, the Proterozoic rocks consist of slates, quartzites, schists, etc., intruded by granite. From the granite intrusions, the largest of which is eight or ten miles long and nearly as broad, numerous dikes penetrate the clastic beds, and furnish good illustrations of the metamorphosing effects of igneous intrusions. In the Adirondack region, pre-Cambrian rocks make up the larger part of the mountain mass. They include both:sedi- mentary (meta-sedimentary) and igneous rocks, the latter partly at least intrusive in the former. The Cordilleran region. The cores of many of the older moun- tain ranges of the west are believed to be of Archean rock. In many of them there are thick series of sedimentary or meta-sedi- mentary rocks (Proterozoic) overlying the Archean and surround- ing its outcrops, overlain in turn by Cambrian or younger strata. Sedimentary formations predominate among these Proterozoic formations, but are associated with igneous rocks which are in part PROTEROZOIC ROCKS IN THE EAST AND WEST 341 contemporaneous. In most of these localities the Proterozoic rocks are unconformable beneath overlying formations, and above the Archean where that is shown. In much of the northwest, however, there is conformity between the Proterozoic and the Cambrian, according to present interpretations. In the Canyon of the Colorado, pre-Cambrian formations are well exposed. The Proterozoic (Grand Canyon) group, more than 10,000 feet in thickness, rests unconformably on the Archean, and is in turn covered unconformably by the Cambrian. Here, as in Montana, a few fossils have been found. In the eastern part of the United States. There are large areas of metamorphic rock in the eastern part of the United States, former- ly classed as Archean. ‘Their position is shown in Fig. 295.° These metamorphic rocks include some that were sedimentary, and some that were igneous. A part of them are probably Proterozoic, but the Proterozoic, Archean, and metamorphic Paleozoic rocks have not been fully differentiated. Summary. While the correspondence of the Proterozoic rocks in these various regions with those of the Lake Superior region is not generally very close, it may be pointed out again that close correspondence is not to be expected, even if the rocks of different localities were contemporaneous in origin. The phases of sedi- | mentation taking place about any land mass at any time depend largely on the height of the land, the exposure of its coasts, climate, and the character of the formation suffering erosion. These various factors were as likely to be dissimilar as similar about the various centers of sedimentation. Igneous rocks form a not inconsiderable part of the Proterozoic systems, and there is no apparent reason why igneous activities in different regions should correspond either in time or in the nature of their products. Even deformations of the crust, which are the basis for the separation of the rocks into systems, need not have been the same in different regions. It fol- lows (1) that the number of Proterozoic systems bounded by uncon- formities may not be the same in all regions; (2) that the thick- nesses of the various systems may vary greatly; (3) that there need have been no close correspondence in the sorts of rock.in different regions at the outset; and (4) that they may have been metamorphosed unequally since their deposition. Dissimilarity of the Proterozoic in different regions was, therefore, to have been anticipated. 342 PROTEROZOIC ERA Proterozoic Formations in other Continents Proterozoic formations are believed to exist in all continents. In more than one country where they have been studied, the pre- Cambrian sedimentary rocks are thought to belong to at least two unconformable systems. In Sweden, as about Lake Superior, iron ore occurs in these formations, and the great bodies of iron ore in Brazil probably are of similar age. LIFE DURING THE PROTEROZOIC ERA The presence of a few fossils in the Proterozoic rocks proves the existence of life during this era.!. The best-preserved fossils are arthropods (p. 686) resembling crustacea. There are-also tracks of two genera of worms. In addition, there are obscure forms which appear to be referable to brachiopods and pteropods. It is signifi- cant that the oldest definite fossils yet found are forms well up in the animal kingdom, and that they occur (in Montana) 9,000 feet below the unconformity between the Proterozoic and the Cambrian. Other lines of evidence indicating life are: (1) Carboniferous shales, slates, and schists, and (2) limestone, some of which occurs near the base of the Lower Huronian. This rock was formerly regarded as demonstrative of the existence of life; but in recent years the belief has gained ground that considerable formations of limestone may have originated by precipitation from sea-water. This origin is suspected for many limestone formations which are free from fossils, and if the hypothesis is applicable to any extensive formation of limestone, it may be applicable to that of the Proterozoic. But even without reliance on this sort of rock, the occasional fossils leave no doubt of the existence of life in this era. CLIMATE Since inferences concerning the climate of any period are drawn largely from fossils, and since fossils are exceedingly rare in the Proterozoic strata, they afford little warrant for conclusions con- cerning the climate of the era as a whole. Conglomerate beds which have been interpreted as glacial” are found at the base of 1 For summary of knowledge concerning pre-Cambrian fossils, see Walcott, Bull. Geol. Soc. Am., Vol. 10, pp. 199-244. 2 Coleman, Jour. Geol., Vol. XVI, pp. 149-158, and Wilson, Ibid., Vol. XXI, pp. 121-141. CLIMATE OF THE PROTEROZOIC 343 the Proterozoic in the vicinity of Cobalt, Ontario. This inter- pretation, long doubted, now appears to be warranted. It may be noted that glacial formations are singularly out of harmony with the conceptions of the climate of early geologic time which have prevailed until recently. They are altogether in harmony with the conceptions of climate which grow out of the planetesimal theory. Map studies. Map studies should be carried on in connection with the chapters on the Archeozoic and Proterozoic. For this purpose, numerous folios of the U. S. Geological Survey are especially serviceable. See also Laboratory Exercises in Structural and Historical Geology, Salisbury and Trowbridge, Exercise VII. THE PALEOZOIC ERA CHAPTERS Aa THE CAMBRIAN PERIOD FORMATIONS AND PHYSICAL HISTORY The crustal movements which closed the Proterozoic era con- verted a large area within the limits of North America into land. This is shown by the distribution of the basal strata of the Cambrian, the oldest system of the Paleozoic era. Where accessible, the base of the system is, in most places, unconformable on underlying formations. The distribution of the successive parts of the system discloses the relations of sea and land throughout the period, for ~ most of the strata are of marine origin. Subdivisions The Cambrian system is divided into three parts, the Lower, the Middle, and the Upper. Georgian (Vt.), Acadian, and Potsdam or Saratogan (N. Y.), names of localities where the several divisions of Cambrian were first differentiated in North America, are syno- nyms for Lower, Middle, and Upper Cambrian respectively. The name St. Croixan (Wis.-Min.) also is used for the Upper Cambrian. Lower Cambrian. Lower Cambrian formations are known in North America only near the eastern and western borders of the continent (Fig. 309). In the east, they occur in the Appalachian belt and at some points farther east; in the west, they are found in various states between the rroth and the 120th meridians. Both east and west, the strata contain marine fossils. Those of the east were accumulated in straits, sounds, etc., rather than on the shores of the open sea. The great tract between the Appalachian Moun- tains on the one hand, and western Montana and Utah on the other, is believed to have been land during the early part of the period, and from it sediments were probably carried to the sea on either hand. 344 Stee eww en . ‘ ‘ ‘ . ‘ ’ news, + he ace Soe Fig. 309. Map showing the outcrops (in black) of Lower and Middle Cambrian tormations. The areas shaded by lines represent regions where the formations are believed to exist, though not exposed. The longer the lines, the better the basis for belief in the existence of the beds. The unshaded areas north of Mexico are believed to have been land during the early portion of the Cambrian period. The unshaded area south of the United States represents lack of knowledge. The shading within the area of the ocean is the same as in Fig. 295. The Middle Cam- brian may be somewhat more extensive than the map shows. The area not covered by the early Cambrian formations, and probably a still larger area, was land at the end of the Proterozoic, 346 CAMBRIAN PERIOD Middle Cambrian. Strata of the Middle (Acadian) Cambrian are found above those of the Lower, and in addition in Texas, Oklahoma, Arizona, parts of Montana, and perhaps elsewhere. Since the Middle Cambrian beds contain marine fossils, their dis- tribution indicates that the continent was being invaded by the sea from the south and west before the close of the Middle Cambrian epoch. Middle Cambrian beds are absent from much of the inte- rior, if present identifications are correct. Where the Middle Cam- brian rests on the Lower, the two are generally conformable. Where the Middle overlaps the Lower, it is unconformable on older forma- tions. . Upper Cambrian. In the Later Cambrian (Potsdam, Saratogan, or St. Croixan) epoch, the sea overspread much more of the continent, for the Potsdam series covers not only the eastern and western bor- ders of the continent, but much of the interior as well. ‘The Upper Cambrian is, as a rule, conformable on the Middle in the east and west, but in the interior it is unconformable on pre-Cambrian formations. Fig. 310 shows something of the distribution of the Cambrian system as a whole. Basis for the Subdivisions We have now to inquire how the Cambrian system may be recognized, and further, the means by which the Lower, Middle, and Upper parts may be distinguished from one another. Superposition. Where a formation or series is conformable on another of known age, as the Middle Cambrian on the Lower, the presumption is that the upper was formed immediately after the lower. In this case, the approximate age of the upper is known. But where one formation is unconformable on another of known age, the stratigraphic relations between them do not show whether the upper is much or little younger than the lower. Fossils. ‘The Cambrian is the oldest system of rocks known to contain abundant fossils. Most of them represent the shells, other hard parts, or tracks of marine animals buried in the sands and muds when they were deposited. The fossils of any division of the Cambrian system constitute the known fauna of that stage, but it is not supposed that fossils of all species that lived have been preserved. . The Lower Cambrian formations contain certain fossils which are distinctive, Among them are species of a genus of trilobites FORMATIONS AND PHYSICAL HISTORY 347 Fig. 310. Map showing the Upper Cambrian formations. The outcrops are shown in black. The continuous lines represent areas where the Upper Cambrian formations are confidently believed to exist, though concealed. |The dashes repre- sent areas where there is some reason for believing them to exist. The dotted areas represent areas from which the Upper Cambrian is believed to have been removed by erosion, The unshaded areas have the same meaning as in Fig. 309. 348 CAMBRIAN PERIOD named Olenellus (Fig. 311, a). Along with representatives of this genus, many other species of various types are found. To the aggregate, the name Olenellus fauna is given, and Olenellus Cambrian is synonymous with Lower Cambrian and with Georgian. It is not to be understood that representatives of the genus Olenellus Fig. 311. CHARACTERISTIC CAMBRIAN TRILOBITES: @, Olenellus gilberti Meek; b, Paradoxides bohemicus Boeck; c, Dikellocephalus pepinensis Owen. These three genera are characteristic of the Lower, Middle and Upper Cambrian, respectively. are found in the Lower Cambrian everywhere, or that other genera of trilobites are absent. | Where formations representing the whole of the period are present, the fossils in the middle beds are not the same as those in the lower. At no single plane is there, as a rule, a striking change in species, but in successively higher beds some of the species found below disappear, and new species comein. ‘These changes show that the inhabitants of the sea changed as time went on. At about that stage in the Cambrian system where the genus Olenellus drops out, the genus Paradoxides (Fig. 311, 6) appears in some places. The species associated with Paradoxides are somewhat different from those associated with Olenellus. The Paradoxides and their asso- ciates constitute the Paradoxides fauna, which includes many species of other genera of trilobites, and many species not related to trilobites. By general agreement, the Middle Cambrian, on both sides of the North Atlantic, is defined by the Paradoxides fauna, so that Paradoxides Cambrian is synonymous with Middle FORMATIONS AND PHYSICAL HISTORY 349 Cambrian and with Acadian (p. 344). In the western part of North America, and on the opposite side of the North Pacific as well, the Middle Cambrian does not contain Paradoxides. Its fauna is known as the Olenoides fauna, which is distinct from fauna of the Lower Cambrian. In like manner the Middle Cambrian fauna is succeeded by another, the Dzkellocephalus fauna found in the Upper Cambrian strata. Geologists have agreed to define the Upper Cambrian as the series of strata carrying this fauna. It is not to be understood that every species of the Paradoxides fauna is unlike every species of the faunas below and above. This is not the case; but so many species of the three faunas are different, that with a considerable number to judge from, their separation is possible by those familiar with Cambrian fossils. Sequence of faunas based on stratigraphy. The sequence of faunas was first determined by superposition of the strata. The Lower Cambrian fauna could not have been known to be older than the Middle Cambrian fauna if beds containing the former did not underlie beds containing the latter. In other words, the primary basis for correlation by means of fossils 1s stratigraphy. Physical Events of the Cambrian Submergence. The distribution of the several series of the system shows that the great physical event of the Cambrian period in North America was progressive submergence of the continent. Theo- rectically, this may have been brought about by a rise of the sea or by a lowering of the land, or by both together. Both the lowering of the land and the rise of the sea may be due to gradation, to dias- trophism, or to the two combined. Gradation as a cause of submergence. Gradation is perpetual and inevitable where land and sea exist. The waves attack the land along its borders, and the agents of land degradation lower its surface. The former is a direct cause of encroachment of sea on land, and the latter is an indirect cause, since all sediments car- ried from land to sea raise the surface of the sea correspondingly. Small as this rise is for any brief period, its effect is to cause the sea to advance on the land, and the lowering of the land by degradation at the same time increases the area of the advance. If continued long enough, shore-cutting about the borders of the lands, down- cutting over the whole surface, and the accompanying rise of the sea-level, must inevitably cause the water to cover the continents 350 CAMBRIAN PERIOD provided there 1s no deformation of the body of the earth in the mean- time. If the earth were to remain without deformation long enough for the continents to be base-leveled, the deposition in the sea of the sediments thus derived would raise the water about 650 feet. This would submerge a large part of the base-leveled land. The evidence of gradation in the Cambrian period is clear and firm. Most of the sediments which make up the Cambrian system of rocks were eroded from the land and deposited in the sea. ‘This lowered the land and raised the sea. Gradation was, therefore, a factor in the submergence of the continent, and there is evidence that great progress was made toward base-leveling before the close of the period. If gradation were the sole agency involved in the submergence of the lands, the advance of the sea should have been steady, though not necessarily equal in rate at all times and places. Without going into details, it seems certain that there were changes in the areas of deposition other than those which can be accounted for by gradation, but none of these changes imply notable warpings such as are recorded in the rocks of the Proterozoic and Archeozoic eras. Deformation as a cause of submergence. Deformations which may cause submergence of land (and emergence of sea-bottom) are of various sorts. Any deformation which causes the land to sink, or the sea to rise, leads to submergence. Such movements and their causes have been discussed briefly (chapter VIII). One special phase of movement which may have especial significance here is noted at this point. | Continental creep. ‘The continents are about 15,o00 feet above the ocean bottom. Their weight causes an average pressure of 15,000 to 20,000 pounds to the square inch on their bases, 15,000 feet down. This pressure tends to cause the continents to spread by creep into the ocean basins, on the same principle that an ice-sheet spreads. Spreading is opposed by the hydrostatic pressure of the oceans against the sides of the continental platforms. ‘This is some 5,000 pounds per square inch at the bottom, so that there remains an unbalanced pressure of 10,000 to 15,000 pounds per square inch, tending to cause creep. Is this enough to overcome the strength of the rock, which opposes creep? Even the lesser of these figures is equai to the crushing strength of some of the weaker rocks, and is a notable percentage of the crushing strength of even the strongest. Under less pressure FORMATIONS AND PHYSICAL HISTORY — 351 than this, rock in some mines is observed to creep. It is not im- probable, therefore, that such a pressure, constantly exerted for a prolonged period, might cause some spreading of the great con- tinental platforms, and hence (1) some lowering of their surfaces, (2) some submergence about their borders, and (3) at the same time some rise of the sea-level. Many phenomena which cannot be cited here seem to lend support to this hypothesis of lateral creep,! but its efficiency is not determined. Sedimentation. in the Cambrian Period Sedimentation in the Cambrian period appears to have followed the general laws that govern deposition in periods of comparative freedom from great deforming movements. Most of the known sediments were deposited in the sea, and their area may be regarded as a rough measure of the area of the Cambrian sea. Sedimentation was probably faster in the early stages of the period when the land- area was largest and highest, and slower in the later stages after the land had been lowered and narrowed. Sedimentation was probably greatest near the land. Sources and kinds of sediments. As in other periods, the land- derived sediments came from all formations exposed to erosion. The sediments along the immediate borders of the land were doubt- less different from those farther out, and even along shore probably there were variations, because of differences (1) in the sources of the sediments, and (2) in wave, river, and current action. The Cambrian system includes all common phases of sedimen- tary rocks. There are conglomerates, presumably laid down near the shores of the time; sandstones, the sand of which was deposited in shallow water; shales, representing the mud deposits in quiet water; and beds of limestone representing, for the most part, the accumulations of shells, etc., where sediments from the land were not abundant. Geographic variations. The distribution of these various sorts of sedimentary rocks shows that various kinds of detrital beds were accumulating in different places at the same time, and at the same place at different times. Not only this, but they were accumulated at very different rates, as the great variations in thickness show. The fact that the Upper Cambrian in the northern interior of the United States is mostly of sandstone, and that this sandstone is 1 Chamberlin and Salisbury, Earth History, Vol. II. 352 '—- CAMBRIAN PERIOD widespread, indicates that the water was so shallow that the waves were competent to roll sand long distances. Furthermore, the structure of the beds, with their cross-bedding (Fig. 199), ripple- marks, etc., shows that the whole of the thick series from bottom to top was deposited in shallow water, and therefore on a surface which was depressed gradually, relative to sea-level, as the sand accumu- lated. The limestone (chiefly dolomite) in the Upper Cambrian of the southern and southeastern interior, points to clear seas, but per- haps not to deep ones. The adjacent lands were perhaps too low to yield abundant sediment. Limestone is also an important part of the Middle and Upper Cambrian of the west, though clastic rocks predominate in the Lower Cambrian. Where the Upper Cambrian is limestone, it is, as a rule, not sharply differentiated from the over- lying Ordovician. Outcrops of Cambrian The Cambrian formations were once as widespread as the Cam- brian seas themselves, but they are not now present over all the area they once covered. ‘The areas where they are exposed are not to be confused with the areas where they actually exist. Cambrian formations are exposed, for example, in Wisconsin, Missouri, and Texas; but the strata of Missouri are doubtless continuous, beneath younger formations, with those of Texas, on the one hand, and with those of Wisconsin, on the other (Fig. 310). Position of outcrops. The map (Fig. 310) showing the areas where the Cambrian system is now exposed reveals several points of significance: (1) Many of the outcrops are in association with outcrops of the Archean and Proterozoic systems (Fig. 295). In places, the exposed Cambrian lies along one border of the exposed parts of these older systems, while in others it completely surrounds them. ‘This distribution is not peculiar to the Cambrian, but is characteristic of most formations as compared with those of greater age. (2) The exposed areas of Cambrian in the Appalachian Moun- tains occur in parallel or subparallel belts (Fig. 310). This is the result of (a) the folding to which the Cambrian and later strata of this region have been subject, and (6) the erosion which the folds have suffered. Fig. 314 will help to explain the repetition of out- crops. In this diagram, A represents pre-Cambrian strata, € repre- sents the Cambrian, and O, S, D, and C, the Ordovician, Silurian, Devonian, and Carboniferous systems, respectively. After the strata were folded, erosion cut the folds down. A fold involving FORMATIONS AND PHYSICAL HISTORY 353 Cambrian beds, if truncated below the level of the bottom of these beds at their highest point, exposes two belts of Cambrian strata, one on either side of a pre-Cambrian axis, as represented in the AVAUAR unt ere ae Fig. 312. Diagram illustrating the relation of Cambrian formations, A, B, and C, to older rocks. The diagram suggests that the Cambrian formations have been eroded back from their original margins, A’, B’, and C’. Fig. 313. Diagram illustrating the general relations of Cambrian beds in the interior. ‘The Cambrian, €, is represented as appearing at the extremes of the diagram, and as dipping below younger beds between. left-hand part of the figure. If the truncation is at a level below the top and above the bottom of the Cambrian (right-hand side of Fig. 314. Diagram showing the positions of outcrops determined by folds. Fig. 314), the strata of that system are exposed in a single belt along the axis of the fold. (3) In some places, Cambrian outcrops are surrounded by older formations. In such Pee eee 1a ae os bs sap nea gs® ay cases the Cambrian t cess Bel aare aN oo ae TS eed outcrops presuma- , Bi eve eee Vm Nad Tee he, tN bly represent rem- nants which have Fig. 315. Figure to illustrate isolated occurrences of escaped _ erosion. Cambrian surrounded by older formations. They may occupy depressions in the surface of pre-Cambrian formations, or may constitute hills (Fig. 315). 354 CAMBRIAN PERIOD Width of outcrops. The widest outcrops of the Cambrian (Fig. 310) are in Wisconsin; yet there the Upper Cambrian only is present, with a thickness of less than 1,000 feet, while in the Appala- chian Mountains, where the system has anaggregate thickness of sev- eral thousand feet, it appears at the surface in narrow belts; that is, the outcrops are narrow in the east where the system is thick, and wide in the interior where it is thin. The explanation of this appar- ent anomaly is found in the attitude of the strata. In Wisconsin they are nearly horizontal, while in the mountain regions, both east and west, they are tilted at high angles. Where strata are vertical, the width of their outcrop on a hori- zontal surface is about the same as S&S the thickness of Fig. 316. Diagram illustrating the influence of dip the beds (€, right- on the width of outcrop. The Cambrian beds, €, to the hand side of Fig. left have a much wider outcrop than the Cambrian beds 316); where they to the right, though the thickness is the same. : are nearly hori- zontal, (€, left-hand side of Fig.) the width of outcrop on a hori- zontal surface is much greater. It is not to be inferred, however, that horizontal strata always have a wide outcrop. The width of outcrop is also influenced by topography, as shown in Fig. 317. Fig. 317. Diagram illustrating the effect of topography on width of outcrop. Here the horizontal stratum between B and C has about the same thickness as € of Fig. 316, but its outcrop is narrow. In general, the width of outcrop, so far as determined by topography, depends on the angle between the bedding-planes and the surface where the formation outcrops. The width of the outcrop decreases as this angle increases. Changes in Sediments Since Deposition The sediments of the Cambrian system have undergone change since their deposition. In most regions they have been compacted and cemented into solid rock. Over great areas in the interior FORMATIONS AND PHYSICAL HISTORY 356 (Figs. 312 and 313) the strata still remain nearly horizontal, while in some other regions they have been tilted, folded, and faulted Bs pete " b frase Fok S eek SrNINe Cee Fig. 318. A section in the Menominee region of northern Michigan, showing the Potsdam sandstone, €s, in unconformity with older formations. (Van Hise, U. S. Geol. Surv.) Fig. 319. Section showing relations of the Cambrian in the Appalachian Moun- tains. ‘The strata are folded and faulted. -€, Cambrian; O, Ordovician; S, Silurian. Length, 13 miles. (Hayes, Cleveland [Tenn.] folio, U.S. Geol. Surv. Ordovician and Silurian not separated in the original.) (Fig. 319). Where close folding has taken place, the rocks have been more or less metamorphosed. In extreme cases the sandstones Fig. 320. Section showing the relations of Cambrian and other formations at a point north of Leadville, Colorado. -® gn, Archean; €s, Cambrian; Cmr, Carbon- iferous; Jw, Jurassic; /p and gp, igneous rocks. (Emmons, U. S. Geol. Surv.) have been converted into quartz schists, the shales into slates and schists, and the limestones into marble. Close of the Period No physical changes of great importance seem to have marked the close of the Cambrian period in America. Nowhere in our continent, so far as now known, were mountains made at this time, and nowhere were great areas of sea-bottom converted into land, though local unconformities between this system and the next record local changes in the sites of deposition. 356 CAMBRIAN PERIOD The Cambrian in Other Continents Europe.! In Europe, as in North America, widespread defor- mation before the beginning of the Cambrian converted large areas - of the present continent into land, and there is evidence that these lands, like those of America, were subjected to protracted erosion before the deposition of the Cambrian system. The Cambrian system of Europe, like that of America, is largely clastic. Ripple-marks, cross-bedding, and sun-cracks are common, showing that a large part of the Cambrian sediments were laid down in shallow water, or on land. In Wales (Cambria), the country from which the system got its name, and in Brittany, the system is very thick. In Scandinavia and western Russia, on the other hand, it is thin, locally no more than 400 feet. These differences probably mean that sediments were being deposited in some places many times as rapidly as in others. The Middle Cambrian of Eu- rope is more widespread than the Lower or Upper, showing that changes in the relation of sea and land were in progress during the Cambrian period, shifting the areas of erosion and sedimentation. The Cambrian of western Europe has been much folded, but in central and eastern Europe, the strata are essentially horizontal. Beds of clay which are still plastic, and beds of sand which are still uncemented, are known in the undeformed part of the system. Geographic changes of great importance seem not to have marked the close of Fig. 321. Glaciated stone é ’ from the glacial beds at the the Cambrian, in Europe. base of the Cambrian in China. Other countries. Cambrian rocks (Willis, Carnegie Institution.) — 4¢cyr in various parts of Siberia, China. India, Australia, and Tasmania, and in the northwestern part of Argentina, but their distribution outside of North America and Europe is but poorly known. Glacial formations. (1) In northern Norway, Lat. 70° 8’ W., 1 The best summary, in English, of the Cambrian of Europe, is found in Geikie’s Textbook of Geology, 4th ed., Vol. II. FORMATIONS AND PHYSICAL HISTORY 357 there is a bowlder-bearing formation (the Gaisa beds) resting on a glaciated surface of crystalline rock. The Gaisa beds have been thought to belong to the oldest part of the Cambrian system, or to antedate it. (2) Recent exploration in China! has made known a thick formation (170 feet) of bowlder-bearing rock of glacial origin, containing many striated bowlders of diverse sorts of rock (Fig. 321) on the Yangtse River, in latitude 30°. This formation lies at the base of the Paleozoic, beneath the beds that carry Fig. 322. A glaciated bowlder from the Cambrian till of Petersburg, South Australia. (Howchin.) . Cambrian trilobites. Glacial formations of early Cambrian age have been found in Australia, and perhaps in South Africa.’ The profound climatic significance of these glacial formations is obvious. The testimony of Cambrian fossils, on the other hand, implies nearly uniform climatic conditions throughout all regions where fossils have been found, and the wide spread of the sea during the later part of the period would seem to point to oceanic, rather than continental, climates at that time. Duration of Cambrian Period There is no reliable estimate of the duration of the Cambrian period. The destruction and removal to the sea of such large volumes of rock as are represented by the sediments of the system 1 Willis, Researches in China, Vol. II. 2 David, Report of International Geological Congress at Mexico, 1907; and Howchin Quar. Jour. Geol. Soc., Vol. LXIV, p. 234, 1908, and Jour. of Geol., Vol. XX, pp. 193-8. 358 CAMBRIAN PERIOD required a very long period of time; but since there is no standard rate at which any sort of sediment accumulates, this long period cannot be reduced to years. It has been estimated that limestone sometimes forms at some such rate as one foot per century. In some parts of the West there are 6,000 feet of limestone, besides thick bodies of fragmental rock. At the above rate of accumula- tion, 6,000 feet of limestone would call for a period of 600,000 years, and if time be allowed for the other formations of the same region, this period would be lengthened greatly. It should be remembered, however, that while one foot per century is a rate at which limestone may accumulate, it does not follow that it is the rate at which Cambrian limestone was formed. Many estimates of geological time, based on various data, have been attempted.! These estimates, so far as applied to the Cam- brian, generally assign to that period a duration of 1,000,000 to 3,000,000 years. It should be distinctly borne in mind, however, that the chief value of these figures is to give emphasis to the fact that the period was one of great duration. For aught that is now known, the largest of these figures might be multiplied by 2 or even by some larger number. LIFE OF THE CAMBRIAN Perhaps no single event in the history of the earth possesses greater interest than the first appearance of life; but the date of its beginning is not known. ‘There is good evidence that life existed before the close of the Archeozoic era, and under the accretion hypothesis, it is not improbable that its beginning antedated, by a long period, the oldest accessible Archean formations. If so, it is quite beyond hope that the earliest forms of life will ever be known from fossils. The known fossils from the Proterozoic rocks give but a very inadequate conception of life before the Cambrian. But in the Cambrian system there is, for the first time, a reasonably adequate record of animal life. Animal fossils. Every great division of the animal kingdom, except the vertebrate, was representated in Cambrian times, and though no vertebrate remains have yet been found, it would be rash to assume that no vertebrates lived. All the known fossils appear to be of marine species. Of land animals there are no traces, but this does not prove that they did not exist. 1 For a general discussion of this topic, see Williams’ Geological Biology, Chap. II. LIFE 350 -Trilobites were easily the most distinguished forms of Cambrian life. They were not only the highest in organization, but the most characteristic of the period. Their successive genera best distin- guish its successive stages, and their distribution is a chief means of correlating the formations of different regions. Figs. 311 and 323 show their three longitudinal lobes, whence their name. Trilobites were kin to the modern crab and crayfish, representatives of the great group Arthropoda (p. 686). They have long been extinct, but the modern horse-shoe crab has some likeness to them. Trilobites were well advanced in the scale of development, possessing nearly all the anatomical systems and _ physiological functions of modern crustaceans. Perhaps their compound eyes are the best index of their development. In this and succeeding periods, the number of eyelets in trilobites’ eyes ranged from a score to several thousands. Some of them, however, had no eyes, while others possessed abortive rudiments, implying that their ancestors had possessed them. The acquisition and abortion of so important an organ seem to indicate change in the conditions of life. This may mean no more than migration to deep dark waters, or the habit of burrowing in the mud, where eyes were useless. The eyes of some were raised slightly on crescentic lobes, with the con- vex face outwards (a and c, Fig. 323). In later epochs, these cres- ae WAbspCE i COLL LLLP ff AN) Wey © SVE Ne f t Fig. 323. CAMBRIAN CRUSTACEA: a, Holmia (Olenellus) bréggeri Walcott, a characteristic trilobite of the Lower Cambrian; b, Olenoides curticei Walcott, a Middle Cambrian trilobite; c, Ptychoparia kingi Meek, a Middle Cambrian trilobite; d, Agnostus interstrictus White, a Middle Cambrian trilobite; e, Aristozowe rotundata Walcott a Cambrian phyllocarid; f, Leperditia dermatoides Walcott, a Cambrian ostracode. 360 CAMBRIAN PERIOD cents became more and more curved, extending the sweep of vision fore and aft, to the animal’s obvious advantage. The upper surface of the body was ornamented variously, and the ornamentation varied as time went on, increasing, in general, until after the climax of the trilobites had been passed. ‘Trilobites possessed a row of slender articulated legs on either side, and deli- cate filaments which served the. function of respiratory organs. The nature of the legs indicates that trilobites both walked and swam. They possessed antennze which doubtless served as organs of touch, and they moulted the shell at successive stages of growth, like modern crabs. Omitting further details, it is to be observed that, at this early day, a highly complex, well-differentiated organi- zation had been acquired, possessing nearly all the organs and functions of arthropods of the present day. Brachiopods (molluscoidea, p. 686 and Fig. 324) were second in geological importance to trilobites; but unlike trilobites, brachiopods still live. They are conspicuous representatives of stability and persistence. Though the species and most of the genera have g h Fig. 324. CAMBRIAN Bracutopops: a and b, Acrotreta gemma Billings, a brachiopod ranging from the Lower to the Upper Cambrian, summit and side views of the ventral valve; c, Billingsella transversa Walcott, a pedicle or ventral valve of a hinged brachiopod of the Lower Cambrian; d and e, Lingulepis pinni- formis Owen, views of the two valves; f and g, Kutorgina cingulata Billings, side and dorsal or brachial views, a Lower Cambrian species; /, Billingsella coloradoensis (Shum.), an Upper Cambrian species. . changed, the class as a whole has been but slightly modified since the Cambrian period. The brachiopod shell is bivalve. The two valves are unlike, but each is bilaterally symmetrical (Fig. 324). LIFE 361 Mollusks (p. 686) were well represented, Cephalo pods (chambered shells), the highest class of mollusks and are found in the upper- most beds of the Cambrian. As they were even then highly devel- oped, there is little doubt that the class had passed through a long history before the end of the period. Pelecypods (bivalves, oysters, clams, etc., 0, Fig. 325) lived throughout the period, though their Fig. 325. CAMBRIAN Mo.tusks: a, Hyolithes americanus Billings, a Lower Cambrian pteropod; 6, Fordilla troyensis Barrande, a Lower Cambrian pelecypod; c, Stenotheca rugosa Hall, a capulid gastropod of the Lower Cambrian; d, Trocus saratogensis Walcott, a gastropod with well-developed spire; e, Platyceras primevum Billings, a Lower Cambrian gastropod; f, Ophileta primordalis Winchell, an Upper Cambrian gastropod. fossils are not abundant. Like brachiopods, pelecypods are bi- valves, but unlike the brachiopods, the valves are not bilaterally symmetrical. Gastropods (univalves, c, d, e, Fig. 325) are rather Fig. 326. CAMBRIAN VERMES: borings and trails. a, a surface of sandstone showing annelid borings, with mounds of sand heaped about their mouths and with trails leading away from some of them, 362 CAMBRIAN PERIOD plentiful throughout the system. The early forms are chiefly of the low conical type, while more amply coiled and spiral forms be- came common later. Some of them resemble modern gastropods closely. Sea worms (Vermes, p. 686) left evidence of their abundance by borings, tracks, etc. (Fig. 326). A few cystoids, the forerunners of the beautiful crinoids (stone lilies), represented the echinoderms. Celenterates were represented by graptolites, meduse and polyps (corals). The eccentric freaks of fossilization are nowhere better illustrated than here. Relics of graptolites, among the most delicate of animal forms, and of medus@ (jelly-fish), among the soft- est of animals, were preserved, while some stronger types left scant record of themselves. Graptolites, now extinct, were slender, plume- Fig. 327. CAMBRIAN C@LENTERATA: supposed corals, meduse, and grapto- lites. a and b, Archewocyathus rensselericus Ford, a problematic fossil referred by some paleontologists to sponges, and by others to corals; c and d, Brooksella alternata Walcott, supposed casts of the gastric cavities of meduse; c, a supposed exumbrella in which the interumbrella lobes are a prominent feature; d, a view of a supposed umbrella with six lobes and a depression over the central stomach; e, een (?) cambrensis Walcott, the hydrosoma of a graptolite. like organisms (e, Fig. 327), consisting of a series of hard cells, in which the individual zodids lived, attached to a common Bender axis. The whole colony appears to have floated free in the sea. The secret of their preservation probably lies in the fact that, being floating forms, they settled in quiet waters off-shore, where fine silts accumulated, and where the conditions were favorable for burial without destruction. The most singular case of fossilization is the preservation of traces of jelly-fish, or at least of what are so identified (Fig. 327, c and d) in the Lower Cambrian. Obscure fossils of corals are found (Fig. 327, a and 0), the forms of which re- semble sponges so much that they long were regarded as such. Corals seem to have been more abundant in some other parts of the world than in North America. | LIFE 363 Sponges lived throughout the period. It is probable that many protozoans existed, but only a few forms have been identified. Implied life. The existence of so much animal life implies much vegetable life to supply the necessary food. Furthermore, various characteristics of the fossils suggest the presence of animals not known from fossils. A large percentage of the known Cambrian animals were provided with shells, tests, plates, or other forms of hard coverings. In the main, these appear to have been protective devices, and imply enemies or rivals against which protection was needed. Perhaps the most significant feature of the protective devices is that they are of the same types as those possessed by similar animals of later times. If there had been a radical change in the character of their enemies or rivals, we might expect some notable change in the defensive devices. It is a natural inference, therefore, that the conflicts of life in the Cambrian seas were similar to those of the present. ‘The inference may be pushed further, and the deduction drawn that the conflicts which led to the evolution of the defensive devices were much like those throughout the period of their retention. Stage of evolution represented. What stage of advancement in the development of life had been attained by the beginning of the Cambrian period? Do the fossils of the system indicate that the life of the period was primitive, or do they imply that it had advanced far beyond primitive forms? For comparison it may be assumed that the first forms of life were as simple as the simplest existing forms. If the plants and animals that consist of a single cell are taken to represent primitive forms, how far had the Cambrian life advanced beyond them? In the early stages of their development, animals pass through a succession of changes in which their structure resembles that which their ancestors had in their maturity; in other words, the individual history of any animal is an epitome of the history of its ancestors. Now the Cambrian trilobites are known to have passed through a series of remarkable changes after the individuals had developed far enough to be fossilized, and it is inferred they passed through other stages previously. There is, therefore, specific ground for believing that they had had a long line of ancestors. On the anatomical and physiological side, it is clear that nearly or quite all the fundamental organs had been developed. ‘There were skeletal systems of several forms, muscular systems, nervous 364 CAMBRIAN PERIOD systems of high development, as implied by eyes and other sense- organs, devices for capturing and ingesting food, organs of digestion, secretion, excretion, and respiration. ‘The Cambrian animals had acquired the various habits of life possessed by existing animals of their kind, as well as the various modes of preserving their lives. The question may be approached in another way. The studies of recent decades have convinced investigators that later forms of life were derived from earlier ones by processes of evolution. The exact methods of evolution are not altogether understood, but the fact of evolution is not now regarded as an open question. As the various forms developed and diverged from a common ancestral stock, many of the intermediate forms disappeared, and the forms which persisted became widely separated. By continued diver- gence, with the loss of intermediate types, a discontinuous series of forms was developed, and those which lived on became more and more unlike. The process was not unlike the evolution of a tree- top, in which the dying out of most of the interior branches leaves a few great limbs which bear the more numerous and more recent branches, while these in turn bear the uppermost and outermost twigs which represent the living phase. In some such way, it is thought that the existing divergence of organisms into kingdoms, branches, classes, orders, families, genera, species, and varieties came to be established. If it is assumed that the whole system of living things was derived from a common primitive form, or from a few primitive forms, a comparison of the primitive state with the degree to which life had advanced in the Cambrian period will give some impression of the amount of pre-Cambrian evolution. If to this be added a compari- son between the Cambrian life and that of today, an estimate of the relative amount of evolution before and since the Cambrian may be made. It is to be noted that not only were all the animal sub-king- doms, save perhaps the vertebrate, present, but that, in many of them, the species had come to have nearly the aspect of living forms. The initiation and divergence of the structures and types that preceded the Cambrian stage mean much more in the way of evolution than all the evolution of later times. These considerations lead to the conclusion that life must have been in existence a very long time prior to the Cambrian period. The succession of faunas. Under the doctrine of evolution, it LIFE 365 is presumed that the life of every past stage has grown out of that which immediately preceded it, and that it has merged into that which immediately followed it. It is usually assumed that if no * exceptional influences came in, there was a continuous series of slow changes without sharp lines of demarkation. If this conception were realized in fact, it would be less appropriate to speak of a suc- cession of faunas than of one continuous ever-changing fauna. It is not yet demonstrated, however, that evolution proceeded solely by very slight changes coming in from generation to generation. It may have proceeded by distinct and abrupt changes;! or at any rate new species may have arisen abruptly, so far as now known. Irrespective of any other specific hypothesis, it is to be noted that the geological record, as now known, does not show complete grada- tions from one species to another. In some cases there is something of a graded series, but the steps of the gradation are not sufficiently close and definite to decide between evolution by an infinite number of small changes, and a smaller number of greater changes. If we turn from species to faunas, a more general point of view must be taken. Observation shows that in some cases one fauna grades into the succeeding one, while in other cases the change appears to be abrupt. If the progress of life the world over could be studied as a unit, it would probably appear that there was a nearly perfect gradation of the life of one stage into that of the next. This gradation probably was more rapid at some times than at others, and it is quite certain that some forms changed more rapidly than others. But when we limit our study to the succession of faunas on any one continent, or to any one province, it is evident that the progress of evolution in the region studied was interrupted by physi- cal changes which affected the depth, temperature, or clarity of the water, and the nature of the bottom, and that these changes brought about variations in the character and distribution of life. There seem to have been rather definite times of notable change, between which faunas changed but slowly. Where the faunal change in a conformable series is abrupt, and there is no evidence of a gap in the record, the explanation is usually sought in the immigration of a new fauna from some other region. In the study of faunal progress, therefore, there is occasion 1 DeVries. Die Mutationstheorie, 1903. See also Bateson’s Material for the Study of Variation, 1894; W. B. Scott, On Variations and Mutations, Am, Jour. Sci., 1894. p. 355; and discussions of Mendel’s theory. 366 CAMBRIAN PERIOD to recognize (1) rather abrupt changes brought about by over- whelming invasions; (2) less abrupt changes brought about by the more gradual ingress of outside species, and the gradual com- mingling of immigrants with resident species; (3) very gradual’ changes due to the slow evolution of resident species when not much affected by immigration or by physical changes; and (4) rapid evolution due to profound changes in the physical conditions or to’ other agencies less well understood. The abrupt appearance of the Cambrian fauna. The apparent suddenness of the appearance of the Cambrian fauna is unexplained. In a general way, it may be said that older formations have been metamorphosed, and that this destroyed most of their fossils; but this suggestion is not altogether adequate, for some of the older formations are not greatly changed, and some younger metamorphic rocks carry fossils. It is also true that some younger formations which seem well suited to receiving and retaining organic impres- sions are without them. Geologists are inclined to refer the scanti- ness of pre-Cambrian fossils, and hence the apparent abruptness of the introduction of the Cambrian fauna, to unfavorable conditions for fossilization in pre-Cambrian time, combined with subsequent changes in the rock. This makes the abruptness a matter of rec- ord, rather than of fact. Map work. Suggestions for work with geologic folios are found in Laboratory Exercises in Structural and Historical Geology, SALISBURY AND TROWBRIDGE, Exercise VIII. CHAPTER XVI THE ORDOVICIAN (LOWER SILURIAN) PERIOD ! FORMATIONS AND PHYSICAL HISTORY The general conformity ? between the Cambrian and Ordovician systems shows that no great change took place in the relations of land and water in North America at the close of the Cambrian period. At the opening of the Ordovician, therefore, an epicontinental sea stood over much of the continent. Sedimentation During the Ordovician Period While the principles of sedimentation during this period were - the same as during the Cambrian, the conditions, so far as our continent is concerned, were somewhat different, chiefly because the smaller areas of land yielded less sediment. During much of the period the deposition of land-derived detritus was confined to littoral tracts. Since the land areas were of various sizes, of various sorts of rock, and presumably of various heights, conditions existed for the deposition of all sorts of clastic sediments about their borders, and for their deposition at very different rates. Sedimentation was doubtless more rapid near the larger and higher lands than about the smaller and lower ones, and more rapid on that side of any land towards which the larger part of its drainage flowed. Where clastic sediments failed, the shells and other secretions of marine animals and plants were accumulating, making limestone. The known formations of the Ordovician period are in keeping with these general principles. Adjacent to the broad, shallow 1 Recently it has been proposed to recognize a system of rocks, the Ozarkian, between the Cambrian and the Ordovician, the Ozarkian would include the lower part of the Ordovician (Beekmantown formation and its equivalents), and the upper formations of certain regions commonly referred to the Cambrian. Ulrich, Bull. Geol. Soc. Am., Vol. XXII. * There are local unconformities between these systems, as in some parts of New York, and the evidence is increasing that they are more wide-spread than formerly was supposed. 307 368 ORDOVICIAN PERIOD arm of the ocean which covered the larger part of the Mississippi basin (Fig. 310) there appear to have been no sources of abundant sediments during most of the period. Along the western base of - Appalachia, clastic materials were being deposited. Alternating beds of coarse and fine sediment indicate either (1) that the adjoin- ing land was higher at some times than at others, or (2) that the climatic conditions or (3) the vegetal covering changed, or (4) that waves and currents varied in their effectiveness. Conditions for the formation of limestone prevailed widely in the epicontinental sea. Plants and animals secreting calcium car- bonate may have been no more abundant far from land than near it, but away from shore their shells, etc., were more abundant relative to the sediments derived from the land. The development of the Ordovician system meant the destruc- tion of an equivalent body of older rock. The material which entered into the new system came from all preceding formations so situated as to be exposed to erosion. Even the limestones of the system had their ultimate source in older formations, for the mineral matter extracted from the sea to make the shells had been dissolved from older formations during their decay, and brought to the sea in solution, largely by the same streams which carried the clastic sedi- ments. Sections of the Ordovician. The Ordovician system of North America was first studied carefully in New York, and the section of that State is, in some measure, the standard to which others are referred. In New York the system is divided as follows: Richmond beds! (in Ohio and Indiana) Lorraine beds Utica shales Trenton limestone Upper Ordovician (or Cincinnatian) Ordovician ; Middle Ordovician Black River mnestaee (or Mohawkian) Lowville limestone Lower Ordovician Chazy limestone (or Canadian) Beekmantown limestone (Calciferous) The classification of New York is not applicable in detail in other parts of the continent. In Wisconsin, Iowa, and Minnesota, for example, the formations commonly recognized, numbered in the 1 Question has been raised as to the propriety of including the Richmond beds in the Ordovician. Hartnagle, N. Y. State Mus. Bull. 107, 1907. In Illinois, beds of Richmond age are unconformable on the older Ordovician. Weller, Jour. of Geol., Vol. XV, p. 519; and Savage, Am. Jour. Geol., Vol. 125, p. 431, 1908, » FORMATIONS AND PHYSICAL HISTORY 369 order of age, are shown below, but it cannot be affirmed that any one of them is the exact equivalent of any one in New York. Upper Ordovician 5. Hudson River ! (Maquoketa) shale Poatie Ordovician } 4 eae ene 3. Trenton limestone ( 2. St. Peter sandstone Lower Ordovician ; ; 1. Lower Magnesian limestone In the mountains of Tennessee, a series of limestone or dolomite beds (Knox, Chickamauga, etc.), is followed by a series of clastic beds (Sevier shale, Bays sandstone, etc.).2 The exact relations of these formations to those of New York and to those of the upper Mississippi basin are undetermined. The section of Tennessee does not correspond in detail with that of other parts of the Appala- chian belt. In the Great Plains, the Ordovician system appears at the sur- face but rarely, though it probably underlies the younger formations. Fig. 328. Trenton Falls, Trenton, N. Y. The locality whence the Trenton formation derived its name. (Darton, U. S. Geol. Surv.) 1 Tt is now held by some that a portion, if not all, of the Hudson River (Maquo- keta) shale of the Mississippi basin is the equivalent of the Richmond beds farther east. 2The subdivisions mentioned here are those of the Maynardsville, Tenn., folio, U. S. Geol. Surv. 340 ORDOVICIAN PERIOD West of the Great Plains, the system is present generally, and the sections are somewhat simpler than in the interior or the east, limestone being a conspicuous part of the system here. General conditions in the eastern part of the continent. At no previous epoch was there anything like such widespread deposition of limestone within the limits of our continent, as in mid-Ordovician time, when limestone was forming from New England on the east, to Georgian Bay on the north- west, to Oklahoma and Texas on the southwest, and Alabama. on the south, as well as in much of the west. It is perhaps equally worthy of note that in the later part of the period, mud (now shale) was deposited over an almost equally exten- sive area. This may mean that the lands were so elevated as to allow the streams to carry more sediment to the sea, or that conditions favored the transportation of mud farther from shore than formerly, or both. All the Ordovician for- = Cambro-Ordovi- (Darton, Monterey €0 eee 2), SD) ANN 2 Sea) WG (a> > PEPE SO IMWMWhil? == AZZ PAWN 43 aS 4 2 3 4 The last four formations are Ordovician, un- Sn, Niagara limestone (Silurian); Di, Hamilton limestone (Devonian, ) t a point in West Virginia. Length of section, 18 miles. Devonian. Siluro-Devonian; D= Length of section, about 135 miles. The section extends from the Archean area, in the north-central part of the state, to Lake Michigan, Silurian; SD in the vicinity of Milwaukee. AR, Archean; -€, Potsdam sandstone (Cambrian); Olm, Lower Magnesian Limestone; Osf, St. Peter sandstone; Of, Trenton and Galena limestone; Ofr, Hudson River shales. less, as recently suggested, p. 368, the last be Silurian. Section showing the relations of Ordovician and other beds a Fig. 329. Section of the formations in southern Wisconsin, showing the position of the beds and the relations of the several Los! o 8 A a — _- fo) vo oO . — —/Y m Quy Oo oO ey nt . . . Ss x ¢,; mations of the interior and ° Ppa a - “4 g 5 35 the east bear within them- > ‘ Y a 3. selves evidence of shallow water O° ‘oO oO a) . . 6 8 mow. OFgin. ~~ to} 8) e =n ay 9 r ir eae Igneous rocks of Ordovician 8 = .-- age attain little importance in Nn . . > = ‘$= North America. Their general FORMATIONS AND PHYSICAL HISTORY 371 absence is in harmony with the quiet which characterized the period. General Conditions and Relations of the Ordovician System Position of beds. As originally deposited, the Ordivician beds probably dipped away from the lands of the period. Over great areas in the interior, this original and simple plan of stratigraphy has been but little modified (Fig. 329). In other regions, deforma- tion of the strata has completely changed their original positions. Thus in the Appalachian Mountains (Fig. 330) and in some parts of Arkansas (Fig. 331), Oklahoma, oot and various mountains of the \ uA | fas west, the strata are folded and in some places faulted. Metamor phism. The sediments have undergone more or less al- teration since their deposition. In ‘ A oO" \ : 4 or ‘T—— We ALY a : \ H ‘ \ % 1} s bi) AA A ‘ 1 a s oO AN \ \ ' ' i . X ‘ ‘ . i \ \ \ some places the changes have ape iN : been slight, and in others great. rage + ee / a 8 3 ¢ Ke Mi See wae we *e The larger part of the Ordovician yo Sn i “s "“ aseoe sands have been changed to sand- Fj oe ; g. 331. Section showing the stone, the larger part of the muds position and relations of the Ordovi- to shale, and most of the lime- cian beds in the mountains of Arkan- stone is still essentially non-meta- Pe ee le nee morphic. But where dynamic action has been great, and where the original position of the strata has been changed greatly, the changes in the rock have been greater.! Thus in the Taconic Mountains (southeastern New York and southwestern New England), the limestone has been changed to marble, the sandstone and quartzite to quartz schist, and the shale to slate and schist. Thickness. The rocks of all systems vary greatly in thickness, and the Ordovician system is no exception. In the Appalachian Mountains it is thousands of feet thick, while in the interior it is only hundreds. In Wisconsin and Iowa, the aggregate thickness is rarely more than 800 or goo feet. Outcrops. In the interior, where the system is relatively thin, 1 See, for example, the New York City, Holyoke (Mass.-Conn.), and Hawley (Mass.) folios, U. S. Geol. Surv. Compare with folios of the Appalachian Moun- tains, the interior, and the western part of the United States. 2 ORDOVICIAN PERIOD Y ‘ Soe man, Fig. 332. Map showing the general condition of the North American continent in Mid-Ordovician (Trenton) time. The black portions represent areas where the Middle Ordovician beds appear at the surface. These areas so nearly correspond with the areas where the Ordovician system as a whole appears at the surface, that no serious error is involved if the black areas be interpreted as Ordovician. The various conventions of the map are the same as in Fig. 310, p. 347. FORMATIONS AND PHYSICAL HISTORY 373 it appears at the surface in relatively wide belts or areas (Fig. 332), while in the eastern mountains, where it is thick, it appears at the surface in a succession of narrow and parallel belts (p. 372). The outcrops are largely adjacent to older rock. Close of the Ordovician Period The close of the period was marked by geographic changes of more importance than those at its beginning. The greatest change was the withdrawal of the epicontinental waters from a large part of North America, converting extensive stretches of shallow-sea bottom into land. The cause of this change may have been the sinking of the ocean bottoms and the drawing off of the epiconti- nental waters. The altitude of the new land must have been slight _ or its exposure brief, for it suffered little erosion before much of it was again submerged and covered by sediments of later age. It is indeed the widespread absence of the lower part of the Silurian system, apparent or real (p. 388), rather than a pronounced strati- graphic unconformity between it and the Ordovician, which indi- cates the extensive emergence of land in the interior at the close of the Ordovician period. Folding movements were limited. The most considerable were in the Taconic Mountains, where both the Cambrian and Ordovi- cian systems were thick. The date of the folding is known, because Silurian formations overlie the Upper Ordovician unconformably in this region. It is not to be inferred that all the mountain-making movements which have affected western New England occurred at this time. There had been folding earlier, in pre-Cambrian times, and there were movements later as will be noted. The principal deformation of the strata in the Appalachians and in Arkansas came much later. Between folding and the more gentle movements already noted there are all gradations. The ‘‘Cincinnati arch” is an example. This arch is a very low anticline with a general north-south course, extending through Cincinnati. The beginning of this arch may have been as early as mid-Ordovician. Another similar arch! may have come into existence at about the same time in Arkansas and Oklahoma. The crustal movements referred to above have been mentioned 1 Branner, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive article has bearings on many questions besides the Ouachita Uplift. 374 ORDOVICIAN PERIOD as occurring at the close of the Ordovician. It would perhaps be more accurate to say that their beginning marks the beginning of the transition from the Ordovician period to the Silurian. The duration of the interval of transition was probably long. Economic Products In Ohio and eastern Indiana the Trenton formation yields much gas and oil.!. Both are commonly held to be products of the decay or distillation of organic matter included in the sediments when they were deposited. The oil is most abundant under low anticlines, where it occurs in the pores and openings of the rock, somewhat as ground-water does. The Galena and Trenton formations in Wisconsin ” and in the adjacent parts of Iowa and Illinois contain ores of lead and zinc, - mainly in the form of sulphides and carbonates. Lead ores are also found in the Ordovician (or Cambro-Ordovician) formations of southeastern Missouri,? and lead and zinc ores in the south- central part of the same state. In all these regions the ores occur (1) in cavities formed by solution, (2) as replacements of limestone, or (3) in crevices. In these positions, the ore was concentrated by ground-water. The metallic substances were doubtless derived from the limestone itself, which, at the time of its deposition, is thought to have contained trifling amounts of lead and zinc, perhaps extracted from sea-water by organisms. The Ordovician limestones of central Tennessee * locally yield Fig. 333.. Shows modes of occurrence of the phosphates (the shaded surface parts of the limestone, ph) in central Tennessee. (Hayes and Ulrich, Columbia (Tenn.) folio, U. S. Geol. Surv.) 1 Orton, 8th Ann. Rept., U. S. Geol. Surv.; Phinney, 11th Ann. Rept.; also the reports of the State Geol. Surv. of Ohio and Indiana. 2 Chamberlin, Geol. of Wis., Vol. IV, 1879, pp. 365-568; Calvin and Bain, Iowa Geol. Surv., Vol. VI, and Grant, Bull. XIV, Wis. Geol. Surv., 1906. 3 Winslow, Missouri Geol. Surv., Vols. VI and VII, and Buckley, Vol. IX. 4Hayes. Columbia (Tenn.) folio, U. S. Geol. Surv. FORMATIONS AND PHYSICAL HISTORY a6 calcium phosphate, valuable as a fertilizer. The workable deposits have resulted from the leaching out of the calcium carbonate from the phosphatic limestone, leaving the less soluble calcium phosphate concentrated at the surface (Fig. 333). The manganese ore of Arkansas had a similar origin. Foreign Ordovician The Ordovician formations appear at the surface in various parts of Europe, and they exist beneath younger formations over considerable areas where not seen. Fig. 334 represents the general Fig. 334. Diagram showing the relations of land and water in western Europe in the Ordovician period. The shaded parts represent areas of marine sedimenta- tion. (After DeLapparent.) geographic relations of land and water in Europe during this period. The submerged area represents in a general way the area where the Ordovician formations are present. In contrast with North Amer- ica, the Ordovician formations of Europe are largely fragmental. In the British Isles Ordovician strata are very thick (something like 376 ORDOVICIAN PERIOD 24,000 feet maximum).! Locally (Wales), nearly half the system is of igneous rock, including sheets of lava and beds of pyroclastic material. This is one of the most extensive, as well as one of the most ancient, volcanic tracts of Europe. From north England and Wales the system thins in all directions. The Ordovician of Europe is generally conformable on the Cambrian, but over considerable areas it is unconformable beneath the Silurian. In other continents the Ordovician strata have not, as a rule, been separated from the overlying Silurian, but they are known in South America, Australia and China. Duration and Climate The duration of the Ordovician is perhaps no better known than that of the Cambrian, but the period was probably somewhat shorter than its predecessor. Neither in Europe nor in America is there decisive evidence of distinct climatic zones. All that is known of the life would seem to indicate that the climate was much more uniform than now where the strata of the period are known. The fact that Ordovician rocks have been identified in the far north by fossils akin to those of low latitudes, indicates that the climatic conditions of North America and Europe must have been less diversified than now. This appar- ent lack of diversity of temperature through wide ranges of latitude is one of the unexplained problems of geology. Its solution is possibly to be found in a much higher average temperature of the ocean.” If the body of the ocean-water was relatively warm (in- stead of cold as now), it would have done much to counteract the effect of slight insolation in high latitudes during the cooler part of the year. LIFE From Cambrian to Ordovician, there was no pronounced break in the succession of life. The time from the beginning of the first to the close of the second of these periods appears to have been one long eon of progressive development and expansion of life. The fossil record of the Ordovician is fuller than that of the Cam- brian. ‘This is due partly to an increase in fossilizable forms, partly 1 This measurement is doubtless subject to the strictures set forth on p. 355. 2 Chamberlin and Salisbury, Earth History, Vol. III, pp. 437-445. LIFE 377 to an increase in numbers of individuals, and partly to better con- ditions of preservation. The general aspect of life was cosmopolitan, though it was not che same everywhere. It varied with the physical evolution of the continent, and largely as the result of it. The variations assumed three general phases: (1) adaptation to the immediate physical environment, particularly the nature and depth of the sea-bottom; (2) modification by auto-evolution within areas isolated by barriers (provincial evolution); and (3) modification toward a universal type through intermigration (cosmopolitan development). (1) Rocky, sandy, muddy, and calcareous bottoms had their appropriate life, as did also tracts of shallow and deep water. The faunas adapted to these special conditions were not altogether un- like, for some animals, particularly free-swimmers, were indifferent to them. (2) Although the sea covered a large part of the continent, affording facilities for the migration and mingling of faunas, there is evidence of some separation into zodlogical provinces. ‘This was probably due partly (a) to barriers in the form of shoals, bars, and spits, (b) to ocean-currents with their attendant differences in tem- perature, and (c) to variations in the saltness of the waters. (3) Notwithstanding local and provincial modifications, the progress of Ordovician life in the American continent seems to have been, on the whole, in the direction of cosmopolitanism, especially in the shallow water faunas of the great interior of the continent. This was due, primarily, to the wide epicontinental seas, which permitted free migration. The Ordovician system contains an exceptionally large number of fossils of free-floating graptolites! (Fig. 343). Their remains are mingled with the fossils of shallow-water life, showing that they swam over the epicontinental seas freely. The Ordovician grapto- lites are nearly identical in Europe, North America, and Australia. The history of individual species was short, geologically speaking, and hence the succession of species marks the progress of events in all parts of the ocean. During the lifetime of the graptolites (limited to the late Cambrian, Ordovician and Silurian), a score of successive zones, each characterized by particular species, have been 1Tt is not universally agreed that all graptolites were floating forms at all stages, but there seems to be little doubt that they usually were in their young stages at least. 378 ORDOVICIAN PERIOD identified. One of these zones falls in the Cambrian, eight in the Ordovician, and eleven in the Silurian. If these be taken as chrono- logical bench-marks, the successive horizons of the different conti- nents may be correlated accurately,.and the progress of life in the various quarters of the globe referred to a common standard. Marine life. The known faunas of the Ordovician consist almost wholly of marine invertebrates, among which trilobites and brachiopods hold the leading places. Brachiopods are most numer- ous, trilobites highest in organization, and cephalopods most power- ful; but the foreshadowings of a new dynasty are at hand, for re- mains of fish are found in this system. cz \\\\ ea He ER 1B Ss ae = oy 0.4 be = a0 o.gas Ex3coe <9.83 6548 OoHOg Zaks S bo% 6 6, O22 n8 SSSA Zuo BH o a 8S opp e-em =| 9 Sg eS gE Ker sd eel as: O..0'0 ws O © issippi basin.) 1SSISS. the lower M 4xO DEVONIAN PERIOD regions were contemporaneous, they were probably deposited in waters which were not connected (Fig. 358). Toward the end of the Hamilton epoch, the barrier which separated their waters seems tc have been removed sufficiently to allow the waters and the life on opposite sides to mingle freely (Fig. 359). General Considerations Outcrops. While the Devonian system is widely distributed in North America, it does not appear at the surface in large areas. _ The reasons are substantially the same as those for the limited exposures of earlier systems. The removal of Devonian from areas it once covered is oddly shown near Chi- cago, where a small remnant of Devonian TTI sediment has been found in a fissure in the Fig. 360. Figure illus Niagara limestone, as shown in Fig. 360. trating the occurrence of The limestone was apparently fissured remnants of Devonian ma- , ‘ terial in fissures in Niagara before the Devonian sediments were de- limestone, near Eliahurst posited upon it. Portions of the sedi- (Cook Co.), Illinois. ments fell into an open fissure, carrying with them distinctive fossils (fish teeth). In this protected position, the fossils escaped removal. Igneous rocks. Igneous rocks have little representation in most parts of the system in North America, but in Nova Scotia, New Brunswick, and Maine, and at some points in the west, there are igneous rocks which appear to be of this age. In many places in the west, Devonian strata have been affected by dikes and intru- sions of later times. Close. The general quiet which had prevailed during the period seems not to have ended at its close. Only in the eastern part of the continent, so far as now known, in Maine, Nova Scotia, New Brun- swick, and the adjacent region to the north were Devonian strata , notably disturbed at the close of the period. Elsewhere the for- mations of the younger system rest on those of the older without stratigraphic break. Economic Products The Upper Devonian is the chief source of oz! and gas in western Pennsylvania and southwestern New York, and is one of the sources in West Virginia. ‘he Middle Devonian is oil-producing in On- tario. Within the regions of their occurrence, oil and gas are more likely to be founda under low anticlines than in other positions, FORMATIONS AND PHYSICAL HISTORY Alt apparently for the reason that anticlines furnish an inverted basin capable of holding these substances against the pressure of the (e] SAE NO Fig. 361. Section showing the relations of the Devonian and other Paleozoic systems in the vicinity of Loudon, Tenn. =Cambrian; O=Ordovician; S=Si- lurian; D=Devonian; aw=age unknown. Length of section, about 7 miles. (Keith, U. S. Geol. Surv.) heavier subterranean water which tends to force them to the sur- face. In all cases it appears that there must be impervious beds above to prevent the escape upward of the oil and gas. The Devonian of central Tennessee is the horizon of black phosphates, which are of importance commercially. Foreign Devonian Europe. At the close of the Silurian there seem to have been more considerable geographic changes in Europe than in America, for the Devonian system there is more commonly unconformable on its base. During the progress of the period, Europe was pro- gressively submerged, for the Middle and Upper Devonian forma- tions are more widespread than the Lower (Fig. 362). In the British Isles the Devonian system has two phases. The first is found in the area which gave the system its name (Devon- shire). The system here is thick and of marine origin. Igneous rocks are associated with the sedimentary, and the system has valuable ore-bearing veins, as in Devon and Cornwall. The second phase of the Devonian is the Old Red Sandstone, widely distributed in Great Britain and Ireland and found at some points on the continent. Concerning the history of this sand- stone there has been much difference of opinion, but it is believed to have been deposited in a series of inland lakes or seas, the waters of which were fresh or brackish. Since species of marine fossils occur at some horizons, the sea had access to the basins at times. It is not improbable that some parts of this singular sandstone are of subaérial, rather than subaqueous, origin. The Old Red Sand- stone has some features like those of the Catskill formation of America. In the British Isles, the Old Red Sandstone has great thickness and includes much igneous rock. 1 Columbia (Tenn.) folio, U.S, Geol. Surv. 412 DEVONIAN PERIOD le = | i ™ a hh GG. A a i Mh it [ | A i ‘i | @ . : Fig. 362. Sketch map of Europe during the Devonian. The horizontal lines represent the Lower Devonian; the vertical lines mark the additional areas where the Middle Devonian occurs. (Z After De Lapparent.) In the Devonian of Germany much igneous rock is interbedded with the sedimentary. The igneous rock occurs in many separate beds, showing that there were many periods of igneous activity separated by intervals of quiet. In not a few places, especially where the sedimentary rocks have been invaded by igneous rocks, mineral veins have been developed, and from them large quantities of iron, tin, copper, and other metals have been obtained. The Devonian of Russia is made up of beds of arenaceous and calcareous rocks, the former containing fossils related to those of the Old Red Sandstone, the latter containing fossils of a marine fauna. The Lower Devonian appears to be wanting in much of Russia, and the Middle and Upper parts of the system are in most places unconformable on subjacent formations. Other continents. The Devonian system has wide distribu- tion in Siberia and China, and is known at many points in southern Asia. It occurs in North and South Africa, in New South Wales, chin. LIFE 413 and Victoria and New Zealand, and the Lower Devonian especially has considerable development in South America. Climate Conclusive evidence of great diversity of climate, or of variations of climate during the period, are not at hand. The Old Red Sand- stone and the Catskill formation perhaps point to aridity, but this can hardly be affirmed. In formations thought to be Devonian, evidences of glaciation have been reported from South Africa,! but the evidence is perhaps not conclusive. LIFE The Marine Faunas At the beginning of the period shallow-water faunas were re- stricted to limited bodies of water about the continental borders. The life of these several bodies of water developed differently. The early Devonian life consisted of the expansions of these provincial faunas. When in the early Devonian the sea invaded the land from these different embayments, the advance from each carried its own somewhat peculiar fauna toward the interior. The faunas invaded the continent more or less simultaneously, but they reached the interior more or less successively. The following faunas have been recognized: (1) the Helderberg, (2) the Oriskany, (3) the Onondaga (Corniferous), (4) the Southern Hamilton, (5) the Northwestern Hamilton fauna, and (6) the late Devonian fauna. They reached the interior in the order named. As each in turn came in contact with the preceding fauna, there was a mingling of the two, resulting in the destruction of some species and the modification of others. A new, composite fauna developed from the survivors. Helderberg fauna. The Helderberg fauna seems to have developed from the late Silurian fauna in the embayment at the mouth of the St. Lawrence and on the border of the adjacent con- tinental shelf, and perhaps also on the border of southern Europe. It appears to have found its way into the Appalachian valley- trough, and thence to have spread westward and northward, but not beyond the eastern part of the great interior region. Perhaps it reached the interior also from embayments on the southern coast. The fauna had much in common with the contemporaneous fauna (Hercynian) of southern Europe, but both differed markedly from 1 Schwarz, Jour, Geol., Vol, XIV, p, 683, and David, Q. J. G.S., Vol, XLIII. 414 DEVONIAN PERIOD the early Devonian faunas of the northern latitudes of Europe and America. | The main features of the Helderberg fauna were great numbers of mollusks and brachiopods, an erratic tendency of the trilobites, Be Bete Fig. 363. HELDERBERGIAN Fossits: a, Polypora lilea (Hall), a fenestelloid bryozoan representative of a group which was of great importance later; b, Miche- linia lenticularis Hall, the earliest member of a genus of corals which became abun- dant inlater Devonian faunas; c, Lepocrinites gebhardii Con., one of the last represen- i tatives of the cystids. d-n, Brachiopods: d, Rensseleria @quiradiata (Con.), a representative of a genus characteristic of the Lower Devonian; e, Spirifer macro- pleurus (Con.), a species closely related to the Silurian species of the genus; f, : Strophonella punctulifera (Con.); g, Schizophoria multistriata (Hall); h, Uncinulus mutabilis (Hall), a representative of a genus which had its greatest development in the Helderbergian fauna; 7, Gypidula galeata (Dal.), one of the most characteristic species of the Lower Helderberg; j, Bilobites varicus (Con.), a type of orthid charac- ; teristic of the Silurian and Helderbergian; k, Eatonia medialis (Van.), a representa- ‘ tive of a genus most characteristic of the Lower Devonian; /, Rhipidomella oblata (Hall); m, Leptena rhomboidalis Wilck., a species which ranges from the Ordovician to the Mississippian; x, Airypina imbricata (Hall), a lingering Silurian type; 0, Actinopteria textilis (Hall), a winged pelecypod of a type which had great expansion in the Devonian; p, Plalyceras gibbosum Hall, a capulid gastropod; g, Dicranurus hamatus (Con.), a trilobite whose closest relative occurs in Barrande’s Etage G, in Bohemia; 7, Phacops logani Hall, a representative of a genus of trilobites which had its greatest development in the Devonian. | LIFE PP er a paucity of crinoids and corals, and a notable absence of jishes. Fig. 363 shows some of the characteristic forms. Oriskany fauna. The Oriskany fauna was a sand-loving fauna which followed the Helderberg into the interior apparently by a similar route. Its place of origin is not known with certainty, but its habitat was probably on the Atlantic coast. It was bound by many ties to the Helderberg fauna, but contained distinctive features, implying a partly separate origin. On the whole, this fauna was essentially an assemblage of well-fed mollusks and mol- luscoids, with but a sprinkling of other types. Brachiopods were, on the whole, the most distinctive forms. Fig. 364. OrisKANY Fossits. Brachiopods: a, Rensseleria ovoides (Eaton); a representative of a genus restricted to the Helderbergian and Oriskany (see Fig. 363, d); b, Hipparionyx proximus Van., one of the most characteristic fossils of the arenaceous Oriskany beds; c, Camarotechia barrandei (Hall), one of the large rhynchonelloid shells of the Oriskany; d, Spirifer murchisoni Castel, and e, S. areno- sus (Con.), two of the most characteristic Oriskany species, the first occurring throughout the fauna, the second mainly in the fauna of the arenaceous beds; f, Stropheodonta magnifica Hall, a species which sometimes grew to be four or five inches across. The genus has its great expansion in the Devonian. The figures are much smaller than the fossils, the largest shells being 4 to 5 inches across. The large size of the Oriskany brachiopods may be appreciated by comparison with Fig. 363, the brachiopods of which are reduced to the same extent as those of this Fig. Onondaga fauna. The Onondaga fauna was distinguished from the preceding by hosts of marine fishes of divergent types. From this time on fishes were abundant in the epicontinental waters of America and Europe, and doubtless ranged widely over the seas. a6 | DEVONIAN PERIOD A feature of the Onondaga formation consists of thin layers (‘‘bone- beds’) made up almost wholly of their plates (scales), teeth, spines, etc. Among the fish were (1) arthrodirans whose necks were so joined to their bodies as to give their heads vertical motion, a rare feature among fishes; (2) sharks of various types; and (3) ganoids with cartilaginous skeletons and bony scales, in contrast with the modern feleosts which have bony skeletons and membranous scales. These fishes seem to have been more fully clothed with spines and defensive armor than their descendants. Compared with existing species, they were doubtless heavy, clumsy, and sluggish. From the degree of development already attained, it may be inferred that Fig. 365. ONONDAGAN Fosstts: a, Zaphrentis ponderosa Hall, a medium- tized, simple horn coral; b, Nucleocrinus verneuili (Troost), a blastoid abundant in one layer of the Onondagan limestone in the Ohio Valley; c-h, brachiopods: c, Stropheodonta concava Hall; d and e, Productella spinulicosta Hall, an early represen- tative of a genus which became abundant in the Upper Devonian, and gave rise to the typical Productus of the Mississippian and Pennsylvanian faunas; f, Spirifer acuminatus (Con.), a characteristic Onondagan brachiopod; g and h, Crytina hamiltonensis Hall, two views of a species having a wide geographic distribution and a great geologic range in the Middle and Upper Devonian; 7, Tornoceras mithrax (Hall), the first goniatite in America. The goniatites are distinguished from earlier cephalopods by their lobed sutures; 7, Conocardiwm trigonale Hall, a dorsal view of a common Onondagan pelecypod; k, Platyceras dumosum Con., a capulid gastropod with large hollow spines; /, Odontocephalus egeria (Hall), a trilo- bite showing ornamentation of the border of the head and tail. LIFE 417 their ancestors had been living for a long time in the region where they originated, probably somewhere in the north. Another significant feature of the Onondaga fauna is the pro- fusion of corals. From the rapids of the Ohio at Louisville, more than 200 species have been collected, embracing both the simple cup form (a, Fig. 365) and the compound type. Some of the cup corals attained a length of 18 inches and a diameter of 3, but the range in size was great. The reef-building habit attained greater development than in Silurian times, the reef at the rapids of the Ohio being the most famous example. Crinoids were rather few, but they do not appear to have lost their vitality, for they were abundant later. Large Brachiopods and cephalopods were plentiful. It will be remembered that in the primitive types of the cephalo- pods, the septa of the shells were plane or symmetrically curved, and that their juncture with the outer shell was a simple curve. In the Onondaga epoch, one form had septa which were bent abruptly, and suture lines which were lobed (7, Fig. 365). This was the first notable step in a remarkable series of crumplings of the septa which developed later. Gastropods similar to those of the earlier Devo- nian faunas were present, and the spines of the shells had now become pronounced in one group of them, perhaps signifying the necessity of defense against the abundant fishes and cephalopods. Pelecy- pods were abundant, many of them descended, no doubt, from Helderberg and Oriskany ancestors. Trilobites were present in more than half a hundred species, some of them being highly orna- mented. It seems clear that some of the species were descendants from the Helderberg and Oriskany faunas. Other prominent elements of the fauna, particularly the fish, cephalopods, and corals, seem, with equal clearness, to have come in from some other source. The striking features of the fauna seem to be explained by supposing that there was a generating tract to the north,' either on the Ameri- can or European continent, and that from this source migration into the interior sea of North America took place as the waters from the north extended themselves over the continent. As the result of the invasion, some part of the Oriskany fauna which already occupied the interior sea was driven out or destroyed, while the rest intermingled with the northern invaders. 1 This conclusion is not universally accepted. See Schuchert, Bull. Geol. Soc. Amer., Vol. XX. 418 DEVONIAN PERIOD Southern Hamilton fauna. At the beginning of the Hamilton epoch, there was a great influx of muddy material into the eastern part of the interior sea, while farther west the formation of limestone continued as before. At about this time, it appears that a fauna whose forbears lived in South America entered the interior sea, and, joining the resident Onondaga fauna, gave origin to the Southern Hamilton fauna. The transformation was not so radical as that which attended the invasion which gave rise to the Onondaga fauna, because the invaders were then the master type. Fishes were a conspicuous part of the new fauna. The arthro- dirans reached their climax, and some of the species were among the largest fish ever known. Some of them had an estimated length of 20 feet, and had strong mandibles 2 feet long (Fig. 366) which, Fig. 366. Diagrammatic front view of the dentition of -Dinichthys herzeri, Huron Shales, Delaware, O. (After Newberry.) in lieu of teeth, had cutting edges that closed, shears-like, after the fashion of the mandibles of turtles. The front part of the body was encased in heavy plates. Some of the fin-spines of sharks were a foot long. - In both groups of fish the devices of warfare make up nearly the whole record, and this doubtless implies the conditions in which the vertebrates lived. Polyps were affected adversely by the muddy waters. Crinoids were abundant locally, certain beds of limestone being composed largely of their remains. Brachiopods reached their climax at about this time. Among them, the spirifers attained their greatest extension of hinge-line (j, Fig. 367) a feature characteristic of the Hamilton epoch. The muddy bottoms favored mollusks. Gonta- tites increased in numbers and size (Fig. 367, 0), and pelecypods still more, the number of known species approaching’ 200. At this time appeared the first known barnacles of the modern sessile type. In losing its pedicel and in fixing itself immovably on other objects, it became degenerate, but it found a lowly place to which it has sate: 5, SES ie 2.1 BA ATs tracker Fig. 367. REPRESENTATIVE HAMILTon Fossits: a, Fenesteila emaciata Hall, a type of bryozoan common in the Middle Devonian; b, Arthracantha punctobrachi- ata Williams, one of a genus of crinoids restricted to the Middle and Upper Devo- nian; ¢c, Eleutherocrinus cassedayi S. and Y., a peculiar, irregular blastoid; during life it probably rested upon one side on the sea bottom. d, Echinocaris punctata (Hall), a crustacean more highly organized than the trilobites. e-j, brachiopods: e, Tropidoleptus carinatus (Con.); f andi, Chonetes coronatus (Con.); g, Vitulina pustu- losa Hall; h, Rhipidomella vanuxemi Hall, a representative of the orthids, which had great development in the Devonian; 7, Spirifer pennatus (Atw.), one of the long-hinge-lined spirifers most conspicuous in the Middle and Upper Devonian; k, l, and m, pelecypods: k, Cypricardella bellistriatus (Con.); 1, Pterinea flabella (Con.); m, Paleoneile constricta (Con.); three pelecypods common in the Hamilton. n, Loxonema hamiltonie Hall, a gastropod common in this epoch; 0, Goniatites vanuxemi (Hall), a characteristic cephalopod of this fauna; », Phacops rana (Greene) the most common trilobite of the Hamilton, and representative of a genus which has its greatest expansion in the Devonian; qg, Crypheus boothi Greene, one of the last of the dalmanites. hung with wonderful persistence, not unlike the debased human ‘class which it has come to typify. Northwestern Hamilton fauna. While the preceding fauna was developing in the eastern interior sea, another fauna was evolving on somewhat different lines in the northwestern sea which over- spread a large part of the northwestern interior (Fig. 358). Fora time this northwest sea was not in communication with the sea in 420 DEVONIAN PERIOD which the Southern Hamilton fauna lived (Fig. 358), but the inter- vening barrier disappeared finally, and the northwestern fauna overran the territory already occupied by the Southern Hamilton fauna (Fig. 359). This northwestern fauna was closely allied to the Devonian fauna of eastern and central Europe. The southward extension of this great arm of the sea took place late in the period, for the strata bearing its peculiar life lie on pre-Devonian formations in Missouri, Iowa, and Minnesota, and overlie the Hamilton in the more eastern region. Later Devonian (Chemung) fauna. The commingling and conflict which attended the invasion of the eastern and southern interior sea by the European and Eurasian faunas may be regarded as the controlling event in the evolution of the Upper Devonian fauna. As in the case of the Onondaga invasion, the northern immigrants were the more virile, and gave character to the com- posite fauna that arose later from the extinction of the weaker species, and the adaptation of the survivors to one another. ‘There were three dominant factors in this development, (1) the resident Southern Hamilton species, (2) the invading European and Eurasian species, and (3) the shallow and rather turbid waters in which these species met and merged. ‘The last of these factors showed itself in a notable rarity of corals. The brachiopods best express the outcome of the commingling of resident and immigrant species. Among them, as in the whole fauna, there was an indigenous set of species developed from the preceding residents, and an exotic set derived from the immigrants and bearing North-European characters. The latter was the more conspicuous. Among the mollusks, however, the case was the reverse, and the majority seem to have been de- scendants of the resident bivalves. Devonian fauna in the Great Basin area. In the Great Basin region of the west, a large area seems to have been occupied con- tinuously by the sea from about the beginning of Middle Devonian time to the later portion of the Carboniferous period. It seems to have been measurably free from both the physical and the biological changes which gave such diversity to the eastern provinces. Its fauna had a slow, continuous evolution, favored, from time to time, it would appear, by accessions from the north, and perhaps from other sources as well. None of the distinctive South American forms appeared in it, nor any of the peculiar Helderberg or Oriskany species. It is inferred, therefore, that it was shut off from the LIFE 421 eastern and southern interior throughout the whole Devonian period. On the other hand, a notable number of species were common to it and to the northwestern province. Life of Land Waters Certain Devonian formations, such as the ‘‘Old Red Sandstone’”’ and the Catskill formation, appear to be composed of deposits laid down in more or less local lodgment basins that were progressively filled by land-wash and fresh-water sediments. These basins appear to have been the home of a fresh- or brackish-water fauna, among which fishes, crustaceans, and ostracoderms were conspicuous. Perhaps the geological record presents no more suggestive combina- tion of ancient life. The type of the fauna was foreshadowed by the eurypterids and fishes, or fish-like forms of the late Silurian; but the record of that time is less perfect than that of the late Devonian. ‘ The center of interest in this fauna is found in the ostracoderms (Figs. 368 and 369), a class of animals between arthropods and Fig. 368. Restoration of Cephclaspis, seen from the side. (After Patten.) vertebrates. Their chief interest lies in their suggestion that vertebrates sprang from arthropods. The ostracoderms bear ex- ternal resemblances, in the head and trunk, to trilobites and king- crabs, while some of them have caudal fins and fish-like bodies. They were formerly classed as fishes, but no vertebre have been found, or appendages or jaws of the vertebrate type. Ostracoderms probably formed the climax and almost the end of their own strange race, for they practically disappeared with this period. This is not surprising in view of the development of powerful fishes, for the ostracoderms were obviously not a masterful race. Besides being small, they were clumsy, and their mouth-parts were weak. They probably plowed the soft bottoms of the sluggish waters, half buried in the mud, above which little beside their peculiarly placed eyes and the backs of the plated bucklers were habitually exposed. Another class of strange organisms related to the fishes, but not 422 | DEVONIAN PERIOD true fish, was represented by the singular little Palgospondylus (Fig. 370), which represents the vertebrate idea in great simplicity. Fig. 369. Reconstruction of the head and trunk of Tremataspis, seen from above. Natural size. (After Patten.) Fig. 370. Paleospondylus gunni, restored by Traquair; from the Old Red Sandstone, Caithness, Scotland. (After Dean.) It had a slender column of vertebrae, modified at one end into a head and finned at the other for a tail, without ribs, paired fins, or any suggestion of limbs. The fishes found in the supposed fresh-water deposits of the Devonian exceed in number and variety those found in contempo- raneous marine formations. Perhaps the strangest of them were the arthrodirans (Fig. 371), probably related to the ancestors of lung-fishes (Dipnot) which reached their climax at about this time. Ganoids were present, with many resemblances to amphibians, of LIFE 423. which they were, perhaps, the ancestors. Like lung-fishes, they appear to have been near their climax at this time, though they lived on till the Cretaceous. Sharks, now chiefly marine, seem to have lived in the open sea in the Devonian period, but their remains Fig. 371. A partial restoration of Coccosteus decipiens; from the Old Red Sand- BRON YY Owe are found also in the Old Red Sandstone and equivalent formations, so that they probably lived in fresh and brackish waters as well as in the ocean. Shells, probably of fresh-water mollusks, and closely resembling living genera have been found in association with land plants and fishes. Land Life Plants, snails, insects, myriapods, scorpions, and amphibians represent the known life of the land. The Devonian period covers much of the early development, though probably not the actual beginning of terrestrial plant life. It saw the origin of ferns, scouring rushes, lycopods, the seed- bearing relatives of the conifers, and probably the ‘‘seed-bearing”’ ferns... Devonian plants had, on the whole, little foliage, their leaves being spinoid and small. The presence of most of the fossil remains in fresh or brackish water or lowland deposits gives a suggestion of the habitats of the flora. It is inferred from the ‘1 David White, Jour. Geol., Vol. XVII, 1909. Many of the statements of the following paragraphs are from this article. 424 DEVONIAN PERIOD fossils that some of the plants were unable to stand alone, but sprawled about on the ground or clambered over other plants. Of the upland vegetation nothing is known. The Middle Devonian flora of Maine is so like a flora of Scotland, Belgium, and the Rhine provinces, as to indicate the probability of the migration of land plants between our continent and Europe, perhaps by way of a land bridge between America and Europe in the high latitudes. The Portage flora of New York is found also in Bohemia. The Upper Devonian flora was very similar from Penn- sylvania to southern Europe, and this flora has something in com- mon with that.of Australia. Devonian fossil woods show no rings indicative of seasons or long periods of drought. The types of Devonian plants were similar to those of the next period. The dominant forms were fern-like plants, some of which were seed-bearing, and the lower gymnosperms. ‘The forerunners of both lepidodendrons and sigillarias' were present before the close of the period. Angiosperms had not yet come into existence, so far as known. The forests were made up chiefly of (1) calamites (Equisetales) the gigantic ancestors of the horsetails, (2) lepidoden- drons, gigantic ancestors of the clubmosses, and (3) cordattes, all of which were better developed later. The record of the lower land plants is almost negative, except that, singularly enough, bacteria have been reported. The identi- Fig. 373. A, Platephemera antiqua, Sc., St. Johns, N. B. (After Scudder.) B, Xenoneura antiquorum, Sc. From St. Johns, N. B. (After Scudder.) fication of such simple forms in fossilized woody tissue of so ancient a period is remarkable, though the presence of bacteria is altogether probable in itself, for the record of plant life should have been more perfect than it is, had decay not been promoted by bacteria. The general aspect of the fern-like, seed-bearing plants was 1 For classification, see p. 685. LIFE 425 very like that of existing ferns. The larger number were herbaceous, but there were tree-forms not unlike tree-ferns in general appear- ance. ‘These plants were already far advanced in their evolu- tion, though little is known of their antecedents. They are gen- erally thought to have been the progenitors of cycads and of most or all other gymnosperms. In numbers, fern-like plants appear to have surpassed all others. Numerous wings and other fragments of insects have been found, chiefly near St. Johns, New Brunswick. Myriapods (thou- sand legged worms), arachnoids (spiders), and scorpions have been reported, and also terrestrial mollusks. CHAPTER XIX THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PERIOD The time from the close of the Devonian period to the end of the Paleozoic era was formerly regarded as the Carboniferous period. But this interval is now divided into two or three divisions, each with the rank of a period. If three divisions are made (as here), the first is the Mississippian (Subcarboniferous, Lower Carbonifer- ous) period. It represents a time of widespread submergence of the North American continent, and was brought to a close by wide- spread emergence. The second, the Pennsylvanian (Carboniferous, Coal Measures, Upper Carboniferous) period represents a time when the area between the Appalachian Mountains and the tooth meridian maintained a halting attitude, being now slightly above sea-level and now slightly below it. West of the Great Plains, sub- mergence was rather general, as during the preceding period. The third division of the old Carboniferous period is the Permian, a time of notable crustal deformation, general aridity, and, during part of the period at least, low temperature. FORMATIONS AND PHYSICAL HISTORY The foliowing subdivisions of the Mississippian system are recognized in the regions indicated: Mississippi River States Pennsylvania 4. Chester (or Kaskaskia) series (including Cypress sandstone below, and Chester beds above). 2. Mauch Chunk © 3. St. Louis series (including Salem limestone below and St. Louis and St. Genevieve limestones above). 2. Osage or Augusta (including the Burlington and Keokuk limestones, and Warsaw shale). 1. Pocono 1. Kinderhook (or Chouteau) East of the Great Plains In the early part of the Mississippian period, coarse sediments (sands and gravels, now a part of the Pocono formation) were gather- ing along the western border of Appalachia, while in the central part of the Mississippi basin the sediments of this stage ( Kinder- 426 FORMATIONS AND PHYSICAL HISTORY 427 hook) were partly calcareous. At the same time, the area of South- ern Michigan was a sort of bay or partly enclosed sea receiving sedi- ment from surrounding lands. Most of these formations are marine, but the Pocono has yielded fossils of land life. The formations of this stage are less widespread than those of later stages. In the second (Osage or Augusta) stage of the period, the sea of the interior was clearer, and the deposition of limestone was general. Submergence extended westward, probably to New Mexico on the one hand and to Montana on the other. The rich deposits of zinc ore (with some lead) in southwestern Missouri and eastern Kansas are chiefly in the Osage beds, though the metallic compounds were concentrated into ores at a later time. East of the Cincinnati arch, which was probably an island at this time, the deposition of clastic sediments continued. Those of eastern Ohio constitute a part of the Waverly series. Farther east, the accumulation of sand and gravel continued, or had been suc- ceeded by the deposition of the mud which constitutes the Mauch Chunk formation. The sediments of at least a part of this formation seem to have accumulated on land, rather than in the sea. In Maryland and elsewhere farther south, a formation of limestone (Greenbrier) lies between the Pocono and the Mauch Chunk. The St. Louis stage marks the time of maximum Mississippian submergence, so far as the western interior is concerned (Fig. 374). Limestone deposition continued in the Mississippi basin. It was at this time that the Bedford limestone! of Indiana (Salem or Spergen formation), famous as a building stone, was deposited. Much of this limestone, long mistaken for odlite, is made up of the shells of foraminifera. Many of the great limestone caves in Ken- tucky and southern Indiana are in the limestone of this epoch. In Michigan, beds containing salt (brine) and gypsum were being laid down, as at certain earlier stages in the period. In the northern part of the Appalachians, the Mauch Chunk shales were in process of deposition. Other names are applied to the contemporaneous deposits in the mountains farther south. Locally, deposits of this time contain both coal and iron ore. The Chester stage of the period was marked by more restricted waters and more varied sedimentation. The deposits of this stage resemble in a general way those of the Kinderhook stage. Those 1This name as applied to this limestone, is a trade name. As a geological term, Bedford is applied to a member of the Waverly series farther east. 428 MISSISSIPPIAN PERIOD Fig. 374. Map showing the areas, in black, where the Mississippian system appears at surface. The map also shows where the Mississippian system is thought to exist, though buried (the lined areas), and the area from which it is thought to have been removed by erosion (the dotted areas). By inference, also, the map shows the relations of land and water during the Mississippian period. FORMATIONS AND PHYSICAL HISTORY 429 were made while the sea was advancing on the land, these while it was retreating. Both are more restricted in their distribution than the beds of the intermediate epochs. In Illinois, the Chester sand- stone bears oil locally.! In summation it may be said that the Mississippian beds are largely clastic east of the Cincinnati arch, and largely calcareous west of it. It should be added also that the history of the Missis- sippi basin in this period is less simple than the preceding sketch might seem to imply, since there are several unconformities in the system, implying repeated emergencies of considerable areas. The extent of these unconformities has not been determined. In Nova Scotia, the system rests, locally, on much older forma- tions, and contains beds of red sandstone and gypsum. In the Great Plains and West of Them The Mississippian system is known in Oklahoma and South Dakota, where deformation and erosion have brought the strata to the surface (Fig. 374). Farther west the distribution of the system shows that the present mountain region, as far west as the 117th meridian, was mostly submerged, though there were perhaps numer- ous islands. North of the United States, also, marine conditions prevailed widely. Much of the system in the west is limestone, though clastic formations are not wanting. ‘The system is exposed about many of the mountains, and over considerable areas in Arizona and perhaps in New Mexico. It rests on the Ordovician in many places, and locally overlaps all earlier Paleozoic systems, lying on the Proterozoic. In parts of Colorado (Leadville) the Mis- sissippian limestone and dolomite constitute one of the richest ore horizons of the state. In many parts of the west the Mississippian system is unconformable beneath the Pennsylvanian.’ Igneous activity. According to present interpretations, there was great igneous activity in the west during this period. The area affected by vulcanism at this time, or soon after, extended from Alaska on the north to California on the south.* Dikes affect the system of Southern Illinois and adjacent parts of Ken- tucky, but the date of their intrusion is not known. 1 Bain, Econ. Geol., Vol. III, and Bull. 2, Ill. Geol. Surv. 2 The Mississippian is not differentiated from the Pennsylvanian on the maps of most of the western folios of the U. S. Geol. Surv., though the two are differ- entiated in the texts especially in the later folios, 3 Dawson, Can. Geol, Surv., 1886, p. 85. 430 MISSISSIPPIAN PERIOD General Considerations Thickness and outcrops. In keeping with the variations in the sediments, the thickness of the Mississippian system varies greatly. In Pennsylvania, there is a thickness of 1,400 feet of sandstone (Pocono), with 3,000 feet of shale (Mauch Chunk) above it; but so rapidly do the formations thin westward, that in the western part of the same state the equivalent formations have a thickness of only 300 to 600 feet. In the region of the Mississippi it reaches a maximum thickness of about 1,500 feet. In Oklahoma, the thickness is about 1,800 feet, in the Black Hills 275 to 525 feet, in Colorado (Crested Butte region) 400-525 feet, and in northern Arizona (Grand Canyon of the Colorado), 1,800 feet. Close of the period. At the close of the period, the eastern interior sea was contracted to narrow limits if not obliterated. Great changes took place in the western half of the continent too, for there is a widespread unconformity above the Mississippian system. In parts of the west, however, so far as now known, marine conditions prevailed uninterruptedly from the early Mississippian period to the later part of the Pennsylvanian. This great unconformity, and the great changes in life which accompanied the emergence which it records, is the basis for regarding the Mississippian a distinct period. Lower Carboniferous of Other Continents + In western Europe, two great series, or systems, are included under the Carboniferous, (1) the Lower Carboniferous, chiefly of marine origin, and (2) the Coal Measures or Carboniferous proper, Fig. 375. Composite diagrammatic section, showing the unconformity between the Mississippian and Pennsylvanian systems in Iowa. (Keyes, Ia. Geol. Surv.) ' The term Lower Carboniferous is here used, instead of Mississippian, because it is the term in common use in Europe, FORMATIONS AND PHYSICAL HISTORY 431 deposited partly in lagoons, marshes, and lakes, and partly in the sea. These systems correspond, in a general way, to the Missis- sippian and Pennsylvanian of North America. In the southern part of the continent the Lower and Upper Carboniferous forma- tions are like the Mississippian and Pennsylvanian of western North America, in that both are chiefly marine. In eastern Europe the Lower Carboniferous is partly non-marine and coal-bearing, while the Upper Carboniferous is largely marine. The Lower Carboniferous of western Europe is largely of lime- stone, which in Great Britain has received the name of ‘‘mountain Fig. 376. Map showing the relations of land and water in Europe in the early Carboniferous period. The shaded parts represent areas of marine deposition. (After DeLapparent.) limestone.’ East of the Rhine the Lower Carboniferous limestone is replaced by shale, sandstone, and even conglomerate, collectively known as the Culm. This phase of the system contains coal in some places. 432 MISSISSIPPIAN PERIOD The Lower Carboniferous of some parts of Great Britain and western Europe contains much volcanic rock. Some of the erup- tions were probably submarine, and some subaérial. The close of the early Carboniferous period was marked, in Europe, by widespread withdrawal of the sea from the area of the continent which it had covered. There were also some mountain- forming movements (folding), as in the Vosges Mountains, in east- ern France, and elsewhere. The development of the Ural Moun- tains appears to have begun at about the same time. These changes shifted the areas of sedimentation notably. In other continents, where geological work is less advanced, the Lower and Upper Carboniferous have not always been separated carefully, but the lower system exists in all of them. Climate and Duration Most of the data at hand indicate the absence of great diversity of climate during the period, and suggest that it was genial. The salt and gypsum in Montana, Michigan, Nova Scotia, and western Australia, imply aridity, but it is not clear that aridity was general. Certain conglomerate formations (in the Culm) of western Europe have been thought to indicate glaciation, but the evidence does not seem to warrant this conclusion. Recently, phenomena which have been interpreted to imply floating ice have been reported from Oklahoma.! The duration of the period probably was not less than the average duration of the Paleozoic periods. LIFE Marine faunas. Just as there was no great stratigraphic break between the Devonian and Mississippian systems in the American continent, so there was no radical break in the succession of life. Conspicuous elements of the Kinderhook fauna were (1) the beginnings of the great deployment of the crinoids, which reached their climax later in the period; (2) brachiopods, which were transi- tional between Devonian and Later Mississippian types, the genus Productus being conspicuous (Fig. 377, d. e.); and (3) abundant mol- lusks, pelecypods (i, 7, Fig. 377) being most numerous. Trilobites were few and small. Their high stage of ornamentation had passed, and the day of their disappearance was drawing near. Fishes, especially sharks, were abundant. 1 Taff. Bull. Geol. Soc. Am. Vol. xx. p. 701. LIFE 433 “4M * “Fig. 377. Krnperuook Fossits: a, Leptopora placenta (White), a compound coral. 6, Actinocrinus senectus M. and G., a distinctively Mississippian crinoid; c, Dichocrinus inornatus W. and Sp., one of the earliest crinoids with only two basal plates. d-h, brachiopods: d, Spirifer biplicatus Hall, a species retaining an elon- gate hinge line characteristic of the Devonian; e, Spirifer marionensis Shum.; f, Productella pyxidata Hall, a genus which had its greatest development in the late Devonian; g, Paraphorynchus striatocostatus (M. and W.), characteristic of Lower Kinderhook horizons of Iowa, Missouri, and Illinois; h, Productus arcuatus Hall, a genus developed from Productella, and characteristic of the Mississippian and later Paleozoic periods; 7, Grammysia hannibalensis (Shum.), a pelecypod; 7, Pernopecten cooperensis (Shum.), a pelecypod characteristic of certain of the higher Kinderhook horizons; k, Platyostoma broadheadi S. A. M., a capulid gastropod; 1, Macrocheilus blairi (M. and G.); m, Prodromites gorbyi (S. A. M.),a widely distributed cephalopod and the earliest form showing secondary lobing of the sutures; 1, Muensteroceras owent (Hall), abundant in the famous Kinderhook goniatite bed at Rockford, Ind.; 0, Préetus ellipticus M. and W. Trilobites were few in the Kinderhook, and this one illustrates their characteristic lack of ornamentation; p, tooth of Cladodus springeri St. J. and W., a shark; g, a spine of Acondylacanthus gracilis St. J. and W. _ The physical conditions of the Osage epoch furnish the key to the character of the Osage fauna. The extended shallow, clear sea 434 MISSISSIPPIAN PERIOD was a favorable field for the evolution of the varied assemblage of forms that had come together in preceding epochs under less favor- able conditions. ‘There is evidence also of rather free migratory communication with the Eurasian continent, since many species were common to America and Europe. No single group so well characterizes the Osage fauna and ex- presses its dependence on physical conditions as the crinoids, whose abundance and diversity were climacteric (Fig. 378). Their rapid Fig. 378. OsSAGE EcHINODERMS: 4-d, crinoids; a, Barycrinus hovevi Hall; 6, Eretmocrinus remibrachiatus (Hall), having spatulate arms; c, Actinocrinus lobatus Hall, shows highly ornamented plates; d, Forbesiocrinus wortheni Hall, a flexible crinoid; e, a blastoid, Oligoporus mutatus Keyes. decline after this epoch is one of the most remarkable incidents in the life history of the invertebrates. In the day of their glory, the crinoids were most prolific, as indicated by the fact that a single genus (Batocrinus), had more than a hundred species. Their orna- mentation was notable, and as in the case of the trilobites, preceded their decline. The repetition of this phenomenon at different times and in different groups of organisms is worthy of notice, though its meaning is not altogether clear. Crinoids made large contributions LIFE 435 to the limestone of the period. Other echinoderms were not very abundant. It is a matter of surprise that corals were so few, in view of the favorable physical conditions. Their paucity probably is to be explained by unfavorable organic conditions or relations, such as unrecorded enemies, or more successful rivals. Brachiopods (Fig. 379) were abundant, and some of their species ranged to the eastern g n i Fig. 379. OsAGE Fossits: a, Zaphrentis centralis E. and H., the most char- acteristic coral of the Osage. 0-7, brachiopods: 6, Spirifer suborbicularis Hall; a closely allied species occurs in Europe. c, Athvris lamellosa L’Eveille, a species common to America and Europe; d, Spirifer logani Hall, the American representa- tive of Spirifer striatus of the European Mountain limestone; e, Productus burling- tonensis Hall, a species abundant in the Lower Osage; f, Leptena rhomboidalis Wilck, a species which persisted from the Ordovician to the Osage; g, Rhipidomella burling- tonensis (Hall); h, Reticularia pseudolineata (Hall), a spire-bearing brachiopod closely allied to species in the European Mountain limestone; 7, Schizophoria swallovi Hall, one of the last of the orthids. continents. Mollusks were very subordinate. There were a few lingering ¢rilobites, an abundance of bryozoans, some supposed sponges, and doubtless many forms not readily fossilized. Marine plants left but an obscure record. The Waverly fauna, east of the Cincinnati axis, was more provin- cial than the Kinderhook and Osage faunas. It was the direct descendant of Devonian faunas that occupied the same ground, and had changed but slowly. It was modified by some immigration of 436 MISSISSIPPIAN PERIOD Kinderhook and Osage types, and took on slowly a Mississippian aspect, while retaining many Devonian characteristics. Its most prominent members were the pelecypods, as might have been antici- pated from the silty conditions. The Great Basin fauna of the first half of the period records a gradual evolution of the Devonian fauna of the same region, with perhaps the addition of a few immigrants from the west. After the Osage epoch, the Basin fauna united with the Osage fauna of the interior, and this union had an important effect on the later Mississippian faunas of the interior. Previous to the union, the salient features of the Great Basin fauna were the (1) rarity of crinoids; (2) among brachiopods the absence of spirifers, so characteristic of the Osage fauna, and the presence of the genus Productus, closely allied to species of the Osage fauna and probably developed by parallel evolution; (3) the pre- ponderance of pelecypods over brachiopods; (4) the abundance of gastropods, among which were air-breathers, the oldest aquatic pulmonates known; and (5) plentiful cor- als, the horn-shaped type predominating. Cephalopods and trilobites were few, and no fishes have been reported. Unless this is due to the imperfection of the record or of present investigation, it adds much to the evidence of the distinctness of the pro- vince, for fish abounded in the eastern sea. The barrier which separated the Great Basin and the Kinderhook-Osage seas é appears to have been an elongated insu- Fig. 380. Upper Mis-_ lar tract lying between the Rocky Moun- SISSIPPIAN ECHINODERMS: tains and the Great Basin. The yielding ita ne? ee of this barrier about the close of the Osage which lost its stem and epoch, by erosion or submergence, permit- became a free swimming ted the singular semi-Devonian, semi- creature, at least in its See eA y i adult condition; 6, Acro- Mississippian fauna of the west to invade crinus amphora W.andSp., the greater eastern sea. The late Missis- a. specialized camerate cri- sip pian (St. Louis) faunas of the interior noid with a large number ; . of supplementary plates include (1) the culmination of the cosmo- introduced between the politan evolution of the marine life of the basal and radials; ¢, Pen- Wrississippian period on the North Ameri- tremites robustus Lyon, a ‘ iS Soa ; blastoid. can continent, and (2) the initiation of its LIFE 437 decline. The most distinctive feature was the commingling of the Great Basin and the Osage faunas. It introduced into the main Mississippian sea what seemed to be a retrograde change, for species of Devonian aspect that still lived in the isolated Great Basin province and elsewhere, migrated eastward, and their relics are found with species whose evolution had reached an advanced Mississippian phase. Crinoids were less plentiful than in the Osage fauna, and notably changed (Fig. 380). Of one group which had upwards of 300 species in the Osage fauna, less than 25 species are known in the later faunas, and among the 25, no Osage species is found. Other groups of Bs ec SASL eri ogt 8 7) ie Ee yi | Baas Fig. 381. CHARACTERISTIC UppeR MIssIssIpPIAN Fossits: a, Endothyra baileyi Hall, a small foraminifer, much enlarged, abundant in the Bedford limestone of Indiana, and often mistaken, in the past, for an odlitic concretion; b, Archimedes swallovanus (Hall), a bryozoan having a peculiar screw-like axis for the support of the colony. c-h, brachiopods: c, Spiriferina spinosa (N. and P.), a genus which developed from Spirifer, and has its greatest development in the late Mississippian and Pennsylvanian; d, Seminula subquadrata (Hall), a species closely related to Pennsylvanian types; e, Spirifer increbescens Hall, a species characteristic of the later Genevieve faunas; f, Eumetria marcyi (Shum.), a representative of a genus abundant in the Genevieve faunas. It was present in the Kinderhook, but has not been found between the Kinderhook and the closing stages of the Osage; g, Produc- tus fasciculatus McCh.; h, P. marginicinctus Prout; 7 andj, pelecypods: 7, Schizo- dus chesterensis M. and W.; 7, Conocardium prattenanum Hall; k-m, gastropods: k, Bellerophon sublevis Hall; 1, Pleurotomaria nodulostriata Hall; m, Eotrocus con cavus Hall. mn and o, cephalopods: x, Orthoceras annulato-costatum M. and W., one of the ancient type of straight cephalopods, occasional species of which per- sisted to the end of the Paleozoic; 0, Goniatites kentuckiensis S. A. M. 438 MISSISSIPPIAN PERIOD crinoids, however, did not show so remarkable a decline, and newand curious forms appeared. Blastoids had their climax here so far as numbers of individuals are concerned, although there was greater diversity in the Osage fauna. A swift decline seems to have fol- lowed this climax, and the beautiful forms disappeared for reasons quite unknown. Polyps seem to have profited by the decline of the crinoids, or for other reasons, for they were more numerous than in the Osage fauna. The simple horn-shaped forms were the most common. Bryozoans made a new departure in their mode of support. The delicate branches of their colonies could not extend themselves indefinitely without special means of support. As one mode of securing this support, the genus Archimedes (Fig. 381, 6), which made its first appearance in the Osage, secreted an axis with a spiral flange upon which the colony spread itself, producing a unique form resembling slightly Archimedes’ screw. Archimedes became so abundant in the Kaskaskia epoch that a part of the series is known as the Archimedes limestone, because of the great abundance of fossils of this genus. A notable change took place in the brachiopods (Fig. 381), though Productus (g and h) continued to be abundant and charac- teristic. An odd feature was the small size of the brachiopods in the Bedford limestone of Indiana. The associated fossils of other kinds also were dwarfed, implying pauperizing conditions of some sort, for the species seem to be identical with those that grew larger else- where. It is not improbable that this limestone was deposited in a partially isolated body of water that was so highly charged with lime and other salts as to be somewhat unfavorable to life. A similar dwarfed fauna is recorded from Idaho. Among mollusks, pelecypods (Fig. 381, 7,7) were rather abundant, and some of them still had a Devonian aspect. Those in the Indi- ana foraminiferal limestone were small, like the brachiopods. Gastropods were more diversified than in the Osage fauna, and some Devonian genera which apparently had been absent from the Osage, reappeared. Sharks (Fig. 382) were important and other fish were present. The most striking peculiarity of the fauna resulted from the invasion of the more conservative fauna of Devonian aspect from the sea of the Great Basin, and perhaps from a similar incursion of lingering forms from the Waverly gulf on the east. The remarkable LIFE 430 thing is that these should have succeeded, so far as they did, in impressing themselves on the composite result, and in giving tone to the whole. It is more natural to expect an antiquated fauna to be overwhelmed by a younger and more progressive one. Fig. 382. Cladoselache fyleri Newb. Restoration by Dean. About 1/5 natural size. From Cleveland Shales, Ohio. With the close of the Mississippian period, the chief center of life interest passes from the sea to the land, first to the vegetation of the Coal period, and then to land vertebrates. The history of the marine invertebrates will hereafter be followed with less full- ness. With the introduction of fishes it had reached its great adjustments, and its further history bears a close likeness to the struggles and adaptations of the history already sketched. Evolution of fishes. Many of the ancient invertebrates were fixed, and their migrations were confined to the early stages of their lives; but fishes were rovers. While restrained by conditions of food, temperature, etc., they were relatively independent of local condi- tions. They appear to have invaded effectually the open sea for the first time in the Devonian period, though at that time, marine fishes seem to have been fewer than those of inland waters. But by the middle of the Mississippian period, marine fishes were in un- questioned supremacy, while the fresh-water forms had declined notably, so far as the record shows. In the seas, the supremacy of the sharks was almost uncontested. They were more abundant, 440 MISSISSIPPIAN PERIOD apparently, than in any later period. Some 600 species are known, more than half of them from North America. The fossils are chiefly teeth, spines, and dermal ossicles. Three-fourths of the species had crushing or pavement teeth, adapted to breaking the shells of mollusks and crustaceans, and the trituration of seaweeds. The arthrodirans and lung fishes had declined, as compared with the Devonian period. Of fishes frequenting inland and coastal waters, probably the culminating type was of the order to which the modern garpike belongs. The curious tribe of ostracoderms (p. 686). had nearly or quite disappeared. ) Land Life The record of land life is poor, but enough fossil plants have been found to show that the plant life of the early Mississippian land was little more than an expansion of that of the preceding period. There were, however, notable changes in detail. The geo- graphic diversity of the Mississippian floras was somewhat greater than that of the Devonian. The mid-Mississippian flora is thought by White ! to have had its origin on the islands of western Europe, and to have spread thence to Siberia and southward, even to South Africa and Australia; but by what route is not known. Seventy-five per cent of the species of a Mississippian flora of Argen- tina are identical with European species, a fact which suggests strongly a land bridge between South America and the continents just named. The flora of the closing stages of the period indicates adverse conditions of life, and prepares the way for the great floral changes which followed. From this stage comes the earliest wood which shows rings. The most interesting suggestion of advance in land life is found in the footprints of a supposed amphibian from the Mauch Chunk shale of Pennsylvania. They imply a stride of about thirteen inches, and a breadth between outer toes of eight inches. Nearly complete specimens of amphibia (labyrinthodonts) have been found in the Lower Carboniferous of Scotland. Probably insects and their allies lived, but their fossils have not been found. 1 Jour. of Geol., Vol. XVII, 1909. CHAPTER XX THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD FORMATIONS AND PHYSICAL HISTORY This system includes the Pottsville conglomerate (Millstone grit) below, and the Coal Measures above. Its most distinctive feature, so far as North America is concerned, is its coal. The Pottsville Conglomerate (Millstone Grit) The lowest formation of the system in the Appalachian region is sandstone or conglomerate, having different names in different regions. From its conglomeratic phase in the east, it grades into sandstone in the interior. It has not been recognized in the western part of America. Over wide areas it is unconformable on the Mis- sissippian system, as already noted. Locally as in parts of Illinois, the formation is oil-bearing. At various points in the east it con- tains thin beds of coal, and in the southern Appalachians, some thicker beds. The formation varies in thickness from a maximum of some 1,500 feet in the Appalachians, to less than 100 feet in some parts of western Pennsylvania. It is so firmly indurated that the outcrops of its tilted beds have become ridges in many places. The Coal Measures Above the Pottsville conglomerate and its equivalents in the central and eastern parts of the continent, lie the formations known as the Coal Measures. ‘They consist of a succession of alternating beds of shale, sandstone, conglomerate, limestone, coal, and iron ore. The succession differs greatly in different regions, but shale perhaps recurs more frequently than other sorts of rock, and in thicker beds. Both the coal and some of the iron ore are in layers interstratified with the other members of the series, and are to be looked upon as strata of rock. Important as the coal and iron ore are from an economic point of view, they make up but a small part of the Coal Measures. There are many beds of coal in some regions, and some 441 442 PENNSYLVANIAN PERIOD Fig. 383. Map showing the areas where the Pennsylvanian system appears at the surface in North America. The map also shows, as in preceding similar cases, the areas where the Pennsylvanian system is thought to exist though buried (lined areas); the areas where it is thought once to have existed, but to have been removed by erosion (dotted); and by implication the relations of land and sea during the Pennsylvanjan period. FORMATIONS AND PHYSICAL HISTORY 443 of them have great thickness (40 to 50 feet); yet the proportion of coal in the Coal Measures is rarely so much as 1:40, and that of iron ore is much less. ‘The classification of the Pennsylvanian system of the east now in common use is as follows?! 4. Monongahela 3. Conemaugh 2. Allegheny 1. Pottsville A twofold division is common farther west. Thus in Iowa the lower division is called the Des Moines, and the upper, the Mis- sourian. Productive coal-fields. The Pennsylvanian system does not contain coal in workable quantity everywhere, though coal is widely distributed as far west as the 96th or 97th meridian in Okla- a &Fipla . homa, and nearly to the rooth | meridian in Texas. The pro- ductive coal areas of the system in North America are six in xs number, as follows:? Tee esuniracite. field; of 1... ——._$ —_- —-— pee eG eastern Pennsylvania, with an Fig. 384. Map showing the areas of area of 484 square miles. It anthracite coal in Pennsylvania. includes several elongate, nearly parallel, synclinal basins (Figs. 384 and 385). From the associated anticlines, and from the neigh- Pennsylvanian \ 0 INA Fig. 385. Section across Panther Creek basin in the anthracite region of Pennsylvania, showing the structure and the coal beds (black). (Stoek, U. S. Geol. Surv.) boring shallower synclines, the coal beds have been worn away. The strata of this field may once have been continuous with those of the next. (2) The Appalachian field, which extends from Pennsylvania to Alabama (Fig. 386), has an area of about 70,000 square miles, of 1 Prosser, Am. Jour. Sci., 4th series, Vol. XI, p. 191, 1901. 2 22d Ann, Rept., U.S. Geol. Sury., Pt, II, p. 15, 444 PENNSYLVANIAN PERIOD ix OS ee ep of me eg 30 $00 Scale 3 Miles Fig. 386. Map showing the known distribution of coal in the United States. The black areas are the areas within which there is coal of the Pennsylvanian system (anthracite and bituminous). The areas marked by dots in Virginia and North Carolina represent Triassic (bituminous) coal. Those with vertical (lignite) and horizontal (anthracite, bituminous, and lignitic-bituminous) lines represent coal of the Cretaceous (Laramie) system, and those with diagonal (lignite) and crossed (bituminous and lignitic-bituminous) lines represent coal fields of Tertiary age. Some of the fields, as those of Washington and California, appear very small on this map. The Cretaceous and Tertiary areas include only those where there is known to be workable coal. (U.S. Geol. Surv.) which about 75 per cent contains workable coal. The western edge of the sharply folded Appalachian belt is the eastern edge of the Appalachian coal-field. With few exceptions, the strata of this field are horizontal, or gently undulating. (3) The Northern Interior field, confined to the southern penin- sula of Michigan, covers an area of about 11,000 square miles. The strata of this field dip gently toward its center. (4) The Eastern Interior field, centering in Illinois, covers an area of about 58,ooo square miles (Fig. 386), and about 55 per - cent of it is productive. This field is set off from the Appalachian field on the east, and from the Western Interior field on the west, by broad low anticlines from which the Coal Measures, if ever present, have been eroded. (5) Lhe Western Interior and Southwestern fields (lowa to Texas) covers an area of about 94,000 square miles. On the west this field FORMATIONS AND PHYSICAL HISTORY A4S is limited by the overlap of younger for- ( Feet mations. Except in Arkansas and Okla- | homa, where the strata are folded, the Coal Measures of this area are nearly horizontal. (6) The Nova Scotia-New Brunswick coal-field, on either side of the Bay of Fundy, contains an area of about 18,000 square miles. The coal is bituminous, of good quality. Non-productive areas. In the vicinity of Narragansett Bay, the Carboniferous system has great thickness, and locally rests on beds of Cambrian age. Coal occurs here, but it is too highly anthracitic a (or graphitic) to burn readily. The beds are much deformed and are associated with igneous rocks. Carboniferous rocks occur at other points in New England, where they are partly igneous (Fig. 389) or meta-igneous, and partly meta-sedi- mentary. West of the Great Plains. The system is widespread west of the Great Plains, and probably underlies the Plains them- selves. With rare exceptions, the western beds are coal-less, the abundant coal of that region belonging to later systems. The coal-less phase of the system, the whole earth considered; is far more wide- spread than the coal-bearing. In some parts of the west, the Car- boniferous system includes formations which resemble the ‘‘Red Beds” of the next (Permian) system. This is the case in the southern part of the Rocky Moun- tain region, and in the plains adjacent, and here the separation of the Pennsylvanian system from the Permian is not very dis- : tinct, or has not been carefully worked out, 0 | 1500 The black Picea 1400 The Pottsville portion Monongahela Series the Allegheny portion from Armstrong Co. (White [David] and Campbell); 1300 f (U.S. Geol. Surv.) a 1200 1100 900 & Conemaugh Series ‘ 800 700 & 600 F 500 B 400 ae Co. (I. C. White); the Conemaugh portion from Fayette Co. (I. C. White); the Monongahela portion from Fayette Co. (Stevenson). Allegheny Series 300 F Composite section of the Pennsylvanian system of Pa., compiled from various sources. the checked pattern limestone, the dots sandstone, and the broken lines shale. Pottsville Series Fig. 387. of the section is from Mercer bands represent coal, 446 PENNSYLVANIAN PERIOD The Carboniferous system of the west includes all sorts of sedimentary rocks, among which are considerable thicknesses of limestone. They are exposed at many points (Fig. 383) and their exist- ence over wide areas where they are now covered Fig. 388. Section showing the position and relations of the Carboniferous section near Estillville, Ky. C=Carboni- ferous (including Mississippian); D=Devonian; S=Silurian. Length of section about 16 miles. (Campbell, U.S. Geol. Surv.) by later deposits is certain. The system is, how- ever, not continuous. Numerous islands of older rock probably maintained themselves throughout the period, and a large area of land existed through- ea Oe Fig. 389. Section in northwestern Massachusetts, showing the position and relations of the Carboniferous system. Cw= igneous rock, Carboniferous; Sc (Conway schist) and Sg (Goshen schist) are Silurian formations; Of (Hawley schist), Os (Savoy schist), and Och (Chester amphibolite) are probably Ordovician, though classed with the Silurian in the Hawley folio. (Emerson, U.S. Geol. Surv.) out the Paleozoic era in western Nevada (west of long. 117°), and had an unknown extension north and south. Figs. 390 to 392 show the positions and rela- tions of the Mississippian and Pennsylvanian sys- <(H} tems at various points in the west. The sections E¢ are from regions where the strata have been much z % disturbed by folding, faulting, and the intrusion of igneous rock. North of the United States, Carboniferous strata (largely Mississippian) outcrop on the west side of the northward continuation of the Great Plains. These strata are probably continuous southward with the contemporaneous formations of the United States. Strata of the same age are found on both sides of the Gold Range of British Rs Ai! oe, Sfmt 4 ~ oe ARI 2 —_—/ ystem (as well as others) in the Yellowstone Park, fay 4 Jurassic, K = Cretaceous. Carboniferous, J (Hague, Iddings, and Weed, U. S. Geol. Surv.) C= Devonian, gth of section about 17 miles. Ordovician and Silurian, D= Cambrian, S$ Section showing position and relations of the Carboniferous s Fig. 390. Archean, € Neocene, and anp=igneous rock. Len AR N FORMATIONS AND PHYSICAL. HISTORY 447 Columbia. West of this range, the system includes much vol- canic rock, the greater part of which was extruded before the close of the period. The system is continued northward into Alaska,’ where it is less widespread than the Mississippian, so far Fig. 391. Section showing the position and relations of the Carboniferous sys- tem at a point in Colorado. AR=Archean; €=Cambrian; O=Ordovician; M= Mississippian; Cw and Cm=Carboniferous; J = Jurassic; Kd, Kb, Kn, and Km= Cretaceous. Length of section about 6 miles. (Eldridge, U. S. Geol. Surv.) \\\ A Fig. 392. Section showing the Carboniferous in the Sierras of central Cali- fornia. C=Carboniferous; J (Mariposa slates)=Jurassic; mdi=metadiorite; ams = Amphibolite schist; V =igneous rock of various sorts, of Neocene age. Length of section about 6% miles. (Ransome, U. S. Geol. Surv.) as present knowledge goes. In the Arctic lands of America, the Mississippian and Pennsylvanian are not differentiated. One or both are widespread. | Thickness. The thickness of the system has a wide range, but like all preceding systems of the Paleozoic, it is thick (4,000 to 5,000 feet) in the Appalachian Mountains. In the interior, it exceeds 1,000 feet in but few places; but in Arkansas, the Coal Measures have been assigned the remarkable thickness of more than 18,000 feet, from which it is inferred that there must have been land close at hand capable of supplying sediments in great quantity. This was probably the axis of the Ouachita uplift. In Texas, the thickness of the system ranges up to 5,000 feet, and in the west it is even thicker. Coal The general conditions under which sandstone, shale, and lime- stone originate have been outlined, but there has been no occasion heretofore to consider the formation of coal. From its economic importance, coal has been studied with more care than most sorts 1 Brooks, Professional Paper 45. 448 PENNSYLVANIAN PERIOD of rock, and geologists are agreed, in a general way at least, as to its mode of origin. . Origin. There is no doubt that coal is of vegetable origin. Except by the accumulation of vegetable matter, no way is known by which such beds of carbon could be brought into existence. Furthermore, the coal and its associated shales contain abundant remains of plants, in places even recognizable tree-trunks in the form of coal, and microscopic study has revealed the fact that much coal is but a mass of altered, though still recognizable vegetable tissues. Concerning the exact manner in which the beds of vege- table matter accumulated, and the conditions under which it was converted into coal, there is some difference of opinion. Much coal is essentially pure, containing little matter of any sort which was not in the plants which gave origin to it. Purity does not mean freedom from ash, since mineral matter, which on combustion becomes ash, is present in all plants. Along with the large amount of coal which is nearly pure, there is much which con- tains some earthy matter. Where the admixture of earthy matter is small, the coal is still usable; but from poor coal of this sort, there are all gradations into carbonaceous shale. The purity of some coal-beds over great areas warrants the conclusion that they were made of vegetation which grew where the coal is. The character of the vegetation shows that it grew on land or in swamps. Had it been washed down from its place of growth to the situations where the coal is, it should have been mixed with earthy sediment, and the product, after the necessary changes in the vegetable matter, would have been very unlike the purer coal-beds. Furthermore, the nearly uniform thickness of many of the coal-beds over great areas, some of them many thou- sand square miles, is a strong objection to the hypothesis that its substance was drifted together by any process whatsoever. Some other facts which support the theory that the vegetation grew where the coal-beds are, may be noted. (1) Beneath many coal-beds there is a layer of clay with roots (or root marks) in the position of growth. The clay seems to have been the soil in which the coal vegetation was rooted. (2) In association with the coal- beds, stumps of trees are found still standing as they grew (Fig. 393). (3) In coal-beds, or in the associated layers of shale, imprints of the fronds of ferns or fern-like plants are found. They are in places so numerous and so perfect as to indicate that they were COAL 440) Fig. 393. Showing a stump standing as it grew in Coal Measures, near Glas- gow, Scotland. buried where they fell, without being drifted by moving waters from one place to another. (4) In many cases, the layer of rock next overlying a coal-bed contains abundant remains of vegetation, especially in its lower part, as if the conditions which brought about its deposition resulted in the destruction of the forest growth which had preceded. In such situations, trunks of trees 50 and 60 feet long, and 2 or 3 feet in diameter, have been found. While it is confidently believed that most of the workable coal represents the growth of vegetation im situ, it is not to be understood that coal was never formed from vegetation which drifted together. In the formation of a coal-bed, three things are to be accounted for: (1) The conditions under which the necessary quantities of vegetable matter accumulated; (2) how it was kept from decay; and (3) how changed into coal. Accumulation of organic matter. Large marshes, or marshes in low surroundings, are the only places where vegetable matter is now accumulating in quantity, with little admixture of sediment. Thus in the marshes along some parts of the Atlantic coast (Fig. 394), there are quantities of organic matter which, locally, is mixed with little sediment. In Dismal Swamp, the stems, branches, leaves, and fruits of the trees, shrubs, and herbs which grow there, 450 PENNSYLVANIAN PERIOD have been long accumulating, and little sediment is mixed with them. In cypress and mangrove swamps, too, there are consider- able thicknesses of vegetable matter nearly free from mud, etc. The multitude of marshes and peat- aean in the United States and Canada are fur- ther illustrations of the accumula- tion of vegetable matter, in some cases mixed with abundant sedi- | ment and in some nearly free from it. The vegetation | in swamps need not be more luxuriant than | that on moist | lands which are } not swampy. On | fertile prairies — ————4 and in some for- ae a 394. Map of the Cape May peninsula, showing ests the annual coastal marshes. The unshaded areas inside the coast line growth of vege- are dry land. tation is great; but since the leaves, fruits, twigs, and trunks decay as they fall, the larger part of their substance is returned to the atmosphere. In a moist region there is more growth (and therefore more death) of vegetation than in a dry one, and a better chance that decay will not keep pace with death. Preservation of vegetable matter. Where vegetation falls into water, as in marshes, it undergoes slow change different from the decay suffered by vegetation on dry land. It is the partial preservation of organic matter in the water of marshes and ponds which converts them into peat-bogs, for peat is nothing more than accumulated vegetable matter undergoing those changes to which vegetable matter in water is subject. Under favorable conditions, the peat of a bog may become very deep, as in the Dismal Swamp. In and about marshes and swamps, therefore, we find the conditions COAL 451 for the accumulation of considerable thicknesses of vegetable mat- ter, some of it nearly free from sediment, and at the same time the conditions which keep it from complete decay. Conversion into coal. While the vegetable matter is not de- stroyed, it is not preserved intact. The approximate composition of wood and peat are shown by the following analyses (ash omitted). Carbon Hydrogen Oxygen Nitrogen SLi ye ae 49 .66 6.21 43.03 I.I0 MEAD ete ct 59.50 Sete 33.00 2000 The relative atomic proportions of carbon, hydrogen, and oxy- gen in cellulose are expressed by the formula (CsHO;),. In the air, the carbon and the hydrogen of the wood unite with oxygen of the air or of the wood itself, forming carbon dioxide and water, the principal products of the decay of vegetation. But under water the atmospheric oxygen is largely excluded, and the elements of the wood are thought to unite with one another to a larger extent, while the oxygen of the air plays but a subordinate part. One of the common products of decay under such circumstances is CH, (marsh-gas), which escapes into the air. The formation of this gas exhausts the hydrogen of the organic matter four times as rapidly as the carbon. If the carbon and oxygen of the wood are given off combined as COs, the oxygen is consumed twice as fast as the car- bon. If the hydrogen and oxygen of the wood are liberated as water, the result is to increase the proportion of carbon remaining. While the exact quantitative relations of the reactions which take place are not known, and are probably not constant, the fol- lowing table ' suggests certain changes which might take place, and the products which would remain at certain stages: 12 CgHi0Os (cellulose) — 8 CO. = Cg2H72004 (peat). Ce2H72004 (peat)— = C57H56010 (brown coa!), C57H56010 (brown coal)— = C54H 420; (bituminous coal). = C4gHjgO0 (anthracite coal). Ww a jo) ee ee et ee ee C54H4205 (bituminous coal)— CO, 5 CHg 1 Prepared by Rollin T. Chamberlin. 462 PENNSYLVANIAN PERIOD From this table it will be seen that the process which converts vegetable matter into coal is characterized by progressive changes in the nature of the chemical decomposition. The elimination of hydrogen and oxygen (H2O) probably is the dominant change in the production of peat from cellulose. Second in importance at this stage is the removal of oxygen in the form of COs, while the libera- tion of methane (CH) is of still less importance. As the alteration of the peaty material progresses through successive stages to coal, less and less water and carbon dioxide are given off, and there is an increase in the proportion of CH, set free. Laboratory investiga- tions have shown that while CO, may constitute an important part of the free gas held in the pores of some of the Cretaceous coals, the gas which escapes from the more advanced stages of Pennsylvanian anthracite coal is largely CH4. The burial of the peat compresses it, and the physical change resulting is a part of the process of coal- making. If coal-beds represent former swamps, as they are believed to, we have still to inquire into the conditions under which such ex- tensive swamps existed, and to seek the explanation of their recur- rence (one for each coal-bed) in many regions. The first condition for a swamp is lack of drainage, and the second a sufficient, but not an excessive amount of water. Enough to stop the growth of vegetation would be excessive, and too little to preserve it after its growth and death, would be insufficient. During the widespread movements which affected the eastern interior at the close of the Mississippian period, great areas appear to have emerged from the sea. Early in the Pennsylvanian period, considerable tracts which were not submerged stood so low as to be ill-drained, or undrained, and constituted marshes. Climatic conditions were such as to permit the growth of abundant vegetation in the marshes, where, after death, the vegetable matter underwent changes of the nature suggested above. The marshes were thus converted into peat bogs. Some of the great coal-swamps prob- ably came into existence along shores, and some in shallow inland basins or undrained areas. Each coal-bed represents the accumulated vegetable growth of a long period. It would appear that the growth and accumula- tion of vegetation was repeatedly brought to an end by subsidence which let the water (sea, lake, or aggrading stream) in over the marshes, drowning the plants, and burying the organic matter which COAL 453 had already accumulated under sediment which the submergence brought in its train.. A second coal-bed in the same region points to the recurrence of swamp conditions, and means either (a) that after submergence and burial of the organic matter, slight emergence reproduced the conditions for bogs; or (0) that by sedimentation the sea or lake bottom where the first bog had been was built up to water-level, restoring swamp conditions. The number of coal-beds is, in many places, great. In some parts of Pennsylvania it exceeds 20; in Alabama, 35 (not all workable); in Nova Scotia (including some dirt-beds) about 80; but in the Mississippi basin west of the Appalachians, the number is in most places less than a dozen. In Illinois the workable beds are nine. Extent and relations of coal-beds. The widespread distribution of coal does not mean that any one marsh necessarily covered the whole of any one great coal-field. Some coal-beds, however, are of great extent. Thus the Pittsburgh bed is worked over an area of some 6,000 square miles! in western Pennsylvania, Ohio, and West Virginia, and has at least an equal extent where too poor to be gen- erally productive. Many coal-beds, on the other hand, are not extensive. From their thicker portions they thin out in all direc- tions, grading into black shale in many places. Many facts sug- gest that within the general area of a coal-swamp there may have been elevations (islands), interrupting the continuity of the swamps, and therefore of the coal-beds. Varieties of coal. The ways in which the different varieties of coal arose have never been determined precisely. In general, anthracite coal occurs in mountainous regions, where the coal and other layers of rock with which it is associated have been subject to much dynamic action. Thus, in the mountains of eastern Penn- sylvania (Fig. 384) the coal is mainly anthracite, while in’ other coal-fields of the same age, where the strata are deformed much less, the coal is bituminous. In Arkansas, where the strata have been subject to some, but not to extreme dynamic action, the coal is semi- anthracitic.2, Where the dynamic metamorphism of the associated rock has been great, as in Rhode Island, the coal has gone beyond the anthracitic stage. Anthracite coal is found also in some places (not in the Coal Measures of the United States) in contact with 1 White, West Virginia Geol. Surv., Vol. II, p. 166. 2 Ann. Rept. Ark. Geol. Surv., 1888, Vol. III. 454 PENNSYLVANIAN PERIOD intrusions of igneous rock. Other sorts of sedimentary rock axe metamorphosed in similar situations. These phenomena suggest that anthracite is metamorphic coal, produced from bituminous coal by processes similar to some of those which metamorphose other sorts of rock. The fact that most metamorphic coal is found in regions where erosion has exposed its beds (Fig. 385) led to the conjecture that exposure of the coal might. be a factor in the problem, the exposure favoring the escape of the volatile constituents, and so aiding in the transformation of soft coalinto hard. Some beds of bituminous coal are, however, exposed freely. Both dynamic action, involving pressure and heat, and exposure would seem to be conditions favoring the development of anthracite, but it does not follow that these are the only factors in the problem, or that anthracite coal has never been produced in other ways. White has advanced the idea that deep-seated, hori- zontal thrust movements are the essential cause of devolatilization!. There are several varieties of bituminous (soft) coal, some of which appear to depend on the nature and extent of the decay of the vegetable matter before its burial, and some on the degree to which the devolatilizing processes have been carried since burial. Recent studies seem to indicate that the kind of vegetation enter- ing into the coal may have an important effect on the product. Some coal seems to be made up largely of alge, or of the spore- cases of certain plants, and such coal has rather distinctive quali- ties, if recent interpretations are correct. Other Products of Economic Value The iron ore of the Coal Measures occurs in layers, or in the form of nodules concentrated at a given horizon, forming a nearly continuous layer. The iron of the Coal Measures seems to have been deposited largely as a precipitate from the waters of inland and local basins while the other members of the system were being laid down. Dissolved by the land waters from the soil and rocks, it was brought to the marshes in some soluble form. In the marshes, it was precipitated in the form of iron carbonate or iron oxide. Sub- sequent oxidation has changed some of the original carbonate into the oxide. The principal iron ores of the system occur in Penn- sylvania and eastern Ohio. The system yields oi] and gas in some places, as in Oklahoma, Kansas, and Illinois. 1 David White. Economic Geology, Vol. III. GENERAL CONSIDERATIONS 455 General Considerations Geographic conditions in the eastern interior. Returning to the system of which the coal-beds form a small part, it is to be recalled that the formations represent an alternation of marine, lacustrine, and marsh conditions. The cause of the alternation was probably geographic, but it is not to be inferred that geographic changes were more frequent at this time than during other periods. Their record is conspicuous because the land was near sea-level, so that extensive submergence and emergence resulted from slight changes of relative level of land and sea. Equally frequent and equally extensive movements would leave no such record of them- selves, if the surfaces concerned were far above or far below sea- level. It was oscillation just above and just below water-level (or base-level) which allowed the record to be so clearly preserved. How far the oscillations were due to warpings of the land, and how far to changes in the level of the sea, cannot be determined; but when we recall that the ocean-level must respond to every deformation which affects its bottom, and to every stage of filling, it is strange that its level is in a nearly perpetual state of change. In general, it may be said that the movements of the crust which have been of most importance, from the point of view of continental or biological evolution, are not those which have affected high land or deep sea bottom, but those which have con- verted sea bottom into land, or land into sea bottom. Such changes are most likely to have taken place where land was low, or water shallow. From the point of view of geology, therefore, the critical level of crustal oscillation is the level of the sea. Duration of the period. So uncertain is our knowledge of the duration of geological time that all sorts of data which can be made to throw light on the subject are of interest, even though they do not lead to trustworthy numerical conclusions. Under favorable conditions, a foot of peat may accumulate in ten years or even less; but the common rate is probably much slower. A vigorous growth of vegetation has been estimated to yield annually about one ton of dried vegetable matter per acre, or 640 tons per square mile. If this annual growth of vegetable matter were all preserved for 1,000 years, and compressed until its specific gravity was 1.4 (about the average for coal) it would form a layer about seven inches thick. But it has been estimated that four-fifths of the vegetable matter in 456 PENNSYLVANIAN PERIOD peat bogs escapes as gas (COs, CHu, etc.), while the peat is being changed to coal. If this is true, the seven-inch layer would be re- duced to less than one and one-half inches, and a layer one foot in thickness would require between 8,000 and g,oo00 years.. The aggre- gate thickness of coal is as much as Ioo feet in many places, and as much as 250 feet in some. At the above rate of accumulation, periods ranging from nearly 1,000,000 to nearly 2,500,000 years would be needed for such thicknesses. It should be borne in mind, however, that much depends on the rate of growth of Carboniferous vegetation, which is not known. On the other hand, these figures refer to the coal only, not to the Coal Measures. The greater part of the Coal Measures is shale and sandstone, and of these formations there are thousands of feet, even where the sediments were fine and their accumulation prob- ably slow. It would hardly seem unreasonable to conjecture that their deposition may have consumed as much time as that of the coal. Doubling the above figures, we get 2,000,000 and 5,000,000 years respectively, figures which must be taken to mean nothing more than that the best data now at hand indicate that the Penn- sylvanian period was very long. Close of the period. After the long period of oscillation above and below the critical level recorded by the Coal Measures, the interior east of the Mississippi was brought above the level of the sea, not to sink beneath it again during the Paleozoic era, and some of it at no later time. This emergence marks the close of the Carboniferous, and the inauguration of the Permian period. It is also probable that the deformative movements which were to develop the Appalachian Mountains began at this time. There were notable changes also in the western half of the continent, for the Permian system is much less widespread than the Pennsyl- vanian. Where the Permian occurs, its constitution and fossils indicate not only different relations of land and water, but different conditions of erosion. Foreign Countries Europe. As in America, the oldest formation of the Upper Carboniferous in Europe is in many places a conglomerate and sand- stone formation, called the Millstone grit, in England. The Coal Measures consist principally of shales, with sandstone and limestone. Associated with these commoner sorts of rock, there are beds of coal IN FOREIGN COUNTRIES 457 and clay-iron-stone, both of which occupy positions corresponding, in essential respects, with those of similar formations in eastern North America. There is workable coal in Great Britain, Ireland, Belgium, France, Spain, Germany, Austria, and Russia, but the total area of productive coal in Europe is much less than in America. In Russia, as already noted, the Lower Carboniferous (Missis- sippian) contains much coal, while the Upper is chiefly of limestone; but in southern Russia (Donetz coal-field) there is coal in the Upper (Pennsylvanian) division. The Upper Carboniferous limestone of Russia (Fusulina limestone) is similar to that of southern Europe. The faunas of the marine part of the system in Europe have much likeness to those of western North America, suggesting that marine life was able to pass between these continents, via northern Asia. Igneous rocks are associated with the Upper Carboniferous formations of sedimentary origin in western Europe. Their extru- sion seems to have been an accompaniment of the crustal disturb- ances which affected western Europe in the course of this period. These movements appear to have been greatest during the Upper Carboniferous period (after the Westphalian epoch). Other continents. The Upper Carboniferous of Asza is repre- sented by both marine and non-marine formations. The non- marine phase, with numerous beds of coal, is found in Asia Minor, on the east side of the Middle Urals, and in northern and eastern China, reaching to northern Tibet on the one side, and to Mongolia on the other. The Carboniferous of some parts of China contains coal-beds of great thickness. The system is also present in India. The Carboniferous formations of northern Africa are similar to those of southern Europe, but in southeastern Africa, a coal basin has been reported in Zambesi.! | The Carboniferous system is well developed in Australia where it is not in all places clearly separated from the Permian. Both the Carboniferous and the Permo-Carboniferous systems contain coal. In South America, rocks of Late Carboniferous age are somewhat widely distributed. In southern Brazil they contain much coal.’ The system is widespread in the lower part of the basin of the Amazon, where it rests on older formations unconformably, and is not generally coal-bearing. 1 Kayser, Geologische Formationskunde, p. 207. 2 White, I. C, Commissdo de Estudos das Minas de Carvaéo de Pedra do Brazil, 1908, 458 PENNSYLVANIAN PERIOD LIFE With this period the chief biological interest shifts from sea to land, and centers in the vegetation and the amphibians. Plants Plant life was very abundant in this period, and its record is unusually full and perfect. Its completeness has doubtless given this flora an undue prominence over those which preceded and suc- ceeded it; yet it was really a great period in the history of plant life. Angiosperms (flowering plants, p. 685), the dominant plants to-day, had not yet appeared, but gymnosperms (the group to which pines belong) were abundant, and pteridophytes (ferns and related plants) probably made their greatest display at this time. All the great divisions of this group (p. 685) were present, and all of them were nearly or quite at their climax. Of lower plants, little is known. The most rapid evolution of floras was perhaps in the Pottsville epoch. Half the genera of that epoch scarcely survived it, and few of them lived after the Allegheny epoch. The early floras were widely distributed. Thus three floras in Asia Minor may be correlated severally with three floras of the Pottsville series. The place of origin of these early floras is not known with certainty, but present evidence points to western Europe and eastern North America, with an Arctic land connec- tion. The late Pennsylvania floras are less sharply separated from the early ones in North America than in Europe. The later floras indicate greater diversity of climate than the earlier. The dominant plants of the period belonged to five groups: (1) the horse-tail family (Equisete), (2) sphenophylls, now extinct, (3) lycopods, or club mosses, (4) fern-like plants (pteridosperms), and (5) Cordaites, a group of gymnosperms. To this list ferns should perhaps be added. Ferns were a minor element of the Pennsylvanian flora, though fern-like leaves are the most abundant of the plant fossils. It is now known that most of them belonged to seed-bearing plants and not to ferns. Nevertheless, true ferns were present. Species still live which, so far as outward form is concerned, might be referred to Carboniferous genera. The horse-tail group ( Equisetales) was represented by calamites (tree horse-tails), a conspicuous element in the Pennsylvanian flora. They must have been graceful trees, of the same general habit as LIFE 459 modern horse-tails, except for their large size. The largest modern tropical representatives of the group have slender stems 30 or 4o feet high, whereas the Pennsylvanian calamites reached a foot or two in diameter and probably 60 to go feet in height. They had hollow stems, or a core of pith, and casts of the interior are common. Branches from the trunk were comparatively few, and in whorls. Fig. 395. A composite group of leading CARBONIFEROUS PLANTs, adapted from restorations by various paleobotanists, by Mildred Marvin. In the fore- ground at the right, Lepidodendron; at the left Sigillaria; in the right center rear, a tree fern; in the left center rear, Cordaites; at the extreme right and left, Calamites. The leaves also were in whorls (Fig. 395) and dwarfed, though larger than in the modern type. Their roots were of the type commonly found under water or in wet places, and the calamites probably frequented swamps and lowlands. Roots are known to have been sent out as much as nine feet above the base of the stem. 460 PENNSYLVANIAN PERIOD They were probably associated in thickets and jungles, like cane- brakes and bamboos. Their history may run far back, as they were well differentiated in the Devonian; but their ancestry is uncertain. The stems of adult calamites are so unlike those of modern horse- tails that their kinship was long unrecognized, and calamites were thought to be gymnosperms; but it is now known that the stems of d Fig. 396. Group oF FERN Fronps: 4a, Neuropieris auriculata, Brgt.; b, N. angustifolia, Brgt.; c, N. vermicularis, Lx.; d, Odontopteris cornuta, Lx.; e, Pecopteris unita, Brgt.; f, Dictyopteris rubelia, Lx.; g, Archeopteris bochsiana, Goepp.; h, Sphenopteris splendens, Lx. the young plants had the same general structure as the horse-tails, and that the gymnospermous features belonged to the later stages of the life of the individual plants. The group is represented to-day by one genus (Eguisetum) and about 20 species. Its evolution LIFE 401 records a continuous decline from the Pennsylvanian period, when it was at its best. Recent studies have shown that the graceful, slender plants with whorled leaves, referred to the genus Sphenophyllum (c, Fig. 397), formerly classed as calam- am x | ites, should be made a class (Sphenophyllales, p. 685) by themselves. Their interest lies chiefly in the fact that while they have certain cal- amarian features, they have others possessed by lyco- pods. This is interpreted to mean that these two yroups (calamarians and ly- copods) were united with Sphenophyllales in a com- mon ancestral form.! The stems were long, slender, and apparently weak, and a climbing habit has been in- ferred. The leaf structure suggests a shady habitat, perhaps one of undergrowth. The class, represented in the Devonian, had its climax in the middle Pennsylvan- a ian, and continued into the Kj : ‘ ig. 397. CARBONIFEROUS EQUISETALES Permian and possibly later. np SpHenopnyiiates: a, Calamites cistii; In size Lycopods (p. 685) 5, Annularia sphenophylloides; c, Spheno- were the master group of phyllum longifolium. the Coal flora. ‘They were represented by trees of large size which had the highest organization reached by the pteridophytes. From this high estate, they have since fallen to prostrate or weakly ascending plants of moss-like aspect (club mosses and ground pines.) The chief genera were Lepidodendron and Sigillaria (Fig. 395), of which the former was the earlier and sim- pler type. Both take their names from the leaf-scars (lepidos = scale, sigilla=seal) which the trunks retained (Figs. 398 and 399). 1 Seward, Fossil Plants, p. 413; Scott, Studies in Fossil Botany, p. 494. 462 PENNSYLVANIAN PERIOD The trunks of some lepidodendrons were too feet in length. They were erect, and branched at a great height. The leaves were linear or needle-shaped, ranging up to six or seven inches in length, and set densely on the branches. Some of them had characteristics Fig. 398. Leaf markings Fig. 399. Leaf markings of a of a lepidodendron. sigillarian. pointing in the direction of seeds, but it is not known that seed- producing plants sprang from them. More than too species of lepidodendrons have been described. They seem to have reached their climax early in the period, and nearly all had disappeared by its close. . The sigillarians differed from the lepidodendrons in being with- out many branches. They were perhaps the largest of the trees, their trunks reaching six feet in diameter, and 100 feet or more in height. The stems were densely clothed with erect, rigid, linear leaves. They were more abundant than lepidodendrons before the close of the period, but were on the wane at its close. The group is essentially Pennsylvanian but initial forms lived in the Devonian and Lower Carboniferous, and a few survived to the Permian. Cordaitales. One of the characteristic trees of the period was Cordaites, which belonged to a remarkable family (now extinct) of gymnosperms. ‘The trees were go feet or more in height, and rather slender. The wood was of the coniferous type, covered, as in so many other plants of the period, by a thick bark. ‘The trunks had a large pith. The leaves were parallel veined, suggestive of mono- cotyls of the yucca type, and in some cases attained a length of six —_" LIFE 463 Fig. 400. One of the Cycadofilices, Lyginodendron oldhamia. (Restoration by D. H. Scott and J. Allen.) feet and a width of six inches. They are preserved in great abun- dance, and make up a large part of some beds of coal. In one form, the leaf had a distinctly fleshy character, as if adapted to xerophytic (dry) life. The florai organs were peculiar to the family, and have been worked out with marvelous success, even the structure of the pollen having been determined. Conifers have not been found in the Pennsylvanian rocks; but the vegetation of the uplands, where conifers probably would have lived, is not known. Climatic Implications of the Coal-plants What suggestions do the Coal-plants give relative to the atmos- pheric conditions under which they grew? ‘Two partly antagonistic views relative to these conditions have been held. ‘The one regards the beds of coal as evidence of a very luxuriant growth of vegeta- tion, which in turn has been thought to imply a warm, moist atmos- phere, heavily charged with carbon dioxide. The great size of 464 PENNSYLVANIAN PERIOD many of the trees, the succulent nature of many of the plants, and the abundance of aérial roots, are appealed to as evidence of mild- ness of climate, while the absence of rings in the wood, and, above all, the distribution of similar floras through diverse latitudes, point strongly to an equable climate, especially in the earlier part of the period. It is clear that this view has much support. The alternative view postulates less warmth and moisture, and more diversity; in other words, a nearer approach to the present conditions. It assumes, however, a somewhat higher percentage of carbon dioxide than now, and a climate milder and more uniform than that of to-day. The basis of this view is found in the following considerations: (1) Great thicknesses of coal do not necessarily imply rapid accumulation, any more than great thicknesses of lime- stone do. Given favorable conditions of preservation, slow growth will produce great thicknesses. (2) At present the accumulation of peat, the nearest analogue of coal formation, is most favored in cool climates, and is taking place chiefly in high latitudes. (3) The dominant plants had narrow leaves with their breathing pores con- fined to deep furrows on the under side, devices common to plants of dry regions. (4) The trees had thick corky bark, as though protec- tion from external conditions was needed. The thickness of the bark, and the form and structure of the leaves, give a xerophytic aspect to the overgrowth made up of lepidodendrons, sigillarias, calamites, and cordaites. This is not the case with the undergrowth, but this would not be expected of shaded plants. The force of the inference from the xerophytic aspect of the overgrowth is much weakened by the fact that the vegetation of undrained swamps and bogs has many xerophytic features. It is clear that a more critical study of the problem is needed before a final conclusion concerning the climate of the period is reached. Land Animals So far as the evolution of air-breathing vertebrates is concerned, this is one of the most important periods in geological history.! Amphibians, insects, spiders, scorpions, and myriapods, lived on the land at this time. The amphibians are perhaps of chief interest, for they were the first land vertebrates. The rise of amphibians. Tracks attributed to amphibians are found in the Devonian and Mississippian, but in neither of these 1 Williston. Faunal Relations of Early Vertebrates, Jour. Geol., Vol. xvii, p. 389. LIFE 465 systems have bones of these animals been found in America, and only imperfect ones in Europe. Fossils of amphibians first appear in abundance in the later Coal Measures, and in such variety as to imply a long antecedent existence. Most of them were rather primitive in structure, but they were ee genuine amphibians, not transition a types. All of them seem to have had elongate forms, and their heads were well roofed over by the bony plates of the skull. On account of this last feature they are called stegocephalians (roofheaded). Some of them have also been named /aby- rinthodonts, from the intricate infold- : ) a ing of the dentine of their teeth. Hee ew aM wl a Labyrinthodonts were doubtless the i Se on . y y largest amphibia of the period, some Ds of their skulls reaching a length of half a meter. The amphibia varied in length and strength of limb, in agility, ability to climb, etc. The elonga- tion of their bodies involved a nota- ble multiplication of the vertebre, one form having no less than 150. Before the close of the period, prob- ably some of them lived on dry land where fleetness, rather than protect- ive armor, preserved them from their enemies. Others were limbless and SS =a i} pit i i inh snake-like, crawling reptiles in every- : ; ; : : Fig. 401. A CARBONIFEROUS AM- thing except certain technical details PPE MT bor. dobbsig uxt of their palates. ley. A microsaurian from Kil- One branch of the amphibia_ kenny, Ireland, about 3/5 natural which reached its highest develop- ‘i: (4ittel.) ment in the Permian is supposed by some paleontologists to be the ancestral stock from which mammals arose. The other branch, which included the labyrinthodonts, is the only group of Pennsyl- vanian air-breathing vertebrates which left no descendants. Not much is known of the habits of the amphibia, but from their 466 PENNSYLVANIAN PERIOD teeth it is inferred that they were predaceous. In Nova Scotia, Dawson took thirteen skeletons of amphibians from a single sigil- larian stump. Since land shells and myriapods are found in stumps with the amphibian skeletons, it has been inferred that some of the amphibians were climbers, and lived on mollusks, myriapods, and similar land life. The amphibians of different continents were so similar as to suggest great freedom of communication and migration, but free 1 Fig. 402. CARBONIFEROUS TERRESTRIAL AND FRESH-WATER LIFE. Plants: a, Callipteridium mansfieldi Lesq., b and c, Callipteridium membranaceum Lesq., species of ferns. Land shells: d, Zonites priscus Carp., e, Pupa vermilionensis Bradley. These land snails have been referred to genera living at the present time, and although this reference may eventually prove to be incorrect, they are at least close relatives of recent genera. Insects, etc.: f, Euphoberia armigera M. and W., a Carboniferous myriapod or thousand-legged worm; g, Eoscorpius carbonarius M. and W., a scorpion very similar in type to living forms; 4, Arthrolycosa antiqua Harger, a spider more primitive than recent forms as seen by the segmentation of the abdomen; 7, Progonoblattina columbiana Scudd., one of the allies of the modern cockroaches which were the most conspicuous members of the Carboniferous insect fauna. Crustacea: j, Anthrapalemon gracilis M. and W., k, Pale@ocaris typus M. and W., types of crustaceans found in the Mazon Creek nodules; /, Prestwichia dane M. and W., an early ally of the modern horseshoe crab. (Weller.) LIFE 467 intercontinental migration seems to have come to an end by the close of the period. Insects. Hundreds of species of insects have been identified from the Coal Measures. They were, for the most part, rather primitive types. Orthopters (cockroaches, 7, Fig. 402, locusts, crickets, etc.) were greatly in the lead, followed by neuro pters (repre- sented by ancestral mayflies). These two orders include about 90 per cent of the known insects. Hemipters (bugs), which had ap- peared earlier, and possibly coleopiers (beetles) were present, but no fossils of bees, butterflies, or moths have been found, and there is little probability that they existed, since flowering plants, on which they depend, had not yet appeared. There is no record of flies. The evolution of insects was therefore one-sided. Curious forms were developed within the orders which lived, and remarkable sizes were attained, spreads of wing of a foot or more being reported. Spiders and myriapods (Fig. 402) were plentiful, and several species of Jand snails (d and e) have been identified. The amount of car- bon dioxide in the atmosphere could not have exceeded that compatible with this varied assemblage of air- breathing life. : - Fresh-water Life Besides fresh-water plants, the life of land waters appears to have consisted of fishes, mollusks, crus- taceans, and doubtless of many other forms. Aside from the development of the fresh-water fish and amphibia, perhaps the most suggestive feature was the association of the arthropods with other forms of life. Eurypterids (Fig. 403) were still in existence, and their relics are so intimately associ- ; y ; Fig. 403. Natural association ated with ferns, calamites, insects, of Eurypterus mansfieldi with spiders, and scorpions as to leave no _ ferns and calamites. (From Dana reasonable doubt that they were atter Hall.) fresh-water forms. ‘There were also crustaceans resembling cray- fish, and others of shrimp-like appearance. 468 PENNSYLVANIAN PERIOD Marine Life Two phases of sea life are worthy of note, (1) that which occupied the shallow water, which, in the form of estuaries, lagoons, and shoals, crept in and out on the borders of the continent as the rela- tions of land and sea oscillated, and (2) the life of the more open seas. No doubt this distinction had existed always, but it had not before reached equal importance. In the coal regions of this period, a large part of the fossils are of shallow water types. In shallow water, where sandy and muddy flats prevailed, pelecypods and gastropods, together with certain fishes, predominated, while in the more open seas the brachiopods, cephalopods, and clear-water types were more plentiful. During the period, there was progress among the fishes in adaptation to swift movement, and in shapeliness of form. It is difficult to tell which of them were marine, which fresh water, and which common to salt and fresh water. It is clear that much the larger number of those in the American Coal Measures lived in fresh water; whether also in salt water is uncertain. Fig. 404 shows a group of Pennsylvanian marine fossils. It may be noted that ancient and relatively modern types of cephalo- pods lived together, the former represented by straight, plain, small orthoceratites (z, Fig. 404), and the latter by closely coiled goniatites (sz), with curved sutures. The former were about to take their final leave, and the goniatites were about to evolve into ammonites, the dominant type of the Mesozoic era. Brachiopods were abun- dant, and their general facies was like that of the later Mississippian. Some species range not only through northern America and Eurasia, but into the Oriént and Australasia. A close relation between sev- eral American and Russian crinoids implies intermigration. Cyst- oids and blastoids were gone, and other forms of echinoderms were rare. Tvrilobites, which commanded foremost attention at the opening of the Paleozoic, are now almost at the point of disappear- ance. ‘The last representative of the group had the chaste beauty of its early ancestors. Bryozoans were not uncommon, but the peculiar devices for support illustrated in Archimedes and Lyropora of the preceding period were abandoned. Protozoans were repre- sented widely by a little foraminiferal shell (Fusulina secalicus, 6, Fig. 404), which had about the size and form of a grain of wheat. Its abundance gives character to the Fusulina limestone which occurs in America, Europe, and Asia. Corals were rare, as might be ex- pected under the conditions of the time. - = > - - Sa ~ - ” NETL LL PIETER iELE iAH Fig. 404. PENNSYLVANIAN MARINE Fauna: a, Eupachycrinus magister M. and G., a crinoid with biserial arms; 6, Fusulina secalicus Say, a foraminifer shell that in places makes up considerable beds of limestone. c-p, brachiopods: , Productus nebrascensis Owen; d, Productus costatus Sow.; e, Seminula argentea (Shep.), a spire-bearing common Carboniferous species; f, Lingula umbonata Cox, a representative of a genus which persisted from the Cambrian to recent times; g, Hustedia mormoni (Marc.); h, Spirifer cameratus Mort., a characteristic member of the Pennsylvanian fauna; i, Productus symmetricus McCh.; j, Derbyia crassa (M. and H.); k, Enteletes hemiplicata (Hall); 1, Pugnax uta (Marc.); m, Dielasma bovidens (Mort.); 2, Meekella striatocostata (Cox); 0, Chonetes granulifera Owen; p, S piriferina kentuckiensis (Shum.). 7, Allorisma subcuneata M. and H. 4q-t, pelecypods: gq, Monopteria longispina Cox; s, Myalina recurvirostris M. and W.,; t, Aviculopecten occidentalis Shum. u-x, gastropods: u, Worthenia tabulata (Con.); v, Meekospira peracuta (M. and W.); w, Bellerophon percarinatus Con.; x, Naticopsis altonensis (McCh.); y, z, and zz, cephalopods: y, Temnocheilus forbesianus (McCh.); 2, Ortho- ceras cribrosum Gein.; 22, Paralegoceras newsomi Smith; «x, Phillipsia major Shum. Map work. No reference to map work has been made since that at the close of the chapter on the Ordovician, p. 387. Experience has shown that if the prin- ciples of stratigraphy, as illustrated by the Cambrian system, are well developed, further map work may be deferred to about this point. See laboratory manual al- ready referred to (p. 387), exercise IX. 469 CHAPTER XXI THE PERMIAN PERIOD FORMATIONS AND PHYSICAL HISTORY At the close of the Pennsylvanian period much of the central and eastern parts of the United States became dry land, and the sea-covered area in the west was greatly restricted. The area of land was perhaps as large as at any time since the beginning of the Paleozoic. The waters which still lay upon the continent were partly in the form of lakes and inland seas, and partly connected with the open ocean; but the areas which the sea overspread at the beginning of the period were largely abandoned before its close. These changes in geography reflected themselves both in the distri- bution of the Permian formations and in their character. East of the Mississippi. During the earlier part of the period fresh-water sedimentation continued much as before in some parts of the east (parts of Pennsylvania, West Virginia, Maryland, and Ohio), and with the commoner sorts of sedimentary rocks there is some coal. There, and in and about Nova Scotia, non-marine Permian strata rest on Pennsylvanian beds in such a way as to show that sedimentation was not seriously interrupted. The systems are separated on the basis of fossils. Recently, a conglomerate forma- tion (the Roxbury) of Eastern Massachusetts has been interpreted as of glacial origin. This origin suggests its reference to the Permian!. West of the Mississippi. West of the Mississippi the system is better developed, being partly marine and partly non-marine. In Kansas and Nebraska its lower part is marine, and the Permian ~ of these states is probably continued northwestward to. Wyoming and South Dakota. The marine Permian of Kansas is overlain by beds containing gypsum and salt, and possessing other features which show that the open sea of the region was succeeded by dis- severed remnants, or by salt lakes whose supply of fresh water was exceeded by evaporation. With the saline and gypsiferous deposits, and above them, are the ‘‘ Red Beds,”’ many of which are Permian. 1 Sayles, Bull. Mus. Comp. Zool. Vol. LVI. (Geol. Ser. X) pp. 141-170, and. Science, Vol. 32 (1910) p. 723. 470 FORMATIONS AND PHYSICAL HISTORY 47t Some of the Red Beds in western Texas, New Mexico, and elsewhere are perhaps later than Permian, and some in Oklahoma, Kansas, Colorado, and perhaps elsewhere, are older. In the Staked Plains of Texas the system has its greatest devel- opment. The oldest part (Wichita formation) is partly of marine and partly of fresh-water origin. The Middle Permian (Clear Fork limestone) is of marine origin, and overlaps the Lower. The Upper Permian (Double Mountain formation) indicates a reversal of condi- tions, for much of Texas was again cut off from the ocean, and con- verted into an inland sea or seas, in which the phases of deposition common to such bodies of water took place. Occasional beds of limestone with marine fossils point to occasional incursions of the sea, while deposits of salt and gypsum point with equal clearness to its absence, or to restricted connections, and to aridity of climate. Throughout much of the area west of the Rocky Mountains the Permian has not been differentiated. There is, in places, con- formity between the Carboniferous below and the beds classed as Trias above, suggesting the presence of unseparated Permian between. In northern Arizona and in southwestern Colorado and perhaps at other points, there is an unconformity at the top of the Permian. The Permian system may have been continuous once from’Texas to the Great Basin, by way of New Mexico and Arizona; but if so, the continuity of the beds has been interrupted by erosion. A very considerable thickness of marine Permian (3,800 feet) is reported from Utah. Many Permian deposits of the far west, and some of those in the longitude of Texas and Kansas, are red. This color characterizes so many formations known to have been made in inclosed basins that the connection can hardly be accidental. Thickness. In the Appalachian region, the Lower Permian beds, sandstone and shale with thin seams of coal, have a thickness of about 1,000 feet. The Upper Permian is wanting. In Kansas the thickness is twice as great, while in Texas it reaches 7,000 feet. Correlation. In the region east of the Mississippi, the Permian is so closely associated with the Coal Measures that the two were formerly classed together, the Permian being called Upper Barren Coal Measures. Were this region only considered, this classifica- tion would appear to be satisfactory. In the western part of the continent the separation of the Permian from the Carboniferous will-probably prove to be more distinct, when details have been worked out, and its relation with the Trias close. The Permian 472 THE PERMIAN PERIOD period is best looked upon as a transition period from the Carbonifer- ous to the Trias, and so from the Paleozoic to the Mesozoic. Its close relationship to the underlying system in some places, and to the overlying system in others, is therefore to be expected. Foreign Permian Europe. In Europe, as in America, the Carboniferous period was brought to a close by very considerable changes, for much of the area which had been receiving deposits during that period was exposed to erosion at its close. Subsequently, much of the same surface was again the site of deposition, partly from fresh and partly from salt waters. ‘The system is here much more distinct from the Carboniferous than in eastern North America. In western and central Europe, the Lower Permian (Rothlie- gende) consists of a series of clastic formations, together with a large amount of igneous rock, in the form of lava-sheets, dikes, and pyro- clastic material. The formations and their fossils show that much of the sediment was accumulated in inland seas, and in salt and fresh lakes. Gypsum, salt, and a meager fauna of dwarfed and stunted species are among its distinctive marks. But the sea some- times had access to the inland areas of sedimentation, as fossils show. The shallow-water or subaérial origin of much of the Per- mian is shown by the sun-cracks, rain-pittings, ripple-marks, tracks of terrestrial and amphibious animals, etc. In keeping with the conditions of its origin the Rothliegende contains some coal. Especial interest attaches to the conglomerates and breccias, because of their likeness to glacial drift. The conglomerate is wide- spread, and in places contains bowlders which have been trans- ported great distances; but its glacial origin has not been proved. The Upper Permian of western and central Europe (the Zech- stein) is unlike the Lower in several important respects. It contains much more limestone and dolomite, but neither coal, igneous rock, nor, except at its very base, conglomerate. From the stunted aspect of the fossils, and from the association of the dolomite with. gypsum, salt, etc., it has been thought that the limestone and dolomite may be largely chemical precipitates. Some parts of the Permian are, however, of marine origin. The Upper Permian of central and western Europe contains the thickest mass of salt known. Near Berlin, one body of salt has been penetrated about 4,000 feet, without reaching its bottom. It FORMATIONS AND PHYSICAL HISTORY 473 may be doubted, however, whether there is a salt Jayer of this thickness. Besides common salt, salts of potash and magnesium are present locally in such quantity as to be commercially val- uable. With the exception of saltpetre, the world’s supply comes from these beds. The system underlies the larger part of Russia (in Europe), and appears at the surface over a large area in the southeastern part of Ai i Mi | | . Re : ¥ . x i . etna be So: Fig. 405. Sketch map of Europe during the later part of the Permian period. The lines indicate areas of marine deposition, the broken lines areas of lagoon de- posits. (After De Lapparent.) that country. Here and in southern Europe generally the Permian is conformable on the Carboniferous, and is partly marine and partly non-marine. In Russia it contains salt, gypsum, etc., and also, at some horizons, marine fossils. Other continents. In other parts of the world the Permian is widely developed. In countries about the Indian Ocean, there is a less distinct break between the Carboniferous and Triassic systems than in Europe, and locally at least, the Permian seems to bridge the interval completely. 474 THE)PERMIAN! PERIOD’ “' Permian glacial formations. The most remarkable fact Fig. 429. The internal shell of a belemnite, restored; the lower, solid, conical portion (at the left in the Fig.), the part most commonly preserved, is the rostrum or guard; the middle portion is the phragmocone, which is a diminutive chambered shell with septa, siphuncle, and protoconch as in the older tetrabranch order; the upper part is the prostracum, which corresponds to the ‘‘pen”’ of living cuttle-fish. In the course of the period the belemnites came almost to rival the ammonites, and were almost as characteristic of the successive stages of deposition. The first known cuttle-fishes also appeared at this time. (3) Pelecypods flourished during the period (Fig. 430), and, took on a markedly modern aspect, the oyster family taking the lead. Gastropods were abundant in some places, but singularly absent in others. Existing genera were represented. (4) Suggestive of shallow clear seas was the reappearance of corals and crinoids in abundance in the later part of the period. The modern type of corals ( Hexacoralla) was in the ascendant and formed reefs, especially in European seas. Crinoids also rose again to prominence, though their diversity was not great. Most of them lived in shallow water, as most of the Paleozoic types had; but there is evidence that deep-water species had appeared, leading toward the prevalent habit of the present. (5) The slow evolution of the sea-urchins in the Paleozoic era was succeeded in the late Trias by the beginning of a rapid evolution, which reached its climax in the early Tertiary. 510 THE JURASSIC PERIOD Fig. 430. Group OF Jurassic PELECyPops. 4d, T'rigonia navis Lam.; b, Gry- phea arcuata Lam.; c, Ostrea deltoidea Sby.; d, Exegyra (Ostrea) virgula D’Orb.; e, Aucella mosquensis Keys. (6) The trilobites of the sea, and the eurypterids of land waters, had been succeeded by decapods which rose to a moderate and pro- longed ascendancy. The prawns and lobsters (Macrura, long- tailed decapods) were the earlier division, and the most numerous in this period; but the first known crabs (Brachyura, short-tailed decapods) appeared before the period was past. ‘The macrurans seem to have frequented embayments and protected locations near the land, or perhaps within it, for terrestrial, fresh-water, and marine species are preserved in the same sediments. Probably macrurans had representatives in terrestrial waters then, as now. (7) Sponges and foraminifera abounded and are well preserved. (8) The marked change in the aspect of the fishes which set in during the Trias was carried farther in this period. Some of the older types declined; but the selachians (sharks) remained abundant, Fig. 431. JURASSIC C@ELENTERATA AND ECHINODERMATA. a and 6b, Tham- nastrea prolifera Becker, a complete corallum, and the lateral surface of a costal septum, enlarged; c, Thecosmilia trichotoma (Goldf.); d, Pentacrinus briareus Mill; e, Cidaris coronata Goldf. : skates and rays began their modern career; the existing family (Chimeride) of sea-cats or spook-fishes made its appearance, so far as fossils show (Fig. 434); the forebears of the living garpikes 512 THE JURASSIC PERIOD Fig. 432. Jurassic Fossirs. a-c, Cephalopods: a, Cardioceras cordiformis M. and H.; b, Neumayria henryi M. and H.; c, Belemnites densus M. and H. d-h, pelecypods: d, Camptonectes bellistriatus Meek; e, Mytilus whitet Whitf.; f, Gram- matodon inornatus M. and H.; g, Pseudomonotis curta (Hall); h, Ostrea strigilecula White. 7 and 7, brachiopods: 17, Rhynchonella gnathophora Meek; j,: Lingula brevirosira M. and H. and sturgeons were numerous, and the initial forms of the bony fishes (teleosts), the dominant type, made their appearance. The class was distinctly more modern than at the close of the Medi, NTN NNN yp Fig. 433. A Jurassic coelacanth, Undina gulo, a crossopterygian, about 1/7 natural size; the outline of the air-bladder is shown just back of the gills and under the axis. (Restored by A. Smith Woodward.) Paleozoic. Though the fishes doubtless suffered from the reptiles which went down to sea in the Trias, it appears that they continued in notable abundance and variety. It will be seen later that they LIFE 513 outlived the invading race, and resumed, in large measure, their former dominance. Fig. 434. A jurassic spookfish or chimeroid, Squaloraja polyspondyla, % natural size; from the Lower Lias, Dorsetshire. (Restored by A. Smith Wood- ward.) Fig. 435. A Jurassic forerunner of the modern Amia, Eugnathus athostomus, about 1/7 natural size, from the Lower Lias, Dorsetshire. (A. Smith Woodward.) - Fig. 436. Outline and skeleton of Ichthyosaurus quadriscissus. (After Jaekel.) (9) Some of the reptiles which had taken to the sea in the pre- ceding period had become extinct, while others made their first appearance in this period. The ichthyosaurs (fish-like saurians) reached their highest development in this period, and seem to have swum every sea. Their adaptation to aquatic life is shown in the complete transformation of their limbs into paddles (Fig. 436), in sT4 THE JURASSIC PERIOD the reduction of the outline of the body to fish-like lines and propor- tions, in the sharp down-bending of the vertebre at the end of the tail for the support of a caudal fin, in the long snout set with teeth adapted to seize and hold slipping prey, but not to masticate it, in the protection of the eye by bony plates, and, interestingly enough, in the development of a viviparous habit that freed them from the necessity of returning to land to deposit their eggs. That their food consisted in part of invertebrates is evident from the fossil contents of the stomachs, the remains of 200 belemnites having been found in a single one. There were small as well as large forms of ichthyo- saurs, some exceeding 30 feet in length. Descended from a different stock, the plesiosaurs (Fig. 437) adapted themselves to sea life in another way. The body took on a D LOA) SY ¥ tn ( | Piet , [i f LY \ \ A OD, ie Ls SEZE" or Fig. 437. Skeleton of Plesiosaurus dolichodeirus Conyb. (Restored by Cony- beare.) form like that of a turtle, while the neck was elongate, giving rise to the epigrammatic description ‘‘the body of a turtle strung on a snake.’”? Swimming was chiefly by means of paddles, though some forms had a fin-like adaptation of the tail. The elongation of the neck was variable, the vertebrz of the neck numbering from 13 to 76. The neck appears not to have been so flexible as familiar illustrations have represented it, nor were the jaws separable and extensible as in the case of snakes. This implies either that they lived on small prey, or tore their food to pieces before swallowing. They were doubtless formidable foes of the smaller sea animals, but probably not of the larger. Like ichthyosaurs, they were without scales. They ranged from 8 to 40 or more feet in length. Marine crocodilians made their appearance late in the period. They had undergone a remarkable adaptation to the sea (Fig. 438). They were fish-like in appearance, their skins were bare, and their tails terminated in a fin like that of the ichthyosaurs. The fore limbs were short and paddle-like. The hind limbs were modified LIFE 515 Fig. 438. Restoration of a Jurassic crocodilian, Geosaurus suevicus. (Fraas.) but slightly from the land type, perhaps due to the recurring neces- sity of visiting the shores for depositing and hatching eggs. Marine turtles, so characteristic of the Cretaceous, had not yet appeared. Land Life Vegetation. The land vegetation of the Jurassic was little more than a continuation and expansion of that of the late Triassic, with slow progress toward living types. Cycads, conifers, ferns, and equiseta were the leading plants, slightly more modernized than their Triassic ancestors, but not changed radically... The conifers were represented by yews, cypresses, arborvitas, and pines, all of which had a somewhat modern aspect, though all the species are extinct. An interesting feature of the European record is the rather frequent occurrence of land plants in marine beds, which implies that many trunks, twigs, leaves, and fruits were floated out to sea, and that the landward edges of the marine deposits have escaped destruction. In the same beds are the remains of many land insects, not a few of them being wood-eating beetles. Animals. Of the early Jurassic land faunas of North America little is known; but in the Morrison beds (perhaps Comanchean, p. 504) there is a fauna composed chiefly of dinosaurs. Some of these reptiles were large, and some small, and the group as a whole had great diversity in many directions. There were not only carniv- orous types, which had appeared in the Trias, but numerous herbiv- 1 Jurassic plants of the United States, with descriptions and illustrations by Lester F. Ward; 20th Ann. Rept., U. S. Geol. Surv., pp. 334-430. 516 THE JURASSIC PERIOD orous forms; but among them all there was not a single type which was distinctively North American. It is therefore concluded that there was freedom of migration between the eastern and western continents at this time. - Of the carnivores, one of the most common was a type (Fig. 439) whose fore limbs seem to have been used chiefly for seizing and hold- Fig. 439. A carnivorous dinosaur, Ceratosaurus nasicornis, about 1/40 natural size; i.e., length about 17 feet; from the Como beds, Colorado. (Restoration of skeleton by Marsh.) ing prey, rarely for walking. The animal’s pose was facilitated by hollow bones. The head was relatively large, an unusual character for a race among which small heads and brains were the fashion. Besides the large ones, there were small leaping forms not larger than a rabbit. The herbivorous dinosaurs, known first in this system, developed so rapidly that they soon outranked the carnivorous forms in both size and diversity. Most of them were massive, with sub-equal limbs and the quadrupedal habit. Some of them (Fig. 440) attained a length of 60 feet (possibly more), taking rank among the largest of known land animals. These enormous creatures were char- acterized by weakness rather than strength, for they were unwieldy, their heads and brains small. ‘‘The task of providing food for so LIFE S17 i large a body must have been a severe tax on so small a head.’’ The largest of all known dinosaurs (Brachiosaurus) had a femur nearly WE MALES Ss Wes » Nihese és Nee Ke \\ YS. — SEF Bx i Fig. 440. An herbivorous dinosaur, Brontosaurus (A patosaurus): Restoration of skeleton by Riggs, nearly 60 feet long; from Wyoming. 7 feet long. There were other genera of similar nature, and of bulk inferior only to these monsters. © The typical ornithopod (bird-footed) dinosaurs were bipedal in habit, like the carnivores. On the hind limbs there were usually only three functional toes, so that they left a bird-like track; the fore limbs, however, had five digits. One of the largest of this group Fig. 441. Stegosaurus, an armored dinosaur of the Jurassic. Interpreted by Charles R. Knight. (Lucas’ Animals of the Past. By permission of the publishers, Messrs. McClure, Phillips and Company.) 518 THE JURASSIC PERIOD measured about 30 feet in length, and 18 in height in the walking posture. The stegosaurs were quadrupedal in habit, and had solid bones. Though not so large as some of the preceding, they were curiously armored, and formed a very remarkable group that frequented England ani Western America. The Stegosaurus of Colorado and Wyoming (Morrison beds) was one of the most unique (Fig. 441). Its diminutive head and brain imply a sluggish, stupid creature, de- pending for protection on its bulk and armor. A unique feature of the period was the development of pierosaurs, or flying reptiles. Appearing at the close of the Trias in a few yet imperfectly known forms, they were at the opening of this period, Fig. 442. Rhamphorhynchus phyllurus, a flying saurian. (Restored by Marsh.) fully developed flying animals, and later formed a diversified group which included long-tailed (Fig. 442) and short-tailed forms (Fig. 443). With little doubt they sprang from some agile, hollow- boned saurian, more or less akin to the slender, leaping dinosaurs. Between the ponderous forms (Figs. 440, 441) and the pterosaurs (Fig. 442), the Jurassic saurians present strange contrasts. Jurassic pterosaurs were small, but their successors ‘attained a wing-spread of nearly a score of feet. They were curiously com- posite in structure and adaptation. Their bones were hollow, their fore limbs modified for flight, their heads bird-like, and ‘their. jaws set with teeth, though toothless forms appeared later. They were provided with membranes stretched, bat-like, from the fore limbs ‘to the body and hind limbs, which served as organs of flight (Fig. 442). The fifth, or as some paleontologists believe, the fourth front digit was greatly extended, and supported the wing-membrane. The sternum was greatly developed, implying true powers of flight, LIFE 510 a conclusion supported by the occurrence of their remains in marine sediments free from other land fossils. Some of them had singular elongate rod-like tails, with a rudder-like expansion at the end. Pterodactyls (Fig. 443) had short tails, and were mostly small and slender. Fully differ- entiated as first found, they underwent no radical change of struc- ture during their career, and the steps of their remarkable evolution are for the most part unknown. Flying rep- tiles are extremely rare among the Jurassic fos- sils of North America. Turtles, which had lived elsewhere since the Middle Trias, made their first appearance in North America in , fe torso. beds, and Fig. 443. Skeleton of pterodactyl, Pterodac- the crocodilians became tylus spectabilis, from the lithographic stone at differentiated into sev- Eichstadt, Bavaria; about 34 natural size. (After eral branches. Primi- H- v- Meyer.) tive lizards were doubtless abundant, but because of their terrestrial habits and small size, they have little representation among the fossils, and none have been found in our continent. A less bizarre, but really greater evolution, was the differentia- tion of true birds. The remote ancestors of the pterosaurs and the birds may have been closely allied, but there is no evidence that birds descended from pterosaurs. The two are examples of analo- gous and parallel evolution, not of relationship. The oldest known bird, Archeopteryx macrura (Fig. 444), shows clear traces of a reptilian ancestry. From this ancestry it retained a long, vertebrated tail, reptile-like claws, teeth set in sockets, biconcave vertebree, and separate pelvic bones. On the other hand, its head and brain were bird-like, its anterior limbs adapted to flying in bird- 520 THE JURASSIC PERIOD (not pterosaurian) fashion, its posterior limbs modified for bird-like walking, and most distinctive of all, it was clothed with feathers. The presence of feathers, while yet the body retained so many reptilian features, is remark- able. But for their preservation, it is. uncertain whether the creature would have been classed as a bird or reptile. \\ The known speci- * men was somewhat ~ smaller than a crow. * The marvelous AY deployment of aquatic and terres- trial reptiles and of birds makes the scanty record of the mammals all the more singular. Only a few jaw bones of the size of those of mice and rats have been found. These Ni i low types are re- Fig. 444. The earliest ferred, without com- known bird, Archeopteryx plete certainty, to macrura. The long verte- marsupials. They brated tail, the clawed digits . of the fore limbs, and the 4PP¢at to have been toothed jaws are ancestral insectivorous. features to be specially not- The insects of the ed. (H. von Meyer.) : Jurassic appear to have included members of nearly all fossilizable groups not depend- ent on flowering plants. SS a > % \ WS \ 4, ARES j SS Ra ew AWE YR Mil WWW ZL > . \ ‘\ NAN \ ee Le VRS vy ; ‘a = i SS \S 4 AY \ SSA : cd ‘ \ \ ! hi, a Z . : AQ! \ 234i ‘i. \ . y lJ ag BX LE. aN Nk \ \S Kent < Sie ES \ \ RY j poe 2 Sa eS aT ea i SAW it } Ce? = -S — as eZ a > S ~ \ a \ \ ‘ \\ Ss NS DS y g oS a ZEEE A WS) » 1h) \) JAA Wa Wessess. c UHYA NY \Hh OB SS N x y S \e \ = \i A > yee f Ai S \\ << e 2. ANN |) K\ Z “Sy x Sg 7, < f ——_ == < == Map work. For suggestions as to map work see Laboratory Exercises in Structural and Historical Geology, Exercise X. CHAPTER XXIV THE COMANCHEAN (LOWER CRETACEOUS) PERIOD Definition. The history of the Cretaceous period, as formerly defined, was complex. At its beginning, the larger part of the North American continent was above the sea. During its progress, the sequence of events in our continent was somewhat as follows: (1) A somewhat widespread warping of the continental surface, resulting in extensive submergence in Mexico and Texas, and a lesser submergence along the Pacific coast. At about the same time the Atlantic and Gulf coasts and some parts of the western interior were sites of deposition, though not submerged. Prolonged sedimenta- tion followed. (2) Geographic changes which inaugurated a long period of erosion that affected the recent deposits as well as older formations. (3) Encroachment of the sea submerging the Coastal Plain of the Atlantic and the Gulf of Mexico, and presently the Great Plains, probably to the Arctic Ocean. On the Pacific coast, too, the sea gained on the land. Few greater transgressions of land by sea are recorded in the long history of the North American con- tinent. A long period of deposition was initiated by the sub- mergence. It was succeeded by (4) a widespread withdrawal of the waters from the continent, leaving the land area nearly or quite as large as now. The formations of the Cretaceous period have been divided, commonly, into two main series, a Lower and an Upper. To the former were referred the deposits of the earlier and lesser submer- gence, and to the latter, those of the later and more extensive sub- mergence. ‘The distinctness of the Lower and Upper Cretaceous is, however, so great that it is more in keeping with the spirit of modern classification to regard them as separate systems, and the corre- sponding divisions of time as periods. What was formerly called the Lower Cretaceous series is here called the Comanchean system. The propriety of this classification is the more striking, since it is applicable to other continents as well as to our own. 521 522 THE COMANCHEAN PERIOD Fig. 445. Map showing the distribution of the Comanchean formations North America. The conventions are the same as in preceding maps. j FORMATIONS AND PHYSICAL HISTORY 523 FORMATIONS AND PHYSICAL HISTORY Atlantic and Gulf border regions. That part of the Coman- chean system along the Atlantic coast is called the Potomac series; the part along the eastern Gulf coast, where conditions of sedi- mentation appear to have been similar, is the Tuscaloosa series. Fig. 445 shows that the system outcrops near the inland margin of the Coastal Plain. It is the lowest of the Coastal Plain formations. Neither the Potomac nor the Tuscaloosa series is believed to repre- sent the whole of the period, and the two are not strictly contem- poraneous. Conditions of origin, and constitution. By the beginning of the Comanchean period, both the Appalachian Mountains and the area to the east had been degraded well toward base-level, so that little warping of the surface appears to have been needed to convert _ portions of the coastal lands into sites of deposition, though more may have been necessary to provide lands high enough to furnish abundant sediments. The peneplanation of the eastern mountains during the Jurassic period was no doubt attended by deep decay of the underlying rocks, and the consequent accumulation of a heavy mantle of residuary earth. The warping which inaugurated the Comanchean period seems to have involved a rise of the Appalachian tract, and a consequent rejuvenation of the drainage from it, while the coastward tract was left relatively low and became a zone of lodgment for the sediments brought down by the quickened drain- age from the west. Lakes, marshes, etc., probably were features of the lodgment area. The deposits consist of gravel (or conglom- erate), sand (or sandstone), and clay, largely uncemented. The gravel and sand came chiefly from formations to the west. Both are arkose (containing particles of crystalline rock, not de- cayed when deposited) locally, showing that erosion sometimes exceeded rock decay. ‘This suggests high land to the west whence the sediments were derived, and is one of the reasons for the belief that it was tilted upward at this time. Beds of clay in the Potomac series have been utilized extensively, especially in New Jersey,! for the manufacture of clay wares. Some of it is notable for its bright and variegated colors, black, white, yellow, purple, and red being not uncommon. White is to be looked upon as the normal color; the others are the result of various impurities, the black being due te organic matter. 1 Cook, Geol. Surv. of New Jersey, 1868, and Kiimmel, 1904. 524 THE COMANCHEAN PERIOD The clay, sand, and gravel are disposed irregularly, doubtless the result of the physical conditions where the sedimentation took place, conditions which might have existed along the lower courses of rivers or at their debouchures, where shore-waters had little effect upon them. In addition to the clastic sediment, there is a little lignite, and some iron ore, and though both are widely distributed, neither is of much commercial value. | Structure and thickness. The Potomac and Tuscaloosa series are nearly horizontal, with a gentle dip seaward. The Potomac Fig. 446. Section showing relations of various members of the Coastal series. C, Comanchean; K, Cretaceous; L, Eocene; M, Miocene; P/, Pliocene; 0, Quater- nary. series rests unconformably on Triassic and other formations (Fig. 446), and the Tuscaloosa on Paleozoic or older strata. Both series are overlain unconformably by the Upper Cretaceous. The Poto- mac formations reach a thickness of 700 feet in but few places. The thickness of the Tuscaloosa series is about twice as great. Western Gulf Region. ‘The system is more fully represented in Texas than farther east, but its stratigraphic relations are the same. The beds appear at the surface over an area distant from the coast, dip seaward at a low angle, and are concealed near the coast by younger formations. The lower part of the system (the Trinity series) is perhaps the time equivalent of the Potomac, while the uppermost series (the Wichita) is probably younger than any part of the system on the Atlantic coast. Some parts of the system, especially the middle (Fredericksburg) are marine, and some ter- restrial. The marine part includes much limestone. The system here is much thicker than farther east, ranging from 1,000 feet to about 4,000. From Texas, the Comanchean formations, or some of them, originally spread northward into Kansas, northwestward to Colo- rado, and westward to Arizona. ‘Though they appear at the surface in small areas only, their extent may be considerable beneath younger formations. FORMATIONS AND PHYSICAL HISTORY 525 The Comanchean of Mexico is mainly limestone, and, though but imperfectly known, has been estimated to have the extraordi- nary thickness of 10,000 to 20,000 feet. Its distribution is such as to show that a large part of that country was beneath the sea. It has been conjectured that the waters of the Atlantic and Pacific met over the site of some part of Mexico at this time, but this is uncertain. If the oceans were connected, it was probably across southern Mexico, or perhaps Central America. At any rate, there does not seem to have been free faunal intermigration between the Gulf coast and the coast of California. Northern interior. The sea is not known to have extended north of Kansas during the period; but clastic beds of terrestrial origin, and perhaps of Comanchean age, are known at various points farther north. The beds in question, the Morrison (p. 504) beds, are best known along the Front Range through Colorado and Wyoming, and in the Black Hills, though they probably reach northward to Mon- tana. If all the beds thought to be the equivalent of the Morrison are really so, the formation is widely distributed. These beds are regarded by some as Jurassic, and this may be their proper classi- fication. In Montana, Alberta, and Assiniboia, there are beds (the Koote- nay and Cascade formations, etc.) similar in character to those just mentioned. ‘They are mainly clastic, and contain some coal. ‘Their fossils are mostly of plants of early Cretaceous types. In Mon- tana, the Kootenay formation overlies the Morrison. To the Morrison and Kootenay formations a lacustrine origin has usually been assigned, and there is perhaps no adequate ground for questioning this conclusion for some parts of the formations; but the character of some of the beds and the nature and distribu- tion of their fossils suggest a fluviatile origin for parts, and perhaps for large parts, of the series. Pacific border. The system (known as the Shastan group) has great development in California, where it attains its maximum known thickness. It is made up of the Kunoxville series below and the Horsetown series above. The deposits are thickest in the Sacramento valley. Most of the thick system, including its basal beds, bears the marks of shallow-water origin. ‘The Shastan group is represented in Oregon also. Where the base of the system has been observed, it is uncon- formable on Jurassic rocks, or on metamorphic rocks of unknown 526 THE COMANCHEAN PERIOD age. It is overlain unconformably in some places, and without apparent unconformity in others, by the (Upper) Cretaceous (Chico series). Farther north, Lower Cretaceous beds (Queen Charlotte series) occur in the Queen Charlotte Islands,' where they have an estimated thickness of between 9,ooo and 10,000 feet. In British Columbia, the coast line was east of the Coast ranges, and extended farther and farther east with increasing latitude, until the ocean swept clean ~ across the site of the Cordillera in the early part of the period, and extended south along the area which is now the east base of the mountains.” The Kootenay formation is perhaps partly contem- poraneous with these marine beds. The Comanchean system of British Columbia generally rests unconformably on the Triassic system, and contains some volcanic material and, locally, coal. Farther north, the Lower Cretaceous has not always been sepa- rated from the Upper, but the former has extensive development in some parts of northern Alaska, where it contains coal. It occurs also on the west coast of Greenland, where the beds are thought to represent some such horizon as that of the Kootenay, or Potomac. Close of the Period Considerable changes in the geography of North America brought the Comanchean period to a close. Along the Atlantic and Gulf borders considerable tracts were converted from areas of depo- sition into areas of erosion. The system was somewhat deformed and faulted in both Texas and Mexico. In the southern Coast Range of California there was folding of the Lower Cretaceous beds, accompanied by volcanic activity, while in other places the sea spread itself over areas which had been land. Still other areas in the west appear to have emerged at this time, and never to have been submerged since. On the whole, the deformative movements at the close of the period were more extensive, so far as present knowledge goes, than — those which occurred in the midst of any one of the Paleozoic periods as here defined. On stratigraphic grounds, therefore, the distinctness of the two systems is clear. The case is hardly less clear on the paleontological side. 1 Dawson, Geo. M., Am. Jour. Sci., Vol. XX XVIII, 1889, pp. 120-127. 2 Dawson, Science, March 15, 1901; Bull. Geol. Soc. Am., Vol. XII, p. 87. FORMATIONS AND PHYSICAL HISTORY 527 Lower Cretaceous in Other Continents } Europe. The deposits in some of the lakes, marshes, estuaries, and other lodgment basins which resulted from the geographic changes at the close of the Jurassic period in Europe, record the transition from that period to the early Cretaceous. ‘The interrup- tion of marine sedimentation in Southern Europe was not so general, and over considerable areas the Lower Cretaceous succeeds the Jurassic conformably, both being marine. During the early stages of the Lower Cretaceous, the areas of sedimentation were more or less isolated; but later, advances of the sea united many of them. The Lower Cretaceous formations include all sorts of clastic rocks, together with limestone, glauconitic beds, beds of coal (northwestern Germany), and iron ore. They embrace, indeed, about all varieties of sedimentary rock except chalk, the rock from which the name ‘‘Cretaceous”’ was derived. In southern Europe, much of the system is limestone. Other continents. In other continents, the Lower and Upper Cretaceous have been less clearly differentiated; yet enough is known to show that the Lower and Upper Cretaceous systems are, in gen- eral, markedly different, both in origin and distribution. Marine Lower Cretaceous is well developed in the western part of the Andes Mountains. It is widespread also in the northern part of South America, but not elsewhere east of the Andes. It is generally absent about the borders of the South Atlantic. On the other hand, marine Lower Cretaceous beds occur in many places about the southern Pacific and Indian Oceans. Lower Cretaceous formations of marine origin are widespread also in Siberia and Japan. The system is believed to have slight development in the mountains of northwestern Africa, where it is really an extension of the Lower Cretaceous of southern Europe, and is unconformable beneath the Upper Cretaceous, and in South Africa. Geographic changes of importance occurred in various parts of the earth at the close of the early Cretaceous period, as shown by (1) the unconformities between the Lower (Comanchean) and Upper Cretaceous systems, as at some points in Europe, north Africa, Australia, and South America, and (2) in the differences in their distribution. 1 The term Comanchean is not applied to the Lower Cretaceous formations outside of North America, 528 THE COMANCHEAN PERIOD Climate In the aggregate, the known fossils of the Lower Cretaceous of America are not such as to indicate great diversity of climate. Even in Greenland, the climate seems to have been as warm as that of warm temperate regions to-day. The fresh-water fossils of those deposits of central Europe which represent the transition from the Jurassic to the Lower Cre- taceous, indicate a climate far from tropical. It would seem to have been comparable to that of the temperate portions of America to-day. The fossils of lower latitudes denote a warmer climate. On the whole, European fossils seem to afford better evidence of the existence of climatic zones than those of America. LIFE Land vegetation. Fossil plants constitute the chief record of the life of the early stages of the Comanchean in America. The earliest flora was akin to that of the Jurassic, the cycadeans (Fig. 447), conifers, ferns, and horsetails being the dominant forms. In most of Europe, this group held possession of the land throughout the period, though angiosperms appeared in Portugal before its close. Descendants of Jurassic types of plants also continued throughout the period in northwestern America. Introduction of angiosperms. This period was marked by one of the most radical evolutions in the history of the plant kingdom. Angiosperms (p. 685), including both monocotyledons and dicoty- ledons, appeared early in the period, and developed so rapidly that by the beginning of the next they had overrun the continent. Their precise time and place of origin is not known, but present data point to the borders of the north Atlantic as the place of origin, and the late Jurassic or earliest Comanchean as the time. About 400 species of Comanchean angiosperms are known from the Atlantic coast. They were in a minority in the lowest Potomac, but increase to an overwhelming majority in the upper beds. The earliest forms are not really primitive, and throw little light on the origin of the group. The majority resemble modern genera, and a few (as Sassafras, Ficus, Myrica, and Aralia) are referred to living genera. Before the end of the period, figs, magnolias, tulip trees, laurels, and other forms referred to modern genera, but not to mod- ern species, had appeared. By this time the cycadeans had dropped LIFE 529 to an insignificant place, and the conifers and ferns, while not equally reduced, were subordinate to the angiosperms. Land animals. The aspect of the vertebrate life was inter- mediate between that of the Jurassic and Upper Cretaceous, and, so Tage § 1 a PR ES a a Roe ia 5 6 == SCENTI MET ERS e a ahs Fig. 447. A cycadean trunk from the Black Hills, Dakota, Cycadeoidea dako- tensis Ward, Lower Cretaceous. (Ward.) far as it is known, has been sketched already (p. 515). Little is known of other forms of terrestrial animal life, but it has been con- jectured that the great development of flowering plants was con- nected with the existence of abundant insect life. Fresh-water fauna. The molluscan fauna of the inland waters had assumed a pronouncedly modern aspect, as illustrated in Fig. 530 THE COMANCHEAN PERIOD 448. It probably had at- tained considerable impor- tance through the extension of the fresh waters, but the record is by no means so ample as would be expected if the deposits were made mainly in lakes and river channels. This is an addi- Fig. 448. FRESH-WATER FossIts OF THE tional reason for the grow- COMANCHEAN (Lower Cretaceous) from 8 Montana. a and Jb, Pelecypods: a, Unio ing opinion that the terres- farri Stanton; b, Unio douglassi Stanton; trial deposits were in con- c-e, Gastropods: c. Viviparus montanensis a Stanton; d, Goniobosis (?) ortmanni Stanton; oat res ie Hae e, Campeloma harlowtonensis Stanton. transient type, due to overflows, storm-wash, sheet-wash, and other forms of more strictly subaérial aggradation. Fig. 449. COMANCHEAN FOSSILS OF THE TEXAN PROVINCE. a-c, Echinoids: a, Holaster simplex Shum.; b, Diplopodia texanum Roemer; c, Hemiaster dalli Clark. d-h, Pelecypods: d, Anatina austinensis Vaughan; e, Homomya austinensis Vaughan; f, Trigonia emoryi Conrad; g, Lima wacoensis Roemer; h, Pecten texanus Roemer. i-l, Gastropods: 7, Fusus texanus Vaughan; j, Turritella budaensis Vaughan; k, Cerithium (?) texanum Vaughan; /, Trochus sp.; m, a coral, Parasmilia texana Vaughan. Marine faunas. Two very distinct marine faunas are found in North America, that of the Mexican Gulf and that of the Pacific LIFE | 531 Coast. The former had its connections eastward with Portugal and the Mediterranean region; the latter, northward and westward with Asia and Russia, though the boreal element is less conspicuous in the upper part (Horsetown). No species common to the two provinces is known. The decline of the boreal aspect of the western b Fig. 450. FossILs FROM THE SHASTAN SERIES (chiefly Knoxville). a-c, Cepha- lopods: a, Lytoceras batesii Trask; 6, Phylloceras knoxvillensis Stanton; c, Hoplites angulatus Stanton. d-h, Gastropods: d, Astresius liratus Gabb; e, Amberleya dilleri Stanton; f, Cerithium paskentaensis Stanton; g, Hypsipleura gregaria Stanton; h, Turbo moyonensis Stanton. i-q, Pelecypods: i, Pecten complexicosta Gabb; j, Corbula (?) persulcata Stanton; k and /, Aucella piochiit var. orata Gabb; m, A. crassicollis Keyserling; n, Astarte californica Stanton; 0, Arca tehamaensis Stanton; p, Nucula storrsi Stanton; g, Leda glabra Stanton; r, Rhynchonella whitneyi Gabb, a brachiopod. (After Stanton.) fauna may have been due to the closing of Bering Strait, thus shut- ting off cold currents from the Arctic.’ —The Comanchean faunas are said to represent three distinct facies, the reef facies, most con- spicuous, the /ittoral, and the deeper water facies. 1 Stanton, Jour. Geol., Vol. XVIi. CHAPTER XXV THE CRETACEOUS PERIOD FORMATIONS AND PHYSICAL HISTORY The Cretaceous period was ushered in, so far as North America is concerned, by a notable encroachment of the sea. Cretaceous formations are found in (1) the Atlantic Coastal Plain; (2). the Coastal Plain of the Gulf; (3) the Great Plains, from the Gulf of Mexico to the Arctic Ocean; (4) at many points in the western mountains; and (5) over considerable areas along the Pacific coast. While its distribution has much in common with that of the Coman- chean, it is much more widespread (Fig. 451), and unlike the Comanchean, this system is chiefly marine. Atlantic border region. Cretaceous formations come to the surface in a belt near the western margin of the Atlantic Coastal Plain (Fig. 451), just east of the outcrop of the Potomac series. The beds have been but little disturbed, and still dip, as when deposited, slightly to seaward, and in that direction pass beneath younger formations. ‘They are largely of unindurated clay and sand, with some greensand marl, which is rather characteristic of the system. The distinguishing constituent of this marl is glau- conite, primarily a hydrous silicate of potassium and iron,! which occurs in grains. Glauconite is now making in some parts of the sea, and from the situations in which it is formed, it is inferred that the conditions necessary for its development on such a scale as to make considerable beds, are the following:? (1) Water of moderate depth, too to 200 fathoms being the most favorable; (2) a meager supply of land-derived sediment; and (3) the presence of forami- nifera. The production of the glauconite seems to be effected by 1Most Glauconite is impure, and, as it occurs in nature, contains several other ingredients. 2 For brief summary concerning the origin of greensand marl, see Clark, Jour. Geol., Vol. II, p. 161. For a fuller account, see Challenger Report on Deep Sea Deposits. 532 Pim FORMATIONS AND PHYSICAL HISTORY 533 Fig. 451. Map showing the distribution of the Cretaceous formation in North America. The conventions are the same as in preceding maps. 534 THE CRETACEOUS PERIOD chemical changes in sediments perhaps as the result of decomposi- tion of the organic matter contained in the foraminiferal shells. The subdivisions now generally recognized are the following, commencing with the lowest: 1. Matawan formation; 2. Monmouth formation; 3. Rancocas formation; 4. Manasquan formation. ‘These formations are not all continuous throughout the coastal region, and all the formations show notable variations when traced along their strikes. Their aggregate thickness nowhere exceeds a few hundred feet. Eastern Gulf border. The outcrop of the Cretaceous formations ~ of the eastern Gulf states is shown in Fig. 452. Near the Missis- Kj : WAY Vda, = N =N =X LLG, Fig. 452. Map showing the positions of the several members of the Comanchean and Cretaceous systems in Alabama and adjacent states. C, Tuscaloosa series (Comanchean); Ke, Eutaw formation; Ks, Selma chalk; Kr, Ripley formation; Tr, Tertiary. (After Smith.) sippl, the belt of outcrops extends northward to Kentucky. Meager remnants (outliers) are found even north of the Ohio, in southern Illinois. In Alabama, where the Gulf Coast part of the system is best known, there are three principal divisions: the Eutaw below il FORMATIONS AND PHYSICAL HISTORY 535 (mainly clays and sands, some greensand, 300 feet), the Selma Chalk (1,000 feet) in the middle, and the Ripley (mainly sand, 200~500 feet) above. The Eutaw is believed to be the equivalent of the Matawan formation of the Atlantic coast, and the Ripley is thought to be older than the Rancocas. The Cretaceous beds of the Gulf coast have been disturbed more than the corresponding beds along the Atlantic coast. They have been bent into low anticlines and synclines in some places (Alabama), and faulted to a slight extent. Western Gulf region. The general stratigraphic relations of the system here are the same as farther east, but deposition seems to have been well under way in Texas before the oldest exposed beds of the system farther east were laid down. The system has a maximum thickness of about 4,000 feet. Three principal subdivis- ions are recognized: (1) The Dakota; (2) the Colorado; and (3) the Montana. ‘The Dakota formation, 600 feet and less thick, is largely of sandstone, with some lignite, and is, for the most part, of non- marine origin. The Colorado series contains much limestone (or chalk) of marine origin. Its thickness is about 1,000 feet. The Montana series is more largely clastic, and from it the oil of the Corsicana oil field of Texas is derived. Locally, the system is much faulted. From Texas it is continued northward into Arkansas, and westward into New Mexico. The Cretaceous of the western Gulf region differs from the cor- responding system farther east in its greater thickness, and in its greater proportion of calcareous matter, largely in the condition of chalk. Of limestone or chalk, the Cretaceous of the Atlantic coast contains little, that of the eastern Gulf region (Alabama and Mis- sissippi), more, and that of Texas much; nor is the chalk confined to the Gulf region, as will be seen. Western interior. One of the standard sections of the Creta- ceous system of the western interior consists of the following sub- divisions, commencing at the bottom: 1. Dakota; 2. Colorado (including the Benton and the Niobrara formations); 3. Mon- tana (including Ft. Pierre and Fox Hills); and 4. Laramie. This classification, however, does not fit all parts of the west. The Dakota formation, mainly of non-marine origin, is wide- spread in the Great Plains, though most of it is buried. It extends westward beyond the Rocky Mountains at many points. The formation is largely sandstone, though it contains much conglom- erate and clay, and some lignite. It is perhaps to be regarded as 536 THE CRETACEOUS PERIOD the joint product of subaérial and fluviatile deposition. The pres- ence of bird tracks in Kansas, and the widespread abundance of fossil leaves of angiosperms, in a condition which precludes much transportation, imply subaérial sedimentation to a notable extent at least. The upper part of the formation carries some marine fossils. North of Texas the formation is in apparent conformity with the Comanchean in some places, though in others it rests on older formations. The Dakota sandstone is an important source of water in the semi-arid plains. The water enters where the sandstone outcrops. near the mountains, and follows the beds down their dip to the east- ward. Along the east base of the Rocky Mountains, where the beds Fig. 453. A group of concretions weathered out from the Dakota sandstone. Near Minneapolis, Kan. (Schaffner.) have been tilted, the less resistant formations associated with this sandstone have been removed or worn down, leaving the outcropping edges of this formation as ridges or “‘hogbacks” (Fig. 93), char- acteristic of the east base of the Rocky Mountains much of the way from New Mexico to Canada. . The Colorado series records an extensive invasion of the western FORMATIONS AND PHYSICAL HISTORY 537 interior by the sea, the invasion going so far, probably, as to estab- lish connection between the Gulf of Mexico and the Arctic Ocean, over the site of the Great Plains. Clastic formations predominate in the Colorado series as a whole, but there are beds of chalk com- parable to those of Europe, from Texas to South Dakota. The aggregate thickness of the series is locally as much as 3,000 feet, as strata are measured, though its average thickness is much less. The origin of chalk. There has been much difference of opinion concerning the origin of chalk. Its resemblance to the foraminif- eral ooze of the deep seas long since led to the belief that it was a deep-sea deposit; but closer examination has thrown doubt on this conclusion, for the differences between the chalk and foraminiferal ooze are as striking as their likenesses. Both consist largely of the shells of foraminifera, but with them are associated shells of other types. The echinoderms, the sponge spicules, and the secre- tions of certain microscopic plants of the chalk correspond in a general way with those of the oozes now forming, and are consistent with the deep-water origin of the chalk. The molluscan shells of the chalk, on the other hand, seem to point with clearness to water no more than 30 to 50 fathoms deep. The distribution of the chalk and its relations to other sedimentary beds indicate its deposition in shallow water, not in water comparable in depth to that in which oozes are now formed. On the whole, the balance of evidence is in favor of the view that the Cretaceous chalk was deposited in rela- tively shallow water. The conditions for its origin seem to have been clear seas, with a genial climate. Its materials may accumu- late as well on the bottom of a shallow sea as on the bottom of a deep one, if clastic sediments are absent.’ Following the Colorado epoch there were changes in the sedi- mentation and in the life of the western interior sea. The Mon- tana series is chiefly clastic, but the area of sedimentation was somewhat contracted. The beds are largely marine, and the water shallowed as the epoch progressed. Land formations also are found in the series. Local beds of coal give evidence of marshy conditions. Like other parts of the system, the Montana series abounds in con- cretions, some of which attain great size. The thickness of the series is variable, and its maximum is great. From 8,700 feet in Colorado, it thins to 200 feet in some parts of the Black Hills. Deposition continued in the Great Plains and to some extent 1¥For fuller statement of this subiect see Earth History, Vol. III, p. r4o. 538 THE CRETACEOUS PERIOD west of them through the last epoch of the Cretaceous period, but most of the sedimentation was non-marine. Fresh-and-brackish- water beds are widely distributed. The Laramie series records the transition from the marine con- ditions of the Montana epoch to the fresh-water and land condi- tions of the Tertiary in the same region. This change did not take place everywhere at the same time. ‘The series consists primarily of sandstone and shale, with some conglomerate; but with these clastic formations there is much coal. Both shale and coal are © more abundant below than above, while in the upper part of the ~ series conglomerate is not rare. The thickness of the Laramie series is estimated at 1,000 to 5,000 feet, exclusive of the transi- tion (Mesozoic-Cenozoic) beds to be mentioned below. In not a few places there is an unconformity in the great group of strata formerly classed as Laramie, and there is difference of opinion as to whether the part above this unconformity should be called Lara- mie. The present tendency is to regard it as Eocene.! In a considerable area of northeastern Wyoming, and in a large area farther north, some of the Laramie lignite has been burned in the ground. The burning was relatively recent, and locally is still in progress. ‘The firing appears to have taken place at the outcrops on hill and valley slopes. The burning was accompanied by fusion, semi-fusion, and baking, resulting in lava-like slag and brick-red banks of indurated clay. Coal. The Cretaceous is pre-eminently the coal period of the west. Coal-beds occur in every one of its principal divisions in this part of the continent. The total amount of coal, chiefly in the Laramie, is perhaps comparable to that in the Pennsylvanian sys- tem, though the coal is not now so accessible, and its quality not so good. It is estimated that along the east and west bases of the Rocky Mountains there are more than 100,000 square miles of coal- bearing lands, and Colorado alone is estimated to have 34,000,000- ooo tons of available coal,? most of which is Cretaceous. The coal is largely lignite, though in Colorado not a little of it has been advanced to coking bituminous coal, and even to anthracite, where | 1The Laramie question is well reviewed by Cross, Washington Acad. of Sci., Vol. XI, pp. 27-45, 1909. Other recent discussions by Veatch are found in Am. Jour. Sci., Vol. XXIV, (1907), p. 18, and Jour. Geol., Vol. XV, 1907. See footnote — Pp. 539- 2 Storrs, 22d Ann. Rept., U. S. Geol. Surv., Pt. ITI. FORMATIONS AND PHYSICAL HISTORY 539 it has been affected by intrusions of igneous rock. The areas of Laramie coal are indicated in Fig. 438. Transition beds between Mesozoic and Cenozoic.! There are divers, more or less local, terrestrial formations in the west which have been referred now to the Cretaceous (Laramie,— or more exactly, to the upper Laramie or post-Laramie), now to the Tertiary _ Fig. 454. An outcropping ledge of clay, hardened by the burning of the coal- bed below. Except in the immediate vicinity of the burnt-out coal-bed, the clay is not indurated. Near Buffalo, Wyo. (Blackwelder.) (Eocene). These formations are, generally speaking, unconform- able on the Laramie, and in some places seem hardly separable from the recognized Tertiary? (Fort Union). Their reference to the Eocene seems to be justified both on stratigraphic and paleontologic grounds, so far as present data are concerned. Pacific coast. The Cretaceous system is represented on the 1 The questions involved in the formations here referred to are discussed in the following recent papers: Stanton, Am. Jour. Sci., Vol. XXX, and Wash. Acad. Sci. Vol. XI; Knowlton, Wash. Acad. Sci., Vol. XI,and Am. Jour. Sci., Vol. XXXV; Stone and Calvert, Econom. Geol., Vol. V; Lee Am. Jour. Sci., Vol. XX XV; and Cross, Wash. Acad. Sci., Vol. XI. 2 Here belong the Arapahoe, Denver, Raton, Monument Creek and perhaps other beds of Colorado, the Carbon, Evanston, and Lance (Ceratops) beds of Wyoming, and the Lance formation, and part of the Livingston beds of Montana. 540 THE CRETACEOUS PERIOD Pacific coast by the marine Chico series. At the time of its origin, this series probably extended along the coast from Lower California to the Yukon. ‘The Chico series rests on the Shastan unconformably in some places, and overlaps it at others. The fauna of the Chico series is littoral. Its oldest portion is older than the fauna of the Colorado series, and its youngest is older than the fauna of the youngest Cretaceous beds. Close of the Period About the close of the Cretaceous period a series of disturbances. was inaugurated on a scale which had not been equaled since the close of the Paleozoic era. These changes furnish the basis for the classification which makes the close of this period the close of an era. These disturbances continued into later times, but the close of the Cretaceous may be said to have been the time when the changes had advanced so far as to make themselves felt profoundly. They consisted of deformative movements, a part of which were orogenic, and of igneous eruptions on an almost unprecedented scale. General movements. In the closing stages of the period, the sea which had lapped over the Coastal Plains of the Atlantic and Fig. 455. Section showing the position of the Cretaceous beds in western Oregon. Mg, meta-gabbro ot unknown age; sp, serpentine; as, amphibolite schist; Jr, Jurassic (?); Km and Kmw, Cretaceous; Eu, Eocene; Hd, Eocene diabase. (Diller, Roseburg, Ore., folio, U. S. Geol. Surv.) : the Gulf of Mexico was withdrawn toward the abysmal basin. At about the same time, the Appalachian Mountains, which had been reduced to a peneplain by this time, were bowed up again. It is probable that most of the Cordilleran region was elevated FORMATIONS AND PHYSICAL HISTORY 541 bodily at this time, though not to a great height. Without further details, it may be said that enough is known to make it probable that a large part of the continent was affected by deformative movements of a gentle sort. Orogenic movements. The growth of mountains by folding probably was in progress in the closing stages of the Cretaceous period from Alaska on the north to Cape Horn on the south,— more than a quarter of the circumference of the earth. At the same time folding movements probably affected the Antillean mountain sys- tem,' between the southern end of the Cordilleran and the northern end of the Andean systems, for in several of the Antillean islands later formations rest unconformably on the deformed Cretaceous beds. Where the Eocene rests conformably on the Laramie,. the disturbances of this time are not clearly distinguishable from those of later date, which increased the folding initiated in this epoch. Some of the folded ranges of the western mountains began their history at this time, others had a new period of growth, and still others date from a later time; yet the close of the Laramie was a time of general orogenic movement in the western part of North ‘America. The Rocky Mountain system may be said to have had ‘its birth at this time. That these mountains are not older is shown by the deformation of the Laramie beds along with those of greater age. That some of the folding was not younger is shown by the lesser deformation of the Tertiary beds in the same region. Faulting. The growth of mountains at the close of the Creta- ceous was accompanied by faulting on a somewhat extensive scale throughout the region of movement, though the faulting of this time Fig. 457. Section in northern Montana, showing Proterozoic rock, A, thrust over Cretaceous, K. Subsequent erosion has removed much of the overthrust beds, but Chief Mountain is a remnant of them. cannot be distinguished everywhere from that of later date. In the Rocky Mountains of British Columbia, one overthrust fault has been located which crowded the Cambrian rocks obliquely up over the Cretaceous. The horizontal displacement is estimated to be 1 Hill, Nat. Geog. Mag., Vol. VII, p. 175. 542 THE CRETACEOUS PERIOD as much as seven miles,' and the throw 15,000 feet. Near the na- tional boundary, the displacement along what appears to be the same fault crowded the Proterozoic up over the Cretaceous ? by a move- ment of equal magnitude (Fig. 457). The exact date of these faults has not been determined, but was, perhaps, mid-Tertiary. Fig. 458. Chief Mountain. (Willis, U. S. Geol. Surv.) Igneous eruptions. The close of the Cretaceous was marked by the inauguration of a period of exceptional igneous activity, con- tinuing far into the Tertiary. During this period, great bodies of igneous rock, both extrusive and intrusive, were forced up. Erup- tions occurred in other lands at about the same time. Upper Cretaceous of Other Continents * Europe. The distribution of the Upper Cretaceous strata of Europe shows that extensive transgressions of the sea occurred at the beginning of the period, for in some parts of the continent marine Cretaceous formations overlap all older Mesozoic systems. During the closing stages of the Upper Cretaceous, fresh-water beds 1 McConnell, Geol. Surv. of Canada, Vol. II, Rept. D, p. 33, 1886. ? Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 307, 331-335. 3’The term Comanchean has not been applied outside of North America, and the Cretaceous system will therefore be referred to as Upper Cretaceous. FORMATIONS AND PHYSICAL HISTORY 543 Fig. 459. Map and section showing relations of igneous rock to the Cretaceous formations in the Crazy Mountains of Montana. ‘The section is along the line AB of the map. Klv, Livingston formation; d2, diorite; gr, granite. The especial feature of the map is the extraordinary number of dikes radiating from the central intrusion, di. Length of section about 20 miles. (Livingston and Little Belt, Mont., folios, U. S. Geol. Surv.) appear in localities (Alpine region) where marine sedimentation had been in progress earlier in the period, showing that the movements which were to mark the close of the era were making themselves felt. Limestone is the dominant sort of rock in the Upper Creta- 544 THE CRETACEOUS PERIOD ceous of southern Europe, showing that clear seas still prevailed, as in the Early Cretaceous period. From a characteristic genus of fossils, much of the limestone is known as Hippurite limestone. In the system farther north, there is more clastic material. The most notable petrographic feature of the Upper Cretaceous of Europe is the chalk. Both in England and France it attains an aggregate thickness of several hundred feet, though much of it is far from pure. It grades into marls and clays on the one hand, and into sandstone on the other. Chalk is, however, by no means co-extensive with the system, for it has little development outside of the Anglo-French area. Greensand occurs in the Upper Creta- ceous as well as in the Lower. Asia. The submergence of Europe and North America at the beginning of the Upper Cretaceous finds its parallel in other conti- nents. There are extensive areas of Hippurite limestone in south- western Asia, closely connected with that of Europe on the one hand, and with that of North Africa on the other. The Himalayan region seems to have been still beneath the sea, for Upper Cretaceous forma- tions are found in the mountains at great elevations. South of these mountains there appears to have been a large tract of land, in- cluding much of the peninsula of India, which has been thought to have stretched southwest to Africa; but the configuration of the sea-bottom does not lend this view much support. Upper Cretaceous beds occur on the coast of China, and in Japan. In many places they rest on formations older than the Lower Cretaceous, and therefore record an increased submergence dating from the beginning or early part of the Upper Cretaceous. On the other hand, northern Asia, which was largely submerged during the earlier Cretaceous period, was largely land during the later. Late in the Upper Cretaceous occurred the extensive lava- flows of the Deccan. These flows, 4,000 to 6,000 feet in thickness, cover an area of something like 200,000 square miles, and are the most stupendous outflows of lava recorded. The fossils in sediments interbedded with the lava show that the flows were subaérial. | Africa. In northern Africa, the Upper Cretaceous beds overlie the Lower unconformably, and spread southward, covering most of the desert, and so indicating great submergence in the north African region. South of the Sahara, no Upper Cretaceous beds are known except in a few small areas about the coast, where they rest on - FORMATIONS AND PHYSICAL HISTORY 545 crystalline schists with no Lower Cretaceous beds beneath, so far as now known. South America. In South America the sea invaded eastern Brazil, where marine Upper Cretaceous beds cover and overlap the non-marine Lower Cretaceous. In some parts of Brazil, however, the Upper Cretaceous is represented by fresh-water beds only. Farther west, marine Upper Cretaceous beds rest unconformably on the Lower Cretaceous, and form the summits of parts of the eastern Andes, occurring up to altitudes of 14,000 feet at many points, and locally even higher. There appears to have been great volcanic activity in the Andean system (Chile and Peru) during the late Cretaceous. Australia. The phenomena of Australia are in harmony with those of other continents. The Upper Cretaceous beds are wide- spread, locally resting on formations older than the Lower Creta- ceous. Furthermore, the Upper Cretaceous (Desert sandstone) is in many places unconformable on the deformed Lower Cretaceous. General. In general it may be said that there was little marine sedimentation in the Late Cretaceous period north of the parallel 60° north, where the Jurassic and Lower Cretaceous systems are more widespread. Between the parallels of 20° and 60°, on the other hand, the zone where marine Lower Cretaceous is but slightly developed, the Upper Cretaceous system is widespread. Outside of China, the Upper Cretaceous system is wanting over no consider- able land-area within these limits. In the equatorial and south temperate zones, the Upper Cretaceous seas were also expanded much beyond the limits of the waters of the preceding period. Climate The climate of North America throughout most of the Creta- ceous period seems to have been rather uniform and warm through a great range of latitude. In Greenland, Alaska, and Spitz- bergen, climatic conditions seem to have been similar to those in Virginia. Toward the close of the period the temperature was perhaps lower, for the Laramie flora is a temperate, rather than a tropical, one. The fresh-water fossils of central Europe indicate a climate comparable to that of Malaysia. As this seems to have been a period of low land, widely extended epicontinental seas, extensive calcareous deposits, and slow consumption of carbon 546 THE CRETACEOUS PERIOD dioxide in the carbonation of rock, there was a combination of conditions regarded as favorable for a mild and uniform climate. LIFE Land plants. Angiosperms predominated in North America at the beginning of the Cretaceous, and during the period genera now living came to be numerous, giving the flora a modern aspect. Among the living genera which made their appearance were those which include the birch, beech, oak, walnut, sycamore, tulip-tree, and maple. Among the gymnosperms there was a notable devel- opment of the sequoias, which now include the giant trees of Cali- fornia. Of special interest was the presence of genera in Europe and the United States which are now confined to the southern hemi- sphere. Toward the close of this period, monocotyledons first became abundant, so far as the record shows. Palms were plentiful, even in northerly latitudes, before the close of the period. Of greater importance because of their relations to the evolution of grazing animals, was the appearance of grasses, which attained prominence later. | It is worthy of remark that the introduction of dicotyledons, the great bearers of fruits and nuts, and of monocotyledons, the greatest grain and fodder producers, was the groundwork for a profound evolution of land animals. A zodlogical revolution, as extraordinary as the botanical one, might naturally be anticipated; but it did not follow immediately, so far as the record shows. ‘The reptiles seem to have roamed through the new forests as they had through the old, without radical modification. But with the open- ing of the next era, the anticipated revolution in the animal life of the land made its appearance, and advanced with great rapidity. The new flora spread widely. The European flora was very much like the American, and there was a close resemblance between the plants of mid-Greenland (70°-72° Lat.) and those of Virginia, indicating climatic conditions of remarkable uniformity. Not only this, but the flora was of a sub-tropical type. Land animals. ‘The terrestrial animals had the same general aspect as in the preceding period. Dinosaurs still retained the lead- ing place among land reptiles, though carnivorous forms were less abundant and varied than before. Among them was a leaping, kai garoo-like form with a length of 15 feet. The most singular Fig. 460. GROUP OF FOSSIL LEAVES OF TYPICAL CRETACEOUS PLANTS FROM THE DAKOTA HORIZON. @, Liriodendron giganteum Lesq.; b, Nyrica longa Heer; c, Magnolia pseudoacuminata Lesq.; d, Sterculia mucronata Lesq.; e, Quercus suspecta Lesq.; f, Viburnum inequitaterale Lesq.; g, Betulites westi, var. subintegrifolius Lesq.; h, Sassafras subintegrifolium Lesq.; i, Ficus inequalis Lesq. dinosaurian development was in the herbivorous branch. Some of the forms were very large, of quadrupedal habit, with enormous 548 THE CRETACEOUS PERIOD Fig. 461. Triceratops prorsus Marsh, from the Laramie Cretaceous. (Froma painting by C. R. Knight in the U. S. National Museum.) Fig. 462. Spoonbill Dinosaurs of the Cretaceous Hadrosaurus mirabilis (Leidy) as interpreted by Knight. (Osborn, Copyrighted by the Am. Mus. of Nat. Hist.) LIFE 549 skulls which extended backwards over the neck and shoulders in a cape-like flange (Fig. 461). Added to this was a sharp, parrot-like beak, a stout horn on the nose, and a pair of large pointed horns on the top of the head. One of the larger skulls measures eight feet from the snout to edge of the cape. This excessive provision for defense was accompanied by a very small brain cavity. Marsh remarks that they had the largest heads and the smallest brains of the reptile race. They were doubtless stupid and sluggish. . The ornithopod division was well represented (Fig. 463). .Their-hinder parts were large, their limbs were hollow, and their footprints indi- - cate that eaey walked in kangaroo-like attitude. ; a) SAY Ker wk OY 277i BK SY Aes SAS YY A a SX D ee on Qh une At Sa Ed - “Fig. 463. A Cretaceous dinosaur of the ornithopod division, Claosaurus annectens. (Restored by Marsh.) Terrestrial turtle remains are found in the Dakota sandstone, and the fossils of species inhabiting fresh waters in the late Cretaceous deposits of Canada. Of true lizards, only one late Mesozoic form is known, and that of small size and uncertain affinities, from the Laramie. Snakes made their first appearance, so far as known, in the later part of the period, but they were small... Crocodiles under- 550 THE CRETACEOUS PERIOD went a marked change early in the period, developing into the modern forms, though some of the old types lived on. Flying reptiles made so distinct an advance in specialization that Williston regards them as having come to excel all other flying Fig. 464. A Cretaceous pterodactyl, Nyctosaurus gracilis Marsh, about 1/10 natural size, Niobrara Cretaceous, Kansas. (Restored by Williston.) vertebrate animals. Some had a wing-spread of perhaps 20 feet. In some of the genera (Fig. 464) the development of the front parts was great, while the hind parts were so very small and weak that it is doubtful whether they could stand on their feet alone. The Cretaceous forms were all short-tailed, and for the most part tooth- less. Their bills resembled those of modern :birds. Terrestrial birds existed, but their record is meager. There were some curious aquatic forms, which will be mentioned with the sea life. The mammals thus far recovered from the Cretaceous Fig. 465. A Cretaceous mosasaur, Platacarpus corypheus Cope, restored by Williston; from Upper Cretaceous, Kansas. indicate little advance on those of the Jurassic, and they appear to have played very little part in the fauna of the period. | Sea life. Vertebrates. The ichthyosaurs and plesiosaurs which LIFE 551 had dominated the Jurassic sea lived on into this period. The former became insignificant soon after its beginning; but the plesio- saurs attained their highest development and perhaps their greatest size at this time. The American plesiosaurs indicate lack of inter- migration between this continent and Europe. The aquatic branch of the scaled saurians (Squamata) became veritable sea serpents. The long-necked, lizard-like reptiles of the Comanchean period were the forerunners, and perhaps the direct ancestors, of a family (the mosasauritans, Fig. 465) which flourished in the Cretaceous, and ranged from the Americas to Europe and New Zealand. Their short career seems to have ended with the period, and no direct descendants are known. Marine turtles seem to have appeared first in this period, and to have had many forms. Some of them had skulls larger than those of horses, and their shells must have been fully twelve feet across. Fig. 466. Champsosaurus, from the Laramie of Montana. Length, about six feet. (After Brown.) In the long interval between the first known appearance of birds in the Jurassic, and the later Cretaceous when they reappeared, important changes took place, among which was the loss of the elongate, bilaterally feathered tail. The Jurassic birds were terres- trial, while the Cretaceous were aquatic. The Cretaceous birds include about 30 species belonging to two widely divergent orders, Hesperornis and Ichthyornis. The former (Fig. 467) were large, flightless divers, with aborted wings and remarkable legs. The legs were not only very powerful, but the bones of the feet were so joined to them as to allow the feet to turn edgewise in the water when brought forward, thus increasing their efficiency as paddles. Fur- thermore, the legs were so joined to the body frame as to stand out nearly at right angles to it, like a pair of oars, instead of being under the body like walking legs. Apparently, walking as well as flying 552 THE CRETACEOUS PERIOD Fig. 467. Restoration of the great toothed diver of the Cretaceous, Hesperornis, by Gleeson. (From Lucas’ Animals of the Past; by permission of the publishers, McClure, Phillips and Co.) had been abandoned, and the bird was adapted to swimming and diving only. The jaws had teeth set in grooves like those of primi- tive saurians, and in other respects were like the jaws of snakes. As some of these strange birds attained a length of six feet, they were doubtless formid- able enemies to the sea life on which they chose to feed. They have been found in Kansas, Montana, North Dakota, New Jersey, and England, and probably frequented epicontinental seas somewhat widely. The second type Ichthyornis (Fig. 468) was scarcely larger than a pigeon, and had great power of flight, as in- dicated by the strong development of the wings and keel. At the same time, their legs and feet were small and slender. They had teeth in sockets. Fig. 468. Ichthyornis victor, a Cretaceous toothed bird of flight, 1/10 natural size. stored by Marsh.) (Re- For explanation of Figure, see top of page 554. 554 THE CRETACEOUS PERIOD Fig. 469. CRETACEOUS Fosstts. a-e, Echinoderms: a, Pedinopsis pondi Clark; b, Cassidulus subquadratus Con.; c, Botriopygus alabamensis Clark; d and e, Salenia tumidula Clark. f, g, and h, Pelecypods: f, Ostrea soleniscus Meek; g, Idonearca nebrascensis Owen, allied to the arcas of to-day; h, Inoceramus vanuxemi M. and H. 7i-l, Gastropods: 7, Neptunella intertextus (M. and H.); 7, A phorrhais ee (White); &, Drepanochilus nebrascensis (E. and S.); 1, Pyropsis bairdi (M. and H. Fig. 470. CRETACEOUS CEPHALOPODS: 4a, Nautilus meehanus Whitf., one of the simplest types of closely coiled cephalopods; b, Helicoceras stephensoni Whitf., an ammonite coiled in a heliciform spiral, and c, its highly complicated suture; d, Prionotropis woolgari (Mantell), a normal ammonite with ornamented shell, and e, complex sutures; f, Ptychoceras crassum Whitf., an ammonite shell which is recurved upon itself, but not coiled; g, suture of f; k, Scaphites nodosus Owen, an ammonite showing as light tendency to uncoil in the last volution; z, Baculites grandis M. and H. Their biconcave vertebre and other skeletal features, as well as their small brains, suggest reptilian relationships. Their habitat was the same as that of Hesperornis, and yet the two were farther apart, structurally, than any two types of birds now living (Marsh). LIFE oR The old types of fishes gave place to new ones (the teleosts) dur- ing this period. This change set in during the Comanchean, and was complete by the middle of the Cretaceous, though representa- tives of the older types lived on. Invertebrates. The most notable departure from the preceding ages is the prominence of foraminifers among the fossils. They made large contributions to the chalk of the period, and they were concerned in the formation of the greensand, scarcely less char- acteristic of the period than the chalk. While some of these mi- nute organisms live on shallow bottoms, on fixed alge, and in abysmal water, they are chiefly inhabitants of the surface waters of the open sea. Sea-urchins (a-e, Fig. 469) were quite abundant, and lent one of its characteristic aspects to the fauna. Corals and crinoids, so long associated with clear seas, were not plentiful. In the clastic formations, pelecypods (f—-h) and gastropods (i-1) abound (Fig. 469). It will be seen by a glance at Fig. 469 that they have a modern appearance. Cephalopods were still abundant, though ammonites were in their decline and showed erratic forms, attended by excessive ornamentation, comparable to that which marked corresponding stages of the trilobites and crinoids. Odd forms of partial uncoiling, or of spiral and other unusual forms of coiling, were common (Fig. 470). Interesting forms, perhaps to be classed here, were the Baculites (i), which resumed the straight form of the primitive Orthoceras, while retaining the very complicated sutures of the Ammonites (c). Map work. Folios of the U. S. Geol. Surv. and Laboratory Exercises in Structural and Historical Geology, Exercise XI. In the folios of the U. S. Geol. Surv. both Comanchean and Cretaceous are classed as Cretaceous, though the two are distinguished, in some cases, in the text and on the maps. THE CENOZOIC ERA CHAPTER XXVI THE EOCENE AND OLIGOCENE PERIODS The remaining periods of geological history constitute the Cenozoic era, or the era of modern life. The earlier part of the era is called the Tertiary, and the later the Quaternary. The Tertiary is variously subdivided, as shown below: Recent or Human. Post-glacial formations Quaternary Pleistocene or Glacial. Glacial formations and non-glacial deposits of glacial age Cenozoic I II Ill Era Pliocene Pliocene N : Miocene Miocene pete Tertiary ' Oligocene Eo E ety cene ocene FORMATIONS AND PHYSICAL HISTORY There is much to be said for a two-fold division of the Tertiary, the first including the Eocene, Oligocene, and Early Miocene, and the second the later Miocene and Pliocene. This division differs from that of the right-hand column above only in putting the lower part of the Miocene in the lower division. Eocene formations appear in widely separated parts of North America (Fig. 471), though they do not appear at the surface over large areas. They include marine formations, brackish-water forma- tions (made in bays and estuaries), and land (lacustrine and subaé- rial) formations. Themarineand brackish-water beds areconfined to the borders of the continent, while the terrestrial deposits are found in the Great Plains and farther west. Many of the formations are not indurated, but locally they are even metamorphosed. The eastern coast.! Eocene formations appear at the surface 1 Dall, 18th Ann. Rept., U. S. Geol. Surv., Pt. IT. 556 Fig. 471. Map showing the distribution of the Eocene formations in North America. The conventions are.the same as in former maps. in an interrupted belt near the coast from New Jersey to Texas. Their structure is similar to that of the Cretaceous system, upon 558 EOCENE AND OLIGOCENE PERIODS which they are unconformable (Fig. 446). Clays, sands, and green- sand marls are the most common materials of the system, and the conditions of sedimentation were much as in the preceding period. The system is thicker (1,700 feet maximum) in the Gulf region than on the Atlantic coast. It contains much lignitic matter in places, showing that marine conditions were not uninterrupted. In Texas, gypsiferous and saliferous sediments recur at various horizons, though most of the beds are of marine origin, and there are Lecenpb MARINE I FRESHWATER 7 | / tty, i / ——S SS —— ——- — eee ———s ==, eran se — a a —— = = — = = = e 2 ° ' | | —— Fig. 472. Map showing supposed distribution of land and water on the Pacific coast of the United States during the Eocene period. (Ralph Arnold.) numerous local unconformities in the system, suggesting repeated changes in the conditions of sedi- mentation. The Pacific coast. Marine and brackish-water beds. Marine Eocene formations are widespread west of the Sierra and Cascade ranges (Fig. 472), and have con- siderable development in Alaska. Throughout Washington and Oregon and in parts of California, the Eocene is unconformable on the Cretaceous (or Shastan), but in much of California it is con- formable on the Chico, the plane between the two being defined by fossils. These relations suggest that just before the Eocene, all of Washington, most of Oregon, and parts of the coastal region of California were land, over which the sea advanced later. The rocks are mostly clastic, sandstone and shale predominating, but there are conglomerates, tuffs, and diatomaceous’ shales, the last thought to be a source of oil. In not a few places, marine beds are succeeded by brackish-water de- posits. 1 Arnold, Jour. Geol., Vol. XVII. FORMATIONS AND PHYSICAL HISTORY 559 By the beginning of the Eocene, the Puget Sound depression, perhaps to be correlated with the great valley of California and the Gulf of California, had begun to show itself. The lands east and west of the sound were high, but not mountainous; and the region of the sound was a great estuary, in and about which deposition was in progress. Some of the sediments accumulated in brackish water and on land, and resulted in the thick coal-bearing Puget series of Washington, the upper part of which is Oligocene or even Miocene. The series is said to contain 125 beds of coal thick enough to attract prospectors. Most of the workable coal is in its lower part. The area of deposition extended south into Oregon, and east toward the Blue Mountains of that state. The system has an estimated thick- ness of 10,000 to 12,000 feet in southern Oregon, and but little less in southern California. British Columbia appears to have been land during the period, but Eocene beds, much disturbed ( Kenai series), have been recog- nized in Alaska, where they are coal-bearing in places. After the Eocene there was a time of temporary elevation, erosion, and volcanic activity along the Pacific coast, with consider- able basaltic flows in Washington and Oregon. The western interior. The warpings, faultings, and the intru- sions and extrusions of lava which marked the close of the Meso- zoic era in the west appear to have developed lands which were relatively high, adjacent to tracts which were relatively low. The steep slopes of the mountain folds, fault scarps, and volcanic piles seem to have afforded the conditions for rapid erosion, while the adjacent lowlands furnished places of lodgment for much of the sediment. Some of it took the form of fans and alluvial plains, and some of it probably lodged in lake basins formed by warping and faulting, or by the obstruction of valleys by lava flows. The wind also made its contribution to the deposits of the time, and the Eocene system contains much pyroclastic material. The result was a combination of lacustrine, fluvial, pluvial, eolian, and vol- canic deposits. The sites of principal sedimentation shifted somewhat from time to time, and among the widely distributed deposits referred to this period there are great differences of age. Several more or less distinct stages of deposition have been made out, the distinc- tions being based partly on the superposition of the beds, and partly 1 Willis, Tacoma folio, U. S. Geol. Surv. 560 EOCENE AND OLIGOCENE PERIODS on their fossils. These several stages are not readily correlated with those of the coasts. 7 1. Reference has been made (p. 539) to certain formations (Denver, Raton, Lance, etc.), formerly classed as Cretaceous, which probably should be regarded as early Eocene. Some of these beds are inseparable from the Fort Union formation (or series), commonly regarded as the oldest division of the Tertiary in the western inte- rior. During the Fort Union stage, there was an extensive area of aggradation in parts of North Dakota', Montana, and farther north. The Fort Union beds are clastic and are said to be locally 2,000 feet or more thick. Parts of the formation may be lacustrine, but parts are subaérial ? as indicated by the abundance of leaves at many places. The Fort Union series contains much coal, including some that was formerly classed as Laramie. Eocene formations of similar age are found in Colorado (Telluride and Poison Canyon formations), New Mexico (part of the Puerco beds), and elsewhere. The sites of early Eocene deposition were finally shifted. In so far as the sedimentation was in lakes, the basins may have been filled or warped out of existence, and in so far as it was subaérial, deformative movements, or the progress of the gradational work of the streams, or both, may have been responsible for the shifting. 2. During the next or Wasatch stage of the period, sediment was being deposited over parts of Utah, western Colorado, and Wyoming, and elsewhere. ‘The beds of this stage have a maximum thickness of several thousand feet, and are now 6,000 to 7,000 feet above the sea. About 77% of the fossils are of land life. 3. The third recognized stage of the Eocene in this region is the Bridger, during which sedimentation was in progress in the Wind River basin north of the mountains of that name, and another, a little later, in the basin of the Green River, both in Wyoming, and in Utah south of the Uinta Mountains. It may have been during this stage, too, that the volcanic tuff (Sax Juan formation, 2,000 feet and less thick) of southwestern Colorado was made. This last formation is of interest as an index of the vigor of volcanic action in this region. Beneath it, glacial drift was found in 1913 by Professor Atwood. Its extent is undetermined, and it maybe 1 Wilder. Jour. Geol., Vol. XII, p. 290, and Leonard, State. Geol. Surv. of North Dakota, Fifth Biennial Rept. * For criteria for distinguishing lacustrine and subaérial formations, see Davia Science, N. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. XXXV, p. 345, 1900. FORMATIONS AND PHYSICAL HISTORY 561 older than the Bridger Stage. It is at the base of the Eocene in this locality, near Ridgway. 4. The Uinta stage followed the Bridger. Deposition was then in progress in southeastern Utah and southwestern Colorado. Some of the Uinta beds now have an altitude of as much as 10,000 feet, though they probably were deposited at a much lower level. The northwest. In the northwest there are Eocene formations not definitely correlated with the preceding stages. In northern Oregon, there are late Eocene beds of terrestrial origin (Clarno formation, largely volcanic tuff). In Washington, two thick sedi- mentary formations (the Swauk, early Eocene, 3,500-5,000 feet, below, and the Roslyn, 3,500 feet) of Eocene age and non-marine origin, are separated by 300-4,000 feet of basalt. The Payette formation of Idaho, said to have been accumulated in a lake formed by the damming of the upper basin of the Snake River by the early lava-flows of the Columbia River region,! is now referred to the Eocene. Eocene beds of terrestrial or volcanic origin are imper- fectly known in many other places west of the Rocky Mountains. The erosion of the Eocene has given rise locally to the topography characteristic of ‘‘Bad Lands.”’ General considerations. It has been customary to regard the Eocene and later periods as much shorter than those of the Paleo- Fig. 473. Section showing the structure of the Eocene in western Oregon. Eb, Eocene basalt; Ep (Pulaski foruation), and Ec (Coaledo formation), Eocene. Length of section about 20 miles. (Diller, Coos Bay, Ore., folio, U. S. Geol. Surv.) Fig. 474. Section a little south of the last, showing the relation of the Eocene (Ep, Pulaski formation) to the Cretaceous (Km, Myrtle formation). as, amphib- olite schist, and Ps, Quaternary marine sand. (Coos Bay folio, U. S. Geol. Surv.) zoic and Mesozoic; but this conclusion may be questioned. On the basis of thickness, the showing of the system is great, both on the Pacific coast, and in the western interior. Furthermore, any just estimate of the duration of the period must take account of the 1 Lindgren and Drake, Nampa and Silver City, Idaho, folios, U. S. Geol. Surv., and Knowlton, Bull. 204, p. 110. 562 EOCENE AND OLIGOCENE PERIODS great erosion which followed the post-Cretaceous deformation. On the physical side, therefore, there is no warrant for assuming that the period was short. The faunal developments of the period were such as to make great demands upon time, and it is not improbable that the period was as long as the average of those of the Paleozoic and Mesozoic eras. Such thicknesses of terrestrial sediment as occur in the Eocene of western North America, if they are really as great as reported, call for explanation. If the areas concerned were in process of more or less continuous warping, low areas going down as surround- ing lands went up, or if troughs or basins of deposition were pro- duced by faulting, the bottoms sinking while their surroundings rose, the conditions would perhaps be met. The relations of the Eocene beds of the western interior indicate that both the attitude and altitude of the surfaces in that part of the continent were very different from those which now exist. That region must have been much lower than now, and, locally and tem- porarily at least, without well-established drainage. The present mountains were certainly not so high as now, though considerable elevations and great relief doubtless existed. Close of the Period in North America The closing stages of the Eocene were marked by crustal move- ments in the west, resulting in considerable changes in geography. Some such movements had taken place during the period, as has been indicated; but the faulting and folding at its close were on a larger scale. ‘The result was the retreat of the sea along the Pacific coast, the development of new areas of high and low lands, and there- fore a shifting of the sites of rapid degradation and aggradation. Along the Atlantic and Gulf coasts the Miocene is in many places unconformable on the Eocene, and it was at the close of the Eocene (or perhaps during the Oligocene) that an island, now in- cluded in the peninsula of Florida, was formed. In the Carolinas, and in the western Gulf region, the conformity between the Eocene and Oligocene formations seems to preclude notable changes of geography along the coast in the southeastern part of the United States at the close of the Eocene. Foreign Europe. Considerable lakes, estuaries, and perhaps other areas of deposition remained over western Europe, at the close of FORMATIONS AND PHYSICAL HISTORY 563 the Mesozoic era. Later, but still early in the Eocene, submergence set in, allowing the sea to cover considerable areas from which it had been excluded temporarily. In western and central Europe the maximum submergence of the Eocene seems to have been accom- plished by the middle of the period. Toward its close, the epiconti- nental waters of the northwestern part of the continent were again restricted. In the south, the Eocene sea spread much beyond the borders of the present Mediterranean, covering much of southern Europe and northern Africa. Eastward it joined the Indian Ocean, cut- ting off the southern peninsulas of Asia from the mainland to the north. A sound east of the Urals probably connected the Arctic Ocean with the expanded Mediterranean. Above this sea rose many islands, some of which corresponded in position to the Alps, Carpathians, Apennines, and Pyrenees. On the bottom of this great body of water, limestone was deposited ee hi on nummulitic Onean extensive scale: Much of tfimestone. it is made up almost wholly of the shells of nummulites (foraminifera, Fig. 475), and is found from one side of the Old World to the other. Since it is thick (locally several thousand feet) as well as widespread, the sea must have swarmed with foraminifera, and the period must have been long. In few other places are there indications of such great num- bers of organisms of one kind. Some idea of the deformative move- ments since the Eocene may be gained from the fact that the num- mulitic limestone occurs at elevations of more than 10,000 feet in the Alps, up to 16,000 feet in the Himalayas, and 20,000 feet in Tibet. In the Old World as well as in the New, the greater relief features of the present are post-Eocene. Other continents. Marine Eocene is known along the northern and western coasts of Africa, and in the Soudan, in South Australia, New Zealand, and Tasmania, and in various islands of the Pacific. The Tertiary formations of South America have not been closely correlated with those of other continents. There is marine Eocene along some parts of the western coast, in Patagonia! (Magellanian 1 Hatcher, Am. Jour. Sci., Vol. IV, 1897, p. 334, and Vol. IX, 1900, p. 97. 564 EOCENE AND OLIGOCENE PERIODS series) and probably elsewhere in Argentina, and along at least a part of the coast of Brazil! Non-marine beds occur in Patagonia. Eocene beds are extensive in the West Indies where limestone is the dominant rock. Formations of this age are said to occur up to elevations of 10,500 feet in Hayti.* It was formerly thought — that the Atlantic and Pacific oceans connected freely somewhere south of the United States during the early Tertiary, but the work of Hill renders it doubtful whether there were more than shallow and restricted connections in the Eocene, and whether there was con- nection of any sort later. | a General Geography of the Eocene The geography of the Eocene was very different from that of the present time, and the differences were perhaps even greater than has been aaioetcdl It has been conjectured that North America was connected with Asia on the west, by way of Alaska, and with Europe on the east, by way of Greenland and Iceland. Land seems to have failed of making a circuit in the high latitudes of the north only by the strait or sound east of the Urals. In the southern hemisphere, it has been surmised that Antarctica was greatly extended, con- necting with South America, Australia, and possibly with Africa, and that Africa and South America were connected across the Pacific from some earlier time until after the beginning of the Eocene. The basis for these conjectures is found in the distribution of life at that time, as shown by fossils. If these conjectured extensions of land were real, it will be seen that the division of land and water in the northern and southern hemispheres was far less unequal than now, that the land was massed in high latitudes to a great extent, and that tropical seas were more extensive. If extensive polar lands were the cause of glacial periods, as some have thought, the geographic conditions of the Eocene were . favorable in the extreme for glaciation, if the relaticns sketched above were the real ones. In spite of this, the climate of the period seems to have been genial, and less markedly zonal than now. Close of the Eocene. During the later part of this period, and 1 Branner, Bull. Geol. Soc. Am., Vol. XIII, and Stone Reefs of Brazil, Mus. of Comp. Zodél., Bull. 44, pp. 27-53. 2 Hill, Geological History of the Isthmus of Panama and Portions of Costa Rica. Bull. Mus. of Comp. Zodél., Cambridge, 1898. Also J. P. Smith, Science, Vol. 30, 19090, p. 348. LIFE 565 at its close, there were some notable deformations in southern Europe. The initiation of the Pyrenees, and of some of the moun- tains farther east, are among the larger disturbances assigned to this time. The greater deformations which expressed themselves in the mountains of Southern Europe were post-Eocene, and most of them considerably later than the close of the Eocene. LIFE Transition from the Mesozoic. Four salient features marked the transition of life from the Mesozoic to the Cenozoic: (1) among marine animals, nearly all Cretaceous species were replaced by new ones; (2) so many species of land plants lived on as to make it diffi- | cult to separate the Mesozoic from the Cenozoic; (3) the great saurlians almost disappeared, and most other reptiles showed pro- found changes; and (4) mammals appeared in force, and promptly took a leading place. The great change in the epicontinental marine life was due, no doubt, to the withdrawal of the sea from the continent, and the great restriction of the area of shallow water. The increase of the land and the establishment of new land connections may well have caused the existing vegetation to spread and flourish, if the climate remained congenial; but the land faunas did not respond in like manner. It is an open question whether the Eocene mammals of North America and Eurasia descended from the primitive types of mam- mals which lived in these continents earlier, or whether they were immigrants. Satisfactory evidence of their descent from the early (non-placental) mammals is wanting, and the suddenness of their appearance in great numbers suggests invasion from some other quarter. The deformative movements which inaugurated the Eocene period quite certainly made new land connections, and fur- nished the conditions for an invasion, if. mammals, developed else- where, were awaiting the opportunity. Perhaps the rise of mammals caused the downfall of the reptiles. The habit of bringing forth relatively mature offspring, and of nour- ishing and protecting them, gave the mammals an immense advan- | tage, to wich were added superior agility and higher brain power. It would not be surprising, therefore, if the rise of mammals drove the clumsy, small-brained reptiles either to extinction, or to the assumption of new and smaller forms. 566 EOCENE AND OLIGOCENE PERIODS Vegetation In plant history the Eocene was not the dawn of the recent, for the change from medieval to modern plants took place in the Comanchean period. The Eocene did not even mark any radical innovation. There was, however, much progress toward living species, and toward present adaptations of plants to climate, soil, and topography, and to each other. Among the plants of the earliest known Tertiary flora of Europe were oaks like those of the present high lands of warm temperate zones. With them were willows, chestnuts, laurels, etc., which have been likened to the flora of southern Japan. The flora of the Denver beds (p. 539), contains figs, poplars, laurels, magnolias, and many ferns. The early Eocene flora of southern Canada included similar forms, together with oaks, beeches, etc., a flora indicating a temperate climate. The Middle Eocene of England records a flora ‘‘the most tropical in general aspect which has yet been studied in the north- ern hemisphere,” ? while a later flora ‘‘suggests a comparison of its climate and forests with those of the Malay Archipelago and tropical America.’’ The mid-Eocene of America in temperate latitudes contains palms and bananas, mingled with many other trees of similar climatic significance. The Eocene flora of Alaska indicates a climate comparable to that of Southern California and Florida. This flora shows a curious commingling of Jurassic and Cretaceous types (cycads), with angiosperms. A similar flora in the island of Saghalien indicates land connection between Alaska and Asia.’ Early Eocene Mammals The mammals of the earliest Eocene included several poorly differentiated groups, in which existing orders were foreshadowed rather than represented. The herbivores were foreshadowed by the Condylarthra, and the carnivores by the Creodonta; but the twogroups were not sharply differentiated. Both were five-toed plantigrades, whose phalanges had horny coverings that were neither hoofs nor claws. Edentates, insectivores, rodents, and lemuroids seem to have been represented or foreshadowed. Evolution was so rapid that before the close of the Eocene, most existing groups of mammals were well defined (p. 686). None of the present genera, however, 1 Geikie, Textbook of Geology, 3d ed., p. 974. 2 Hollick, Am. Jour. Sci., Vol. 31, 1911, pp. 327-30. LIFE 567 existed then. In general, the mammalian faunas of the Eocene of North America were closely similar to those of western Europe, while during the Middle and Late Eocene there seems to have been faunal separation between these continents.! Main herbivore line. While the condylarths and creodonts were near each other at the beginning of the period, the hoofed herbivores and the clawed carnivores developed from them soon became distinct. The condylarths (Fig. 476) were small generalized Fig. 476. A primitive ungulate or condylarth of the Wasatch epoch; Phenaco- dus primevus Cope, about 1/13 natural size (about the size of a tapir), from Big Horn basin, Wyoming. (Cope.) forms with five toes and forty-four teeth, not yet developed into true herbivores. Condylarths did not live beyond the Eocene, but one branch adapted to forests and marshes seems to have diverged early, and perhaps to have given rise later to the ungulates. In the course of the period many of them became fitted for life on grassy plains. To this end, the flat, heavy, palmate form of foot adapted to marshes, gave place gradually to the light, springy, digitate form, adapted to a quick start and swift flight. At the same time hard hoofs, and grinding teeth were developed. The evolution of hoofs and grind- ing teeth has been thought to be connected with the prevalence of grassy plains, the firm turf of which«is in contrast with the soft soil 1 For references to important literature on the American Tertiary Mammalia, . see the authors’ larger work, Vol. III, p. 228. See also Osborn, Bull. 361, U.S. Geol. Surv., and The Age of Mammals; and Scott, A History of Land Mam- mals in the Western Hemisphere. 568 EOCENE AND OLIGOCENE PERIODS of forest and marsh. Forests perhaps helped to preserve a section of the evolving order in its more primitive form. Back of these influences lay the physical conditions that pro- moted them. In western America, where the evolution is best known, the lakes and rivers were undergoing changes. As they shrank or shifted, they left behind them borders of grassy or sedgy ground which, on fuller drainage, may have become prairies. Such changes were suited to theevolution of herbivorous prairie life, and. this in turn must have invited predaceous animals. If these con- siderations are valid, the prime factors in the evolution of the un- gulates were (1) an undifferentiated plastic animal group susceptible of modification; (2) a plant group (grasses and fodder-furnishing angiosperms) affording appropriate food for the new type; and (3) the shrinkage and shifting of lakes, marshes, and lodgment plains, and the drying up of the plains of the continent, resulting in prairies whose hard turf favored the development of foot and limb modifica- tion in the interest of speed. The era of bulk and heavy armor, such as had been possessed by the reptiles, had passed, and an era of agility and dexterity had begun. No small factor in this progress was the increase in intel- — ligence indicated by the larger brains. The lighter and more agile frame was accompanied by the development of smaller, simpler, but more effective weapons of attack and defence. Nevertheless size continued to be important, and some species in almost every sub-order reached and passed the limit of bulk-advantage, and then declined. In the course of the early evolution strange forms appeared, and soon became extinct. Among them were the Dinocerata (Fig. 477), grotesque monsters whose skulls were armed with three pairs of protuberances, perhaps horn cores, and a pair of enormous canine teeth or tusks projecting below (at least in the male), and an extravagant attempt at armature on both upper and nether sides. Their brains were singularly small for such ponderous bodies. In them, brute mass and low brain-power seem to have reached their climax among mammals. Divergence of ungulates into odd- and even-toed. Early in the Eocene, hoofed animals began to diverge into odd-toed (peris- sodactyls) and even-toed (artiodactyls) types. In the former, the main line of support is in the axis of the middle toe; in the latter, between the third and fourth toes. In the course of time the lateral — LIFE 569 re) = ~ = >a é kL Fig. 477. Dinoceras mirabile, restoration of skeleton by Marsh; about 13 feet long. Middle Eocene, Wyoming. toes fell out of use and were atrophied. The first class reached its extreme type at length in the horse, and the second in our cloven- hoofed cattle; but these perfected types were not attained in the ‘ocene. The horse has become a classic example of evolution. The =e ee es Oe © Fig. 478. An early ancestor of the horse family, Hyracotherium (Protorohip pus) venticolum, from the Lower Eocene (Wind River formation) of Wyoming; about 4 natural size. (Cope.) earliest recognized form was the Hyracotherium (Fig. 478), which resembled the horse but little. The Orohippus (Epihippus) repre- sented a greater advance. It was four-toed (three functional) in 570 EOCENE AND OLIGOCENE PERIODS front, and three-toed behind. It had about the size of a small dog, and was as much canine as equine in appearance. ‘True horses did not appear till the Pliocene. Artiodactyls emerged from their generalized beginnings more slowly. Suina (pigs, peccaries, hippopotamuses) were represented in the Bridger epoch by a small hog. Strangely enough, the camel family seems to have had its beginning in America in the middle and later Eocene, and to have flourished here until the Pliocene. Then, Fig. 479. Mounted skeleton of Patriofelis, a Creodont from the Middle Eocene of Wyoming; 1/18 natural size. (Osborn.) having previously sent a branch to South America to evolve into llamas and vicunas, and another into the Old World to become the present camels, the tribe died out in its primitive home. Carnivore line. It has been thought by some paleontologists that the creodonts were more primitive than the condylarths, and that the latter diverged from the former, as also the edentates and the rodents. If this is so, it gives the creodonts the central position among the primitive mammals. They lived throughout the period and into the next, gradually giving way to their own more progres- sive descendants. Toward the end of the period, modern types began to emerge definitely from the ancestral forms. Primitive representatives of the dog family appeared in Europe late in the period. Scott states that “clawed mammals long antedated the LIFE 571 hoofed types, and that the latter arose, either once or at several separate times, from the former.! Edentates, rodents, and insectivores. The similarity of the ancestral edentates to the condylarths and creodonts of the earliest Eocene seems to imply that the three orders had but recently di- verged from common ancestors. By the middle of the period, rodents became a notable element in the fauna. The squirrel appeared in Europe in the latter part of the period. Even to-day, the rodents retain many primitive characters, and since the Miocene have undergone few radical changes. Their derivation is not yet determined. Most living families of im- sectivores can be traced back to the Eocene. They still retain many primitive characters, and are the least altered of the great mamma- lian branches. Non-placentalmam- mals. During the period, opossums ap- Fig. 480. The skull and jaw of a large Eocene peared in both hemi- rodent, Tillotheriuwm fodiens Marsh, from the Bridger formation, Wyoming; about 1/6 natural size. spheres. They retained this wide distribution until the Miocene, when they disappeared from Europe, but they have persisted in North and South America to the present. It isa singular fact that the monotremes, the lowest of the mammals, are not known until after the Tertiary. The primates. No traces of apes have been found in the Eocene, but lemuroids appeared in the Wasatch epoch in America, and in a similar horizon in Europe. ‘This is the more notable, as the lemurs are now confined to Madagascar, Africa, and southern Asia. They have many affinities with the insectivores, and were possibly derived from them. Apes probably descended from the early lemuroids. Mammals go down to sea. Some mammals took to the sea by choice or necessity, as land reptiles did before them. Thus arose cetaceans (whales, dolphins, porpoises), sirenians (manatees, du- gongs), and pinnipeds (seals, sea-lions). In parts of Alabama, verte- bre of primitive whales (Zeuglodons) were originally so abundant 1A History of Land Mammals in the Western Hemisphere. 572 EOCENE AND OLIGOCENE PERIODS as to attract popular attention, and call forth legends of divers catastrophes. Birds. Fossils of many types of birds, such as gulls, herons, eagles, owls, quails, plovers, and flightless birds of great size, show great deployment of this class. Reptiles and’ amphibians. One of the greatest contrasts in geological history is found in comparing the size, power, and multi- tude of the Cretaceous land reptiles with those of the Eocene. Of the great saurians, only a few lived on into the early Eocene. Land — reptiles seem to have become rare early in the period, though there a, Fig. 481. EOCENE FoRAMINIFERA. @, Nodosaria bacillum Defrance; 6, N. communis (d’Orbigny); c, Anomalina ammonoides (Reuss); d, Cristellaria gibba d’Orbigny; e, C. radiata (Bornemann); f, g, and h, Globigerina bulloides d’Orbigny; i, Vaginalina legumen (Linné); j7, Discorbina turbo (d’Orbigny); k, Truncatulina jobaiula (Walker and Jacob); 1, Textularia subangulata (d’Orbigny). Magnified 8 to 40 times. (Maryland Geol. Surv.) were turtles on the land and in the sea, and some of them attained large size. ‘There were crocodiles which belonged about equally to land and water; also snakes, some of them large. Amphibians were present, but apparently not abundant. Insect life. There has been little important change in the in- sect world since the beginning of the Cenozoic. Few new families have appeared, though genera and species have changed, LIFE aaa a-h, Gastropods: a, Fusus (?) interstriatus Fig. 482. EocENE MOoLtusks. Heilprin; b, Mitra potomacensis Clark and Martin; c, Pleurotoma tysoni Clark and Martin; d, P. potomacensis Clark and Martin; e, Scala potomacensis Clark and Martin; f, Tornatellea bella Conrad; g, Turritella mortoni Conrad; h, Lunatia mary- landica Conrad. i-w, Pelecypods: 1, Glycimeris idoneus (Conrad); j, Dosiniopis Continued at the bottom of p. 574. 574 EOCENE AND OLIGOCENE PERIODS Marine Life The name Eocene (dawn of the recent) was meant to imply the presence of less than 5% of living species among the marine inverte- brates of the period; but most existing orders, families, and genera were established. The changes of later times are considerable, and are valuable as criteria for correlation, climatic changes, migrations, etc., but they are not profound biological transformations. They are in striking contrast with the radical and rapid evolution of the mammals. Geologically, the most striking feature of the marine Eocene life was the extraordinary abundance and size of the foraminifers (Fig. 481). Most types of marine invertebrates had assumed their modern forms. The American Eocene faunas were rather pronouncedly pro- vincial, though some species have a rather wide range. So pro- nounced is their provincial character that much difficulty is experi- enced in making correlations between formations along different parts of the Atlantic and Gulf coasts, and greater difficulties arise in regions more widely separated. The variations are, however, variations of detail, not of broad features. The marine fauna of the Pacific coast,! and the flora as far north as Puget Sound, indicate a subtropical climate. OLIGOCENE FORMATIONS North America. Formations corresponding to the Oligocene of Europe have not been differentiated completely in North Amer- ica;? but certain formations along the Atlantic and Gulf coasts, formerly classed as late Eocene or early Miocene, may be regarded as equivalent-:to some part of the Oligocene of Europe. In the Gulf region the Vicksburg (below) and Grand Gulf formations of Alabama, Mississippi, and Louisiana, and the Fayette formation of Texas, belong to this category. The early Oligocene is repre- sented generously about the Caribbean sea, where its association lenticularis (Rogers); k and 1, Venaricardia marylandica Clark and Martin; m and n, Corbula aldrichi Meyer; 0 and p, Protocardia levis Conrad; q, Ostrea compresst- rostra Say; r, Modiolus alabamensis Aldrich; s, Lucina aquiana Clark; t, Leda parilis (Conrad); «, Crassatellites aleformis (Conrad); v, Nucula ovula Lea; w, Pecten choctawensis Aldrich. (Maryland Geol. Surv.) 1 Arnold, Jour. Geol., Vol. XVII, p. 509, and Knowlton, Tacoma, Wash., Folio. 2 Dall, 18th Ann, Rept., U. S, Geol. Surv., Pt. IT. LIFE 575 with the Eocene is close,! and its separation from the Miocene distinct. This is in keeping with the phenomena of the Gulf States. Limestone is the dominant rock in the Antillean region. The Oligocene stage is also recognized among the terrestrial deposits of the western part of the continent. The White River formation, now classed as Oligocene, occupies an extensive area in northeastern Colorado, southwestern Wyoming, western Nebraska (Brule and Chadron formations), and South Dakota, and perhaps in Kansas. In the light of present knowledge, it seems probable that all phases of land aggradation, lacustrine, fluvial, and eolian, Fig. 483. Chimney Rock, a detail in the Bad Lands of the White River coun- try. The base of the column is Brule clay. (Darton, U.S. Geol. Surv.) are represented in this formation.” Even thin beds and lenses of limestone and volcanic ash enter into it. The formation is said originally to have covered most of the Black Hills region, and pos- sibly all of it.2 Remnants are found up to elevations of more than 6,000 feet, and the highest points of the hills are but little higher. The Florissant beds in South Park, Colorado, consisting largely of volcanic ash, and famous for their fossil insects, are classed as Oligocene. So also are some of the beds of the John Day Basin of Oregon, unconformable above the Eocene. Marine Oligocene beds 1 Hill, Geology and Physical Geography of Jamaica, and Geological History of the Isthmus of Panama and portions of Costa Rica. Bull., Mus. Comp. Zodl., Vols. XXVIII and XXXIV respectively. 2Fraas, Science, Vol. XIV, N. S., p. 212, and Matthew, Am. Nat., Vol. XXXIITI, p. 403, 1899. 3 Darton, 19th Ann. Rept., U. S. Geol. Surv., Pt. IV; 21st Ann, Rept., U. S. Geol. Surv., IT. 576 EOCENE AND OLIGOCENE PERIODS are found on the Pacific coast, but the record of the period here is found chiefly in the unconformity between the Eocene and the Miocene. Considerable geographic changes occurred during the Oligocene, or at its close, especially in the Gulf and Caribbean regions, where the Oligocene (early Oligocene) is commonly conformable on the Eocene, and unconformable beneath the Miocene. Fig. 484. Oligocene Bad Lands of South Dakota. (Williston.) Europe. Toward the close of the Eocene, the epicontinental sea of northern Europe was greatly restricted, but considerable areas stood so near sea-level that slight changes served greatly to diminish or extend the epicontinental waters. The oldest Oligocene deposits of central and western Europe are largely of terrestrial, fresh- and brackish-water origin. Local deposits of salt, gypsum, and coal are suggestive of the physical conditions at various times and places. The Oligocene of southern Europe is chiefly marine. In Europe, as in North America, there were considerable igneous eruptions during the Tertiary, and especially during the Oligocene. Between eruptions, vegetation grew in marshes and shallow lakes LIFE and over the surtace of the lava. 577 The substance of this vegetation is locally (Faroe Islands and Iceland) preserved in the form of coal between the lava beds. Amber. One of the peculiar accessories in the Lower Oligocene is the amber of northern Ger- many, principally in the vicinity of Konigsberg. While amber in small quantities is found in Sicily and a few other places, that of the Baltic region is more abund- ant than that of any other part of the earth, so far as now known. Amber is fossilized resin, ap- parently from certain varieties of coniferous trees. Its original position in the Baltic region ap- pears to be in ‘certain beds of a clayey nature, but parts of. this formation have been worn by the waves, and the amber distributed. Some of that which finds its way into commerce is picked up on the Baltic shore, while some is taken from the beds in which it was originally entombed. One of the interesting features of the amber is the fact that it contains numer- ous insects. was soft, and to, have. become completely immersed in’ it, and perfectly preserved. About 2,000 species have been found thus embedded. Considerable deformative movements made themselves felt They seem’ to:have | alighted upon the resin while it . Lecenpn MARINE cn =p FREsHWwATER -— | | ; | | ) al = 4 Ui Pi od au = So u O LIGOCENE TRadph Arn ela, i404 i NX Fig. 485. Map showing supposed distribution of land and water on the Pacific coast of the United States during the Oligocene epoch. (Ralph Arnold.) in southern Europe at or about the close of the Oligocene, as in the Balkan and Carpathian Mountains.’ Other continents. 1 Willis. In other continents, the Oligocene has not Carnegie Institution Year Book 4, 1905. 578 EOCENE AND OLIGOCENE PERIODS been generally differentiated, but it is known in northern Africa and in Patagonia,! where it is partly marine and partly non-marine. OLIGOCENE LIFE Vegetation. The forests of the Oligocene were similar to those of the Eocene, especially in Europe, where palms continued to be abundant and varied, growing even in north Germany. The Floris- sant beds of Colorado contain a variety of angiosperms, representa- tive of orders now found in the latitude of the middle and southern states. Land animals. All species of imsects in the Florissant beds (over 700) are extinct. This indicates that although the types had Fig. 486. Titanotherium validum Marsh, , photograph of a mounted specimen in the Carnegie Museum. (Holland.) become modern, the species continued to change with relative rapidity. Fish fossils are abundant in the same beds. Mammals continued their rapid evolution. The Carnivora came into clear definition, and were represented in the White River beds by ancestral dogs, cats, raccoons, and weasels, while some creo- 1 Hatcher, Geol. Mag., 1902, p. 136. LIFE 579 donts remained. Rodents were represented by squirrels, beavers, pocket-gophers, rabbits, and mice. Among perissodactyls, the rapidly developing horse family was represented by Mesohippus and Anchippus. The rhinoceros tribe had deployed into three branches, one a lowland form, ancestral to the existing family, one aquatic, and a third an upland running form. The tribe had a cos- mopodlitan range. An erratic branch (the ¢titanotheres) of the odd-toed ungulates which arose late in the Eocene reached its climax in the Oligo- Fig. 487. An interpretation of the elotheres, or giant pigs, of the White River epoch, drawn by Charles R. Knight. (From drawing in American Museum of Natural History. Copyrighted by the Museum.) cene (White River), and then disappeared. Its representatives were distinguished by a long, depressed skull, armed with a pair of horns near the end of the nose (Fig. 486). They reached some fourteen feet in length and ten in height. They were American and apparently rather local. Another odd type was the elothere, which 580 EOCENE AND OLIGOCENE PERIODS appeared in North America in the White River stage, and con- tinued into the Miocene. An interpretation of their general appearance is shown in Fig. 487. Artiodactyls were prominent, represented by various extinct forms, and by ancestral peccaries, camels, ruminants, and other forms. Marine life. The fauna of the Oligocene on the Atlantic coast of North America has the same general aspect as that of the Eocene. Later, however, provincialism became pronounced. By this time, the foraminifers had declined greatly, and the fauna was over- | whelmingly molluscan. On the Pacific coast, the Oligocene fauna shows closer relation to the Miocene fauna than to the Eocene, and suggests a climate intermediate between the climates of those periods. “ CHAPTER XXVII THE MIOCENE PERIOD ! FORMATIONS AND PHYSICAL HISTORY . The geography of the North American continent during the Miocene period was similar to that of the Eocene. The slight emergence of the coastal borders after the Eocene (or early Oligo- cene) was followed by a slight submergence of the same regions during the Miocene. In the western interior, terrestrial aggrada- tion of all phases continued, but the sites of principal deposition differed somewhat from those of the preceding period. The Atlantic coast. In its surface distribution, the Miocene sustains the same relation to the Eocene that the latter does to the Cretaceous (Fig. 446), though in places the Miocene overlaps the Eocene, completely concealing it. There is generally a slight unconformity at the base of the Miocene. Like the other forma- tions of the Coastal Plain, the beds dip seaward and are concealed by younger beds some distance to landward from the present shore. The system originally extended inland far beyond its present border, as shown by numerous outliers. The Miocene of the Atlantic coast is composed chiefly of un-: consolidated sand and clay. In places there is shell marl, and local- ly beds of diatomaceous earth of such thickness (30 or 40 feet) as to be valuable commercially. At the north, the Miocene has a thick- ness of 700 feet, but it thins southward. The Miocene of this coast is generally called the Chesapeake formation. It was formerly regarded as Upper Miocene, the former Lower Miocene being now classed as Oligocene. ‘The fauna of the Chesapeake series has been interpreted to indicate a climate somewhat cooler than that which had preceded. _ The Gulf coast. The Miocene of the Gulf coast is rather thin, and sustains the same general relations to older formations as that * Dall and Harris, Bull. 84, U.S. Geol. Surv., and Dall, 18th Ann. Rept., U.S. Geol. ‘Surv. % Pt. dl. 581 582 THE MIOCENE PERIOD \ Jonzecnnpteeeee Fig. 488. Map showing the distribution of the Miocene formations in North America. Conventions as in preceding maps. of the Atlantic, except that it is not known to be so generally uncon- formable on tormations below. In Florida, Miocene limestone has FORMATIONS AND PHYSICAL HISTORY 583 been changed locally to lime phosphate.! The alteration appears to have been effected through organic matter, especially the animal excrements accumulated about bird, seal, and perhaps other rook- erles. The organic matter furnished the phosphoric acid, which, carried down in solution, changed the carbonate of lime to the phos- phate. The phosphate is used extensively as a fertilizer. In Texas part of the Miocene is non-marine. Much of the oil of Texas and Louisiana comes from dolomized limestone which is probably Miocene.’ | The Pacific coast. At the beginning of the period, the sea encroached upon the Pacific coast, covering considerable areas which were .land during the Oligocene. It flooded the southern part of the central valley of California early in the period, and later the northern part as well. At about the beginning of the period, faulting seems to have affected considerable parts of Cali- fornia, and some of the planes of movement at that time have served as planes of movement since. This was the time of the first definitely recognized movement along the great earthquake rift of California. ‘Though subsidence was the rule in central and southern California, local fault-blocks seem to have had notable elevation. The Miocene history of the Pacific coast is divided into two somewhat distinct epochs, separated by diastrophism and vulcan- ism. During the first epoch, besides clastic formations and vol- canic ash, there is a formation (Monterey) containing much diato- maceous material which is an important source of oil. The amount of siliceous material ascribed to diatoms is prodigious, and seems credible only when the extraordinary rate of reproduction of diatoms is recalled. It has been estimated that a million individuals might come from one, in the course of a month. If this is the fact, it is perhaps not strange that large amounts of siliceous material accumu- lated where conditions favored. After the early Miocene there were extensive igneous eruptions in eastern Washington, Oregon, and the Coast ranges of California. South of San Francisco, this was the time of the last important 1 Penrose, Bull. 46, U. S. Geol. Surv. 2 Hayes, Bull. 213, U. S. Geol. Surv., p. 346. 3 Arnold, Ralph, Jour. Geol., Vol. XVII. 4 Eldridge, Bull. 213, U. S. Geol. Surv.; Arnold and Anderson, Bull. 322, U. S. Geol. Surv., 584 THE MIOCENE PERIOD eruptions in the Coast ranges, though farther north vulcanism con- tinued later. The igneous eruptions were accompanied by diastro- phism, which consisted in the readjustment of fault-blocks and folds throughout the Pacific coast region. Even high mountains were Leaennd Lecenp Marine HN FResHwaTeR F= Marine es AZ UZ = Z «= e ize MMM Fresswarer Fh nis S 4 iy y | I mu |) i| All | ii il | ! | = a — — —— ———s — = —" — 1 23- aT li lk il aati) | | lk jit | | Lower Miocene : Ralph Arnola, 1904 Fig. 490 Fig. 489. Map showing supposed distribution of land and water on the Pacific coast during the early Miocene period. (Ralph Arnold.) Fig. 490. Map showing supposed distribution of land and water on the Pacific coast during the late Miocene period. (Ralph Arnold.) developed locally, as shown by the coarseness of the sediments which followed. The diastrophism resulted in the extension of the sea, for the Upper Miocene is more widespread than the lower. If a two-fold FORMATIONS AND PHYSICAL HISTORY 585 division of the Tertiary were adopted, the earlier part of the Miocene should go with the early Tertiary, and the jater part with the late Tertiary. The marine part of the system has great thickness, the Lower Miocene having a maximum thickness of some 8,000 feet, and the Upper hardly less. | | By the end of the period, the peneplanation of the Klamath and Sierra Nevada Mountains seems to have approached completion. Fig. 491. Contorted beds of Monterey shale. Mouth of Vaquero Creek, Cal. (Lippincott, U. S. Geol. Surv.) Much of the material eroded from them had been deposited in the central valley of northern California, making the thick Miocene beds of that valley. In western Oregon, Miocene (Empire) beds a few hundred feet thick, containing volcanic ash, rest unconformably on the deformed and eroded Eocene. In British Columbia, there ‘are both clastic and volcanic rocks referred to this period. The Miocene of the western coast has not the simple structure of the corresponding beds along the Atlantic and Gulf coasts. The strata have been deformed so as to stand at high angles (Fig. 491) in many places, and locally (Mount Diablo range) they have 586 THE MIOCENE PERIOD been folded, and the folds overturned so that Cretaceous and Eocene formations overlie the Miocene. Non-marine deposits. In the northern part of the central val- ley of California there are deposits of estuarine, lacustrine, and probably subaérial origin (Jone formation) partly contemporaneous with the early Miocene marine beds farther south. They consist of the common sorts of clastic sediments, with some coal, iron, etc. PST Tes Sb ert rte i te HHT Fig. 492. Section showing the structure and relations of the Miocene system in the San Luis Obispo region of southern California. Js/, San Luis formation, Jurassic; Nm, Monterey shale, Miocene; N7rt, rhyolite tuff; Np, Pismo formation, Miocene (?); Vpr, Paso Robles formation, Pliocene; Pal, recent alluvium, etc. Along the east side of this valley, auriferous gravels,! brought down by streams from the Sierras, were being deposited during at least a part of the period. These gravels seem to have been laid down ona surface of slight relief, interpreted as a peneplain.? The Sierra Mountains are thought to have been at least 4,000 feet lower than now when these gravels were deposited. Non-marine Miocene beds are rather widespread in south- eastern California and Oregon, reaching great thicknesses at some points in the vicinity of the goth parallel. They include clastic sediments, volcanic debris, infusorial earths, and fresh-water lime- stones. Farther east, on the western part of the Great Plains, the deposi- tion of the White River beds may have continued for a time after the beginning of the Miocene. Late in the period, aggradation seems to have been renewed in the same general area, and the Loup Fork formation, thin but extensive, was spread over great areas from South Dakota to Mexico. The lacustrine phases of this formation are probably less extensive than the subaérial.* Like the White River formation, the Loup Fork beds have been eroded into ‘‘bad- land” topography (Figs. 68 and 69). Turner, 14th Ann. Rept., U. S. Geol. Surv., 1894; Lindgren, Jour. Geol., Vol. IV, 1896, pp. 881-906; Diller, Jour. Geol., Vol. II, pp. 32-54. See also folios of the Gold Belt of Calif., U. S. Geol. Surv. 2 Diller, Jour. Geol., Vol. II, pp. 33-54. ’ Haworth, Univ. Geol. Surv. of Kan., Vol. II, p. 281, FORMATIONS AND PHYSICAL HISTORY 587 Non-marine deposits, largely of volcanic material, occur in British Columbia between the Coast and Gold ranges. Miocene deposits are known in Alaska, but erosion rather than deposition was the dominant process there, so far as present data show. Igneous activity during the Miocene. The widespread igneous activity which began with the close of the Cretaceous, perhaps reached its climax during the Miocene. Igneous materials abound in the sedimentary formations of the system throughout the west, and igneous | activity affected nearly or quite every state isa urg west of the Rocky Mountains, the erup- Formation : 1000-1500 ft. tions being from fissures as well as volcanoes. Among the conspicuous centers of activity the basin of the Columbia | and the Yellowstone National Park may | be mentioned. Locally, forests were Yakima | buried by the volcanic ejecta, and in 1000-2000 f,” favorable situations their trunks were petrified (Fig. 495). The lavas of at least a considerable part of 200,000 or 300,000 Tanewm ; | Andesite square miles of lava-covered country in | the western part of the United States = | Manatash : S Formation MIOCENE issued during this period, or during the time of crustal deformation which brought it to a close. Volcanoes were active in the Antillean region of Central America ae AYR and the West Indies, and the Andean | system of South America, as well as in Stes d Fig. 493. Columnar sec- North America. tion showing the succession Close of the Miocene. Slow warpings of formations in central of the surface seem to have been in prog- U.S Gea Sure) Smith, ress throughout the Cordilleran region during the Miocene period, accompanied by faulting and vulcanism, and locally, by pronounced orogenic movements; but toward the close of the period movements were more general. Pronounced deformation affected the coastal regions of Oregon and northern California, tilting and folding the Miocene and older formations. The principal folding of the existing Coast Ranges of both these states has been assigned to this time, but it now appears that some of the deformations formerly referred to the end of the Miocene took PRE-TERTIARY 588 THE MIOCENE PERIOD place earlier (p. 583). The Cascade Mountains of Washington were in process of growth at this time.! ! Similar movements were widespread east of the coast, as in the Great Basin region and elsewhere. In some places, they deformed strata heretofore horizontal, but more commonly they affected formations and areas which had suffered deformation earlier. The later part of the period was perhaps the time when the greater relief features of the rugged west, as they now exist, were Fig. 494. Courthouse and Jail Rocks. Buttes of the Arikaree soca) formation of western Nebraska. (Darton, U.S. Geol. Surv.) initiated. The great relief features of earlier times appear to have lost their greatness before this time. After the movements of the late Miocene had been accomplished, it is probable that the western part of the continent had a topography comparable, in its relief, to that of the present, though by no means in close correspondence with it. The details, and many of the larger features, of the present topography are of still later origin. In the eastern part of the continent, the geographic changes were less, though the Atlantic and Gulf regions seem to have emerged, shifting the coast-line to some such position as it has today. Foreign Europe. As compared with the Eocene, the sea on this conti- nent was somewhat restricted in the north, and somewhat extended 1 Willis, Professional Paper 19, U. S. Geol, Surv. FORMATIONS AND PHYSICAL HISTORY 589 in the south. As in most other post-Paleozoic systems, non-marine formations have much representation in this. The marine beds are Fig. 495. Petrified tree-trunks, Yellowstone National Park. (Iddings, U. S. Geol. Surv.) chiefly along the Atlantic and Mediterranean. In the north, much of the system is buried beneath glacial drift. Thick conglomerates (3,900-5,900 feet) of early and middle Miocene age are found 590 _ THE MIOCENE PERIOD along the north base of the Alps, and tell something of the relief of the Alpine region at the time. Southern Europe appears to have been an extensive archipelago, the plateau of Spain, parts of the Pyrenees, the Alps, and the Carpathian Mountains, and portions of adjacent lands being islands. The sea of southern Europe extended east far beyond the limits of the present Mediterranean, but late in the period it was much restricted. The Miocene formations include all the common sorts of sedi- mentary rocks common to marine and non-marine deposits. The — latter include not a little limestone of fresh-water origin, made partly from the secretions of alge. In Italy the system is said to have a thickness of nearly 6,000 feet. Considerable disturbances occurred in the later part of the period, and at its close. Before its end, the Alps had had a period of growth, usually placed at the close of the Lower Miocene. The Apennines and other mountains of southern Europe also were in process of development during the later Miocene. In the Caucasus Mountains, Miocene beds occur up to heights of 2,o00 meters. As in America, widespread movements which were not notably deformative attended the growth of the mountains, with the result that the sea which had overspread southern Europe was greatly restricted, though not reduced to its present size. Igneous activity appears to have attended the movements, but not on such a scale as in North America. Other continents. The Miocene of Asza has not been generally separated from the other Tertiary formations, but is known to be widely distributed in the southern part of the continent. In Africa, Miocene formations occur in Algeria and Lower Egypt, and are well represented in Australia and New Zealand. The beds are found up to heights of 4,000 feet, giving some clue to the extent of post-Miocene crustal deformation here. In South America, Miocene beds probably occur on the western coast, and are known to have extensive development on the eastern plains of the southern part of the continent,! where the distinction between Upper Oligocene and Miocene is not sharp. The lower part of the Oligocene-Miocene series (Patagonian beds) is marine, while the upper part (Santa Cruz) is of fresh-water origin. The terrestrial 1 Hatcher, Sedimentary Rocks of Southern Patagonia, Am. Jour. of Science, — Vol. IX, 1900; and Ortmann, Princeton Univ. Repts. of Expedition to Patagonia, Vol, IV, Pt. II, LIFE 591 faunas of this region are strikingly similar to the Miocene and later faunas of Australia and New Zealand. Arctic latitudes and climate. Miocene beds are somewhat widely distributed in the Arctic regions and seem to be largely of terrestrial origin, with fossil floras indicating a warm temperate climate. LIFE Land Plants The mid-latitude flora of the Miocene records the gradual dis- appearance of subtropical types, and an increase of deciduous trees. This is particularly true of North America, where the flora came to resemble that of to-day in somewhat lower latitudes, and is indeed its predecessor. An important feature in North America was an increase in the grasses, with its appropriate effect on mammals. Land Animals Earlier fauna. The early Miocene land fauna of-North America was very distinct from the late Miocene. The former resembled Fig. 496. A Miocene Mastodon, Tetrabelodon angustidens Cuvier. (Restora- tion by Gaudry.) the Oligocene (White River) fauna. True carnivores, chiefly of the cat and dog families, had succeeded the primitive forms. Several branches of the perissodactyls had disappeared, reducing them 592 THE MIOCENE PERIOD essentially to their three persistent lines, exemplified by the horse, the tapir, and the lowland rhinoceros. The even-toed branch also had developed into modern lines. Rodents were abundant, includ- ing squirrels, beavers, gophers, rabbits, etc. Later fauna. Elephants. A notable addition to the mam- malian fauna of North America in the late Miocene, was the probos- cidians. Primitive proboscidians lived in Egypt at least as early as the Middle Eocene, and in Europe in the early Miocene. Ele- phants reached North America in the late Miocene, and South America in the Pliocene. Much more important was the immigration of the modern ruminants. The great ruminant group that later formed so im- portant a part of the fauna does not seem to have descended from early North American forms, but to have come in from Eurasia. Their remains are found in the Loup Fork beds. The first immi- grants belonged to the deer and ox families. The earliest known deer (excluding Protoceras) were in Europe. They were hornless, as are their surviving relatives in Asia. By the middle of the Miocene, some of the males had acquired small two-pronged decidu- ous antlers. It was at this stage that they appeared in America. About the close of the period, three or four prongs were added, and in the Pliocene the antlers were variously branched. The Miocene skeletons imply lightness and speed, but not to the same degree as now. There is some doubt as to the precise stage to which the remains of bison found in Nebraska and Kansas are to be assigned. They usually have been referred to the Lower Pliocene; but Matthew assigns them to the Upper Miocene, and Williston to the early Pleistocene. The earliest known bisons on the Eurasian conti- nent were found in the Siwalik Lower Pliocene formation of India. The earlier genera of camels were gone, but 15 species of more modern type have been identified from the Loup Fork formation. The family seems to have been confined still to North America. Evolution of the horse. The Miocene was a great epoch in the evolution of the horse; Anchippus, Protohippus, Pliohippus (Mery- chippus), Hipparion, and other genera flourished, and forty or more species. They were still three-toed, but the two lateral toes were dwarfed and did not usually touch the ground, while the central one was strengthened and bore all the weight. A large group of struc- tural features were being modified concurrently with the feet, to fit LIFE | 503 and Teeth like those of Monkeys etc. Three Toes Side toes not touching the ground Three Toes Side toes touching the ground; Splint of 5" digit Four Toes Four Toes Splint of 1* digit [Fore Foot [ Wind Foot [Teeth i Splints of ; ' haigi 2% ond 4' digits Three Toes Side toes Three Toes Side toes touching the ground Three Toes Splint of Stdigit Hypothetical Ancestors with Five Toes on Each Foot Hyracotherium (After William D. Matthews, Am. Mus. Jour.) 9 Mesohippus ae Protorohippus Kf . ‘ye it i I } tl Wiley lycitia ih i ty inv sea (4) <2) oa >) am) ca ae be fu. oO Zz = = — S = aa <2 E~ The evolution of the horse. Fig. 497. oar ae fore Formations in Western United States and Characteristic Type of Horse in Each 504 THE MIOCENE PERIOD the evolving horse to dry plains and grassy food (Fig. 497). The elimination of the side toes, the lengthening of the limbs, the con- centration of the limb muscles near the body to reduce the weight of the parts most moved, and the consolidation of the leg bones, were modifications in the interest of speed and strength. An elongation of head and neck was necessary to reach the ground. The front teeth were reduced to chisel-like, cropping forms, while the molars, by developing ridges, became suited to grinding. The teeth also grew in length to provide for the great wear caused by the dry siliceous grasses. It is probably as safe to infer a development of dry, grassy plains from this evolution of the horse as to infer climatic and topographic conditions from plants and other organic adaptations. Other orders. Tapirs were but meagerly represented, but rhinoceroses were prominent. Most of the American species were hornless, but two-horned species appeared during the period in Europe. Carnivores were abundant, and had assumed forms re- ferred with some doubt to the living genera. The dog family in- cluded numerous wolves and foxes; the cat family, panther-like . animals and saber-toothed cats; weasel-like and otter-like forms, and an ancestral raccoon represented another family. The genera of the late Miocene were nearly all different from those of the early Miocene, indicating rapid evolution. Rodents were abundant, but neither insectivores nor primates are among the North American fossils. ‘The development of the plains, which favored horses, deer, and cattle, was obviously unfavorable to the lemuroids. Primates. In the Old World, apes had appeared. One type was rather large, combining some of the characters of apes and monkeys; another was related to the chimpanzee and gorilla, and about as large as the former. It is the view of some paleontologists that the ancestral branch of the Hominide (man) must have diverged from its relatives at least as early as this; but on the origin of man the geologic record throws no direct light. Lower vertebrates. Little of moment is recorded rele to the lower vertebrates. Not much is known of American Miocene birds, but their advancement in later stages implies that they continued their evolution with measurable rapidity, a conclusion supported by the European evidence. Reptiles were represented by turtles, snakes, and crocodiles. Amphibians came again to notice in the ! For a recent illustrated statement of the evolution of the horse, see Matthew, Supplement to Am. Mus. Jour., Vol. III. LIFE 595 form of a large salamander, whose remains, found at Oeningen, Switzerland, formerly attained an unworthy celebrity from false identification as a human skeleton, and from the application of the pretentious name Homo diluvit testis. Summary. A general view of the American Miocene land fauna shows that the great order of ungulates took precedence in evolu- tion, and that both the odd- and even-toed branches participated actively. Closely following these in importance, and dependent on them for the conditions of their evolution, came the carnivores. Rodents occupied a middle position, and insectivores and lemuroids declined notably. The European record bears a similar general interpretation, with the ungulates somewhat less pronouncedly in the lead, the carnivores somewhat better deployed, and the proboscidians a conspicuous factor. The important evolution of the higher pri- mates seems to have been confined to the Old World. Marine Life Provincialism dominant. The pronounced provincialism that had been inaugurated in the Oligocene epoch continued throughout the remainder of the Cenozoic era, being favored by the shallow seas about North America, and the bays and straits of Europe. Even the narrow border tracts that were geographically continuous show signs of having been cut into biological sections by interrupting barriers. The land being extensive, large rivers reached the coast here and there, and poured great volumes of fresh and muddy waters across the shore belt, doubtless forming barriers to some species. The warpings of the crust probably developed submarine ridges on the continental shelf. These were not only barriers in themselves, but had an influence in directing the courses of the coast currents. Differences of climate in different latitudes had been developed, apparently, and cold and warm currents were probably more pro- nounced than in earlier times, and their shiftings had still graver effects upon the faunas. So, too, the lower temperatures in the northern shore tracts of the Atlantic and Pacific prevented their serving longer as migratory routes for warm-water species, and this tended further to intensify the provincial nature of the shallow- water faunas. According to Dall, the Chesapeake Miocene was ushered in by a marked faunal change due to a cold northern current driving out 596 THE MIOCENE PERIOD Fig. 498. MrocENE PELECypops: a and b, Arca (Scapharca) staminea Say; c and d, Corbula idonea Conrad; e, Crassatellites marylandicus (Conrad); f, Phacoides (Pseudomiltha) foremani (Conrad); g, Tellina (Angulus) producta Conrad; h, Leda concentrica (Say); 1, Modiolus dalli Glenn; 7, Astarte thomasit Conrad; k, Ensis directus (Conrad); 1, Spisula (Hemimactra) marylandica Dall; m, Isocardia markoéi Conrad; », Cardium (Cerastoderma) leptopleurum Conrad; 0, Pecten (Chlamys) madisonius Say; p, Venus ducatelli Conrad; q, Ostrea carolinensis Conrad. (Mary- land Geol. Surv.) | Fig. 499. M1ocENE Gastropops (one Scaphopod): a, Turritella variabilis Conrad; b, Scala sayana Da!l; c, Nassa marylandica Martin; d, Terebra unilineata Conrad; e, Solarium trilineatum Conrad; f, Cancellaria alternata Conrad; g, Surcula biscatenaria Conrad; h. Calliostoma philanthropus (Conrad); 7, Acteon shilohensis Whitfield; 7, Oliva litterata Lamarck; k, Retusa (Cylichnina) conulus (Deshayes); 1, Conus diluvianus Green; m, Polynices (Neverita) duplicatus (Say); n, Fissuridea Continued on next page. 508 THE MIOCENE PERIOD or destroying the previous warm-water fauna of the region, and bringing with it a cold-water fauna. There was a complete change of species, and even some genera were displaced. The fauna re- tained, however, a general molluscan aspect. Both the bivalves and the univalves gave proof of better adaptability to the vicissi- tudes of the coastal tracts than most other forms, and held their dominance. Figs. 498 and 499 show a few characteristic types. Compared with the Eocene group, Fig. 482, the resemblances will be found more striking than the differences. The marine fauna of the Pacific coast indicates a climate but little warmer than that of the present, and this conclusion is re- enforced by the plants of the Puget-Sound region, which record a transition from the subtropical climate of the Eocene to the tem- perate climate of the present. The fauna of the Upper Miocene indicates a still closer approach to the present. alticosta (Conrad); 0, F. griscomi (Conrad); p, Xenophora conchyliophora (Born); q, Crepidula fornicata (Linné); r, Fulgar spiniger (Conrad) var.; s, Ecphora quadri- costata (Say); t, Siphonalia marylandica Martin; u, Ilyanassa (?) (Paranassa) porcina (Say). Scaphopod: v, Dentalium attenuatum Say. (Maryland Geol. Surv.) CHAPTER XXVIII THE PLIOCENE PERIOD FORMATIONS AND PHYSICAL HISTORY Subaérial Formations The most distinguishing feature of the Pliocene formations, so far as the present continents are concerned, is the predominance of terrestrial deposits. This is a consequence of (1) the exceptional deformations which took place during the period, and before its beginning, and (2) the recency of the period, which has saved its deposits, to a large extent, from removal. Similar deposits after earlier periods of comparable deformation have been largely removed by later erosion. These deposits of the Pliocene are perhaps most obvious in intermontane regions such as the Great Basin. They have by some been interpreted as lacustrine deposits, and such no doubt exist; but over areas much greater than those oc- cupied by Pliocene lakes, and over tracts which were never parts of well-defined flood plains, broad aprons of detritus accumulated. Most of the western mountains of America are flanked by such deposits of Pliocene age, or younger. Pliocene deposits of this type are doubtless concealed beneath later accumulations of a similar sort in nearly all the large basins, and at the bases of nearly all the steep slopes in the western mountain region. In the Mississippi basin, far from all mountains, there are patches of gravel on various hills and ridges which are interpreted as the remnants of a once more or less continuous mantle of river detritus. Definite correlation of these gravels is not now possible, and they may not all be of the same age. They are not older than Cretaceous, and are older than the glacial drift. Their similarity to the Pliocene gravels farther south suggests their correlation with that formation. The material of these gravels, almost wholly quartz, quartzite, and chert, is partly local, and partly from the north. The leading topographic features of the Mississippi basin 599 600 THE PLIOCENE PERIOD have been developed since their deposition, for their remnants are on the highest lands within the area where they occur. The Lafayette formation.! About the Atlantic and Gulf coasts similar deposition gave rise to the Lafayette (Orange Sand) forma- tion, which seems to have had a history somewhat like that of the Pliocene beds of the west, though this interpretation has been challenged. This formation has an extensive distribution (1) between the Piedmont plateau and the Atlantic, (2) on the inland part of the Coastal Plain of the Gulf of Mexico, and (3) in the south- ern part of the Mississippi basin, and is represented, if our inter- pretation is correct, (4) in some of the valleys of the Appalachians and west of them. On the Coastal Plain of Texas the formation is connected with analogous deposits on the Great Plains, and through them with the intermontane deposits of the west, already mentioned. The term Lafayette has been applied only to the formation on the slope between the Appalachians and the Atlantic, about the Gulf, and in the Mississippi basin below the Ohio, where it lies upon the eroded edges of older formations, and extends in- land from the coast up to altitudes of 1,000 feet? near the Rio Grande, 800 feet in Tennessee, and 300 to 500 feet on the Atlantic slope. At its mountainward edge, ragged belts of the Lafayette formation follow the valleys up into the mountains. At its seaward margin, it is more or less completely concealed by younger beds, and it is not to be doubted that it passes out to sea beneath them. No part of the formation on land is demonstrably marine. Within the general area of its distribution the formation is not continuous. Over considerable areas, it caps divides, but is absent from the valleys between them, obviously the result of stream ero- sion. The base on which the formation rests has but little relief, and a gentle dip seaward. In general, the formation thickens seaward. Its known thick- ness ranges from o to 200 feet or more, sections of 20 or 30 feet being common. 1 The fullest sketch of this formation as a whole is that of McGee in the Twelfth Annual Report of the U.S. Geological Survey. A few references to other accounts of the formation in special localities, some of them under other names, are as follows: Safford, Am. Jour. Sci., Vol. XX XVII, 1864; Hilgard, Agric. and Geol. of Miss., 1860, and Am. Jour. Sci., Vol. XLI, 1866, and Vol. IV, 1872; Salisbury, Geol. Surv. of Ark., Report on Crowley’s Ridge, 1889; Dumble, Jour. Geol., Vol. I, 1894, p. 560; Smith, E. A., and Johnson, L. C., Geol. Surv. of Ala., 1894. 2 McGee, loc. cit. FORMATIONS AND PHYSICAL HISTORY 6o1 Fig. 500. Map showing the distribution of the better-known parts of the Pliocene system. The area of the Lafayette, along the Atlantic and Gulf coasts, is marked by vertical dashes. This formation doubtless is more widespread than the map shows. Relatively little of the exposed Pliocene is marine. 602 THE PLIOCENE PERIOD It is composed of gravel (and occasionally bowlders), sand, silt, and clay, variously related to one another. It may be said to be both heterogeneous and homogeneous; that is, there is consider- able variation in composition in short distances, and but little more in great ones. In the lower Mississippi basin, whence the name is derived (Lafayette County, Miss.) it is of sand and gravel chiefly, having in many places the distinctive characteristics of fluvial sand and gravel. Over a broad tract of the uplands east of the Missis- sippi and away from valleys generally, it is composed largely of silt and clay. Its constituents are chiefly the insoluble residues of older formations farther up the continental slope on which it lies, chert and quartz pebbles making up its gravels, and other insoluble matter its fine constituents. These constituents replace one another at short intervals and in various ways, and no systematic succession is observable. Irregular stratification is the rule, but some portions are not bedded. Certain lenses of sand suggest an eolian origin, and pebbly-earths that find their analogue in subaérial and flood-plain deposits are common. The color of the formation ranges from brick-red through various pinks, purples, oranges, and yellows, to white. The color is more irregular than the composition, bands, blotches, and mottlings diversifying the structural units. Fossils are rare. In its representative parts they are all of land plants and animals (except, of course, the fossils derived from earlier formations). Origin. ‘The preferred interpretation of the Lafayette formation is as follows: At the opening of the Pliocene, the Appalachian tract is supposed to have been affected by broad, flat, intermontane val- leys, mantled by a deep residual soil and subsoil. The Piedmont tract to the east is supposed to have been a peneplain near sea-level. It is assumed that the upward bowing was felt first in a relatively narrow belt along the axis of the mountain system, that the rise was gradual, and that the rising arch increased in width as time advanced. The first up-bowing rejuvenated the head waters of the streams from the mountain tract, and the surface, with its heavy mantle of residual earth, readily furnished load to the streams. When they reached that portion of the peneplain not yet affected, or less affected, by the bowing, they dropped part of their load (at b, Fig. 501). With continued rise, the zone of deposition is sup- posed to have been shifted seaward, and the deposits already made were eroded and the eroded material was redeposited farther from FORMATIONS AND PHYSICAL HISTORY 603 the mountains and nearer the sea (at b’, Fig. sor). Thus the process is presumed to have continued till the border of the upraised tract passed beyond the present sea-coast. The whole deposit within the area of the present land was then eroded, and the erosion had reached a notable degree of advancement before the first known glacio-fluvial deposits were laid down. This hypothesis of the origin of the formation postulates that the shallow valleys of the coastal plain were filled with sediment, and that later the deposits spread rather generally over the low divides between them. In the region of deeper valleys, such as the Tennessee, the valleys were only partly filled. It has been assumed generally that the formation was once $< << a Fig. 5or. Illustrating the progressive stages of arching described in the text, and the attendant shifting zones of deposition; s-s, sea-level; a, original peneplaned surface with graded slope to sea-coast; a’, a’’, a’’’, successive stages of arching; , b’, b’’, b’”’, successive sites of deposition corresponding to stages of arching a, a’, a”’,a’"’. In the stage of arching represented by a’, the right hand portion of the previous site of deposition is lifted and becomes a part of the area of erosion. The same process is carried farther in the next stage represented by a’”. continuous east of the mountains where patches only now remain; but it may be that the higher divides were never covered by the formation. The removal and re-deposition of material as suggested by Fig. 501 is regarded as an important part of the interpretation of the formation. Erosion and re-deposition of the material did not cease with the Lafayette epoch, but have been in progress ever since, and the derivatives so closely resemble the parent formation in structure and material that their separation is difficult. If it shall be shown ultimately that the seaward portions of the Lafayette, now concealed or unstudied, are marine, the preceding hypothesis would need to be modified only by supposing that as the sources of the streams was bowed up, the coastal border of the plain was submerged. In this case, there should have been estuarine formations in the seaward ends of the valleys. The chief alternative view relative to the origin of this forma- tion regards it as marine,' deposited during a stage of submergence essentially co-extensive with the area of the formation. This hypothesis has been tried out by geologists of wide familiarity with 1 McGee, 12th Ann. Rept., U. S. Geol. Surv, 604 THE PLIOCENE PERIOD the phenomena, and abandoned as untenable even where conditions seem most to favorit. The objections to it are (1) the absence of marine fossils; (2) the presence of structural features not indicative of typical marine deposits; (3) the chemical condition of the formation, particularly the high and very unequal oxidation and the meager hydration; (4) the topographic relations of the formation, especially the lack of any approach to horizontality in its upper limit; and (5) the absence of shore phenomena. Marine Formations The Atlantic coast. If fossils be the test, Pliocene beds of marine origin have little development on the eastern side of the continent. In Florida only (Caloosahatchie beds) have beds con- taining marine fossils any considerable extent at the surface, though small patches are known farther north. They may be parts of a continuous formation, chiefly concealed. The time relations of these marine Pliocene beds to the Lafayette are undetermined. Pliocene beds of marine origin have not been identified certainly between Florida and Texas, but they cover considerable areas farther south, as in Yucatan. The Pacific coast... Marine sedimentation along this coast was confined to narrow limits (Fig. 502). The deposits are chiefly clastic. Their maximum known thickness is found south of San Francisco, where about 4,000 feet of strata (Merced series) are exposed.” The non-marine part of the system (Paso Robles forma- tion) is as thick in the San Joaquin valley. Crustal Movements * The tendency to crustal movements, both warping and faulting, which had characterized the western part of the continent since thé close of the Mesozoic, seems to have continued at least inter- mittently through the Pliocene. Perhaps these movements were in many places no more than continuations of those begun earlier. About the close of the period, movements were extensive and 1 Arnold, Ralph, Jour. Geol., Vol. XVII. 2 Lawson, Science, Vol. XV, 1902, p. 410, and Hershey, Am. Geol., XXIX, p. 359, give the Pliocene of California greater thicknesses. 3 LeConte, Am. Jour. Sci., Vol. XXXII, p. 167, 1886, Bull. Geol. Soc. Am., Vol. II, p. 329, Jour. Geol., Vol. VII, p. 546, 1899; Hershey, Science, Vol. III, p. 629, 1896, and Dutton, Mono, I, U, S, Geol, Surv. FORMATIONS AND PHYSICAL HISTORY 605 great, resulting in increased height of land. The region covered by the Lafayette formation was elevated relatively, and perhaps some- what deformed. ‘The coast line was probably farther east than now, perhaps at the edge of the con- tinental shelf. To this epoch the SPSS submerged continuations of the SS eth weve St. Lawrence, Hudson, Delaware, (OU ree suwarer C= Susquehanna, and Mississippi valleys are commonly referred. From these submerged valleys it. was formerly assumed that the land along the Atlantic seaboard must have stood 2,000 to 3,000 feet, or perhaps even 7,000 to 12,000 feet! above its present level, to allow of their excavation; but it may not be necessary to postulate such extraordinary changes of level. Continental creep (p. 350) along the slope be- tween the continental platforms and the ocean basins may have lowered the valleys notably as it carried them seaward, if such creep is a fact. “i ==V | (ei Aas! In the Mississippi basin also == there was notable elevation at the ——— close of the period, though prob- a ably less than has sometimes been |.) Priocene. estimated. It seems possible, or Raph Amaaraca F ‘perhaps even probable, that the Fig. 502. Map showing supposed evolution of the principal physio- distribution of land and water on the graphic features of the interior, so Pacific coast of the United States ; ; during the Pliocene period. (Ralph far as due to erosion, is post- Arnold.) Pliocene. In the west, too, there were notable closing-Tertiary movements. The plateau region was in process of uplift, periodically, through- out the Tertiary, during which it has been estimated to have under- gone an elevation of 20,000 feet (Dutton), and a degradation of 1 Spencer, Am. Jour, Sci., Vol. XIX, 1905. 606 THE PLIOCENE PERIOD 12,000, leaving it 8,000 feet above sea-level. How much of this is assignable to the close of the Pliocene is uncertain. It was Dutton’s view that the Colorado plateau was so elevated at this time as: to rejuvenate the Colorado River, and that the cutting of its inner gorge some 3,000 feet (maximum) below the outer (Fig. 73), was the work of later times. More recent studies indicate that even the outer and broader part of the valley is younger than formerly was thought, and raise a question as to whether the inner gorge is not the result of rock structure, rather than of a distinct and later uplift. If the whole of the canyon is post-Pliocene, the elevation of the region since the close of the Tertiary must have been several thou- sand feet. The later elevations in this region, largely by blocks, were so recent that many of the fault scarps are distinct, and in- dependent of stratigraphy and drainage. In the basin region, faulting and deformation ” gave rise to de- pressions between the Sierra Nevada and the Wasatch Mountains, preparing the way for two great Pleistocene lakes (Bonneville and Lahontan). It is probable that many other faults between the Rockies and Sierras were developed at the same time, and in many cases the movement seems to have been along fault planes estab- lished earlier. In the Sierra region, the post-Tertiary (or late Tertiary?) up- lift was still more marked.* Not only the deep canyons of these mountains, but all the scenery of the high Sierras is post-Tertiary.* Still nearer the Pacific, notable changes marked the transition to the Pleistocene. In some parts of southern California (Los Angeles County) marine Pliocene beds are said to occur up to alti- tudes of 6,000 feet, and in others (San Luis Obispo), there was fold- ing (Fig. 492) and faulting, while the shore-line was pushed out toward the edge of the continental shelf. There are submerged valleys along the Pacific coast, as along the Atlantic, but their excavation has been referred to a time earlier than the close of the Tertiary. In Washington, present knowledge points to the early Pliocene as a time of prolonged erosion. The crests of the Cascade Moun- ! Huntington and Goldthwaite, Bull. Mus. Comp. Zodél. Geol. Ser., Vol. VI, p. 252; and Davis, ibid., Vol. XX XVIII. 2 King, U. S. Geol. Expl. of the 4oth Parallel, Vol. I, p. 542. 3 LeConte, op. cit., and Diller, 14th Ann. Rept., U. = Geol. Surv. 4 The beginning of the re- -elevation of the Sierras, after peneplanation, was mid-Miocene, FORMATIONS AND PHYSICAL HISTORY 607 tains seem to represent remnants of a deformed peneplain, which, carried to the east and south, is continuous with an erosion plain which cuts across strata (Ellensburg formation) of late Miocene ! age. The planation must, therefore, have been later than that part of the Miocene period represented by the Ellensburg formation. At least the early part of the Pliocene period, if not most of it, would seem to have been necessary for the accomplishment of this great planation, so that the peneplain can hardly be thought to antedate late Pliocene time. If this is correct, the main features of the present topography of this rugged region are the result primarily of Pleisto- cene erosion on the peneplain uplifted and deformed in Pliocene time, or later, and secondarily of vulcanism, which has built up the great volcanic piles (Rainier and others) which affect the region. In British Columbia also, the Pliocene is thought to have been pri- marily a time of erosion. Deformative movements of the orogenic type seem not to have been common at the close of the Pliocene, but such movements affected the Santa Cruz Mountains of California, where Miocene (Monterey) and Pliocene (Merced) beds were deformed together.” On the whole, the close of the Pliocene must be looked upon as a time of great deformation, a critical period in the history of North America. New lands were made by emergence from the sea, and old lands were deformed and made higher; new mountains were made, and old ones rejuvenated; streams were turned from their courses in some places, and nearly everywhere started on careers of increased activity. The fact that such notable changes, with increased elevation of land, occurred during the epoch next pre- ceding the glacial period, is one of the considerations which led to the once widespread belief that elevation was the cause of the climate of the latter period. While there may be a connection between the two things, it was probably not in the simple and com- monly accepted sense. Volcanic Activity The volcanic activity of preceding periods continued into the Pliocene, and became somewhat pronounced near the end of the period in different parts of the western Cordillera. Some of the 1 Smith, Ellensburg, Wash., folio, U. S. Geol. Surv.; and Willis and Smith, Professional Paper 19, U. S. Geol. Surv. * Ashley, Jour. Geol., Vol. IIT, p. 434. 608 THE PLIOCENE PERIOD late igneous formations of the Sierras, and perhaps of northern California,! belong to this time, and probably some of those of nearly or quite every other state west of the Rocky Mountains. Many of the prominent volcanic peaks of the west date from this time or later, and represent the later phases of the prolonged period of volcanic activity, just as the great lava flows and intrusions represent the earlier. Many lesser cones belong to the same period. Foreign From considerable areas of Europe covered by water during the Miocene, the waters retreated late in the period or at its close; but the sea covered southern and southeastern England, Belgium, and parts of France during at least some portion of the Pliocene, and still more extensive areas of the present continent about the Medi- terranean. Beyond the inland margins of the marine Pliocene, there are contemporaneous beds of terrestrial origin. In southeastern Europe, brackish and salt lakes came into existence, as shown by the fossils and the local deposits of salt and gypsum. In some places, as in the Vienna basin, brackish water beds below grade up into fluviatile beds above. In Italy only do Pliocene beds attain massive development. Along the Apennines their thickness has been estimated at from 1,600 to 3,000 feet, and in Sicily 2,000 feet. Limestone as well as clastic beds enter into the system, which occurs up to heights of 3,000 feet. Marine Pliocene is known in Egypt, where the sea is thought to have extended up the Nile to Assuan. The formation of the basins of the Red Sea and the Gulf of Suez has been assigned to this period. These depressions have been thought to be down-faulted blocks. LIFE Land plants. During the Pliocene there was a further sort- ing out of the mixed flora of previous periods, and the southerly segregation of what are now tropical and subtropical plants contin- ued; but in Europe generally there was still much commingling of species now separated geographically. | Land animals. Three important features characterized the Pliocene history of mammals: (1) A notable intermigration between the continents, including North and South America; (2) the begin- 1 Hershey, Jour. Geol., Vol. X, pp. 377-392, LIFE 609 ning of the present divergence between Old and New World types; and (3) the culmination and perhaps initial decline of the mammals, except those domestic species protected by man. The intermigrations of the early part of the period were made possible by the land connections brought about by deformative movements. The extent of the connection of North America with Asia at the northwest and with Europe at the northeast respectively, is uncertain, but there is conclusive evidence that there were good migratory routes for land mammals in both directions during a part of the period. There are also strong hints that the connection afforded passage for some species, but not for others, due perhaps to the increasing cold toward the end of the period. This low tempera- ture, with its effect on intermigration, was perhaps the chief factor in developing the difference between the inammals of the Old World and the New. The connection between North and South America introduced a biological movement of much interest. There appears to have been no effective isthmian thoroughfare for land animals between the earliest Eocene and the Pliocene. During the Eocene con- nection, a few North American mammals seem to have sent repre- - sentatives into South America, and these had evolved there on distinctive lines in the interval. A remarkable group of sloths, armadillos, and ant-eaters had developed from an edentate stem; strange hoofed animals of orders unknown elsewhere had arisen from some very primitive ungulate form; and the monkeys of the South American type had evolved probably from a North American Eocene lemuroid. That the connection of the continents in the Eocene was only partial or temporary seems to be implied by the absence in South America of most of the great North American groups. The absence of proboscidians in South America implies lack of con- nection between that continent and Africa, where these forms de- veloped during the Eocene and Miocene; but the many marsupials of South America, similar to those of Australia, imply either land connection between those continents, or striking parallel evolution. The South American mammalian fauna at the beginning of the Pliocene is a striking instance of evolution on a large scale in com- parative isolation, and in relative freedom from the severe stimulus of effective competition, powerful carnivores, and shifting geo- graphic relations.' 1 Reports of the Princeton University expedition to Patagonia, 1896-99 610 THE PLIOCENE PERIOD When connection between the two Americas was made in the Pliocene, the fauna of each continent invaded the other. Horses, mastodons, deer, carnivores, and tapirs from the northern continent went to the southern, while gigantic sloths from the south came to our continent, though they did not maintain themselves long. The herbivores had the foremost place among mammals; both the odd- and even-toed ungulates evolved their present orders, and many of their present genera. They were represented also by many genera and species which are now extinct. The evolution of the - horse was advanced to the existing genus Equus. Giraffes and giraffe-like animals, some of them of great size, invaded southern Europe and Asia, probably from Africa. The giants of the period were the proboscidians. ‘The extinct Dinotherium was widely distributed in Europe and has been found in India, but is not known to have reached America. Mastodons seem to have lived in all the continents, but it is doubtful whether elephants reached America before the Pleistocene. They appear to have flourished in Europe, and, with the associated rhinoceroses and hippopotamuses, gave the European Pliocene fauna an African aspect. Carnivores throve and perhaps gained on the herbivores; at any - rate they put a severe tax on the herbivores, forcing further progress in the line of alertness, sagacity, speed, and defense, and gaining similar qualities themselves. Great interest attaches to the development of the primates (monkeys, apes, man), but the data on this point are likely to re- main limited until the tropical regions of the Old World, where the chief evolution of this group seems to have taken place, are more fully studied. No remains of lemuroids or of their descendants have been found in the Pliocene of North America, and those of Europe are from the middle and southern parts of the continent, per- haps implying that northern Europe was too cold for these animals. Some years ago a man-like skeleton was found in what were then regarded as Pliocene deposits in Java, and named Pithecanthropus erectus. ‘The find included the roof of a skull, two molar teeth, and afemur. The form of the femur indicates that its possessor walked erect. The forehead was low and the frontal ridge prominent, and in general the characteristic features were intermediate between those of the lowest men and the highest apes, as shown in Fig. 503. The size of the brain was about two-thirds that of an average man. LIFE 611 The interpretation of this find has elicited much difference of opin- ion. By some the bones are thought to be those of an abnormal man; by others, those of an ancestral type between man and his remote ancestry. Recent studies throw doubt on the Pliocene Fig. 503. Profile of the skull of the Pithecanthropus erectus (line Pe) compared with profiles of the lowest men and highest apes; Spy J and Spy JJ, the men of Spy; Nt, the Neanderthal man; H1/, a gibbon (Hylobates leuciscus); Sm, an Indian ape (Semnopithecus maurus); and At, a chimpanzee (Anthropopithecus troglodytes). (After Marsh.) age of the beds in which the fossil was found.t They may be Pleistocene. Marine life. The record of marine life on the Atlantic coast of America is meager, but it appears that species which then ranged from Bering Sea to the north Atlantic are now confined to temperate latitudes.?, On the coast of California the early Pliocene faunas indicate a temperature lower than that of the Miocene, while the later Pliocene faunas point to sub-boreal conditions.* On the other hand, Pliocene fossils from Alaska (vicinity of Nome) indicate for this locality a climate similar to that of north Japan and the 1 Berry, Science, Vol. XXXVII, p. 418. 2 Dall, Jour. Geol., Vol. XVII. 3 Arnold, Ralph, Jour. Geol., Vol. XVII. 612 THE PLIOCENE PERIOD Aleutian Islands, where the sea remains unfrozen. Pliocene fossils from the northwest coast of Iceland indicate a temperature no colder than 42° (mean), where conditions are now arctic. The apparent lack of harmony between the phenomena of California and higher latitudes may perhaps be due to the different horizons from which the fossils come, the fossils from the different places recording the climate of different parts of the period. Certain fossils of Japan and California indicate intermigration, or migration from a common center, some time during the period. CHAPTER XXIX THE PLEISTOCENE OR GLACIAL PERIOD FORMATIONS AND PHYSICAL HISTORY The distinguishing feature of this period is its extensive glacia- tion. Thick sheets of ice, having the slow movement of glaciers, covered six or eight million square miles of the earth’s surface where climates had been mild not long before. More than half the area known to have been glaciated during this period was in North America, and more than half of the re- mainder in Europe. North America. Nearly half of North America was covered by ice (Fig. 504), and strangely enough it was the plain, rather than the mountainous part, which had most ice. Three principal centers whence ice moved have been recognized on the continent,+ the Labradorean, the Keewatin, and the Cordilleran. Spreading from these centers, ice-sheets covered some 4,000,000 square miles. From the Labradorean center, the extension was notably greatest to the southwest, and in this direction the limit is some 1,600 miles from the center of dispersion, in latitude about 37° 30’. The exten- sion of the Keewatin ice-sheet to the southward was scarcely less. It found its limit in Kansas and Missouri, about 1,500 miles from its center, while to the west and southwest it extended 800 to 1,000 miles toward the Rocky Mountains. One of the notable features of the ice dispersion was the great extension of the Keewatin sheet westward and southwestward over what is now a semi-arid plain, rising in the direction toward which the ice moved, while glaciers from the mountains on the west pushed eastward but nee beyond the foothills. The Cordilleran ice-sheet is less simply defined. iach of it occupied a plateau hemmed in by mountains; but plateau glaciation was complicated by extensive mountain Biseition of alpine type. 1A fourth center (Patrician) has been suggested by Tyrrell, southwest of Hudson Bay, and still another by Wilson, in the extreme East. Wilson, The Glacial History of Nantucket and Cape Cod. 613 614 THE PLEISTOCENE PERIOD The southerly lobes of the complex body of ice crossed the boundary of Canada into the United States. The plains of Alaska seem to have been largely free from glaciation even when the waters of the hy on) ‘ yy yITHOY OY, yyy hy GUS g SES Y s c q 9 iX} “> (Ce ac ( ye nee hd ~ ~, ‘ ‘ Fig. 504. Sketch-map showing the North American area covered by ice at the stage of maximum glaciation. Ohio and the Missouri, 2,000 miles farther south, were being turned from their courses by the ice-sheets. South of the more or less continuous Cordilleran glaciation of the north, local glaciers were widely distributed in the western moun- tains, even down to New Mexico, Arizona, and southern California. DISTRIBUTION OF ICE 615 They were larger at the north and smaller at the south. Of gla- ciation in the mountains of Mexico little is known. Greenland was glaciated more extensively than now. Newfound- land seems to have had its own ice-sheet, and the same was probably true of Nova Scotia, and probably of the peninsula between the Bay of Fundy and the lower St. Lawrence. Other continents. South of the ice-sheet of Europe (Fig. 505), great glaciers descended from the Alps to the lowlands in all direc- Sia F ie. 505. een showing the area of Europe covered by the continental glacier at the time of its maximum development. (Jas. Geikie.) tions. Iceland was buried in ice, and even Corsica had glaciers. In Asia glaciers larger than those of to-day affected all the higher mountains, and ice-sheets existed in some of the more northern lands. In tropical regions, there were glaciers in mountains where none exist now, and in mountains where there are glaciers now, the ice descended to levels 5,000 feet or more below its present limits. The southern hemisphere was affected less than the northern, but the higher mountains generally bore glaciers, and even mountains which were not very high, as the southern Andes, had glaciers which 616 THE PLEISTOCENE PERIOD reached the plains outside the mountains. Antarctica is assumed to have been buried beneath ice as now. The Criteria of Glaciation The area of North America which was overspread by ice is covered by a mantle of clay, sand, and bowlders which, taken to- gether, constitute the drift. The various lines of evidence which have led to the general acceptance of the glacial theory have to do with (1) the drift, (2) the surface of the rock which underlies Fig. 506. ‘‘Pilot Rock,” a glacial bowlder near Coulee City, Wash. (Garrey.) it, and (3) the relations of the drift to the bed. Some of the prin- cipal considerations are the following:! 1. Constitution. One of the distinctive characteristics of the drift is its heterogeneity, both physical and lithological. It is made up at one extreme, of huge bowlders (Fig. 506), and at the other of fine earthy matter. Between these extremes there are materials of all sizes, and the proportions of coarse and fine are subject to great variations. Coarse materials are, on the whole, most abundant in regions of rough topography, where the underlying and neighboring formations in the direction from which the drift came are resistant; ‘ Jour. Geol., Vol. II, pp. 708-724 and 807-83 5, and Vol. III, pp. 70-97. CRITERIA OF GLACIATION 617 fine materials are most abundant where the underlying formations and neighboring formations in the direction from which the drift came, are weak. The fine part of the drift is made up largely of the same materials as the gravel and bowlders, but of these materials in a fine state of subdivision. The coarse and the fine materials are, as a rule, mixed without trace of assortment or arrangement. ‘The drift of any locality is likely to contain rock material from every formation over which the ice which reached that locality had passed; but the larger part of the drift of any place is from formations near ape Fig. 507. A large bowlder in northwestern Illinois. (Carman.) at hand. Over large areas it is probable that 75% of the drift was not moved 50 miles.' No agent except glacial ice makes de- posits with these characteristics. 2. Bowlders of the drift. Many of the bowlders and smaller stones of unstratified drift have smooth surfaces, but they are not generally rounded. Many are subangular, and the wear which they have suffered was effected by planing and bruising, rather than by rolling (Figs. 147 and 508). Some of these planed, subangular bowlders and stones are distinctly marked with one or more series of lines or stri@ on one or more of their faces. The lines of each series are parallel, but those of different sets may cross at any angle. By no means all the stones of the drift show stria. They are rarely seen on those which have lain long at the surface, and they are more common on the less resistant sorts of rock, such as limestone. No 1 The Local Origin of the Drift, Jour. Geol., Vol. VIII, p. 426. 618 THE PLEISTOCENE PERIOD depositing agent except glaciers habitually marks the stones which it deposits in this way. 3. Structure. The larger part of the drift is unstratified, but a considerable part is stratified, some of it irregularly. The un- stratified drift (Fig. 509) or dill (for some of it the name bowlder-clay is appropriate) has no orderly arrangement of its parts. The structure of the stratified drift (Fig. 511) shows that it was deposited Fig. 508. Stones of the drift, striated and beveled by glacial wear. (U. S. Geol. Surv.) by water, which doubtless sprang, in large part, from the melting of the ice. Either of the two great types of drift, the stratified and the unstratified, may overlie the other, or the two may be interbedded. The association of the two is such as to demonstrate their essential contemporaneity of origin. No agents but glacial ice and glacio- fluvial waters could have brought about such relations between the stratified and unstratified drift over such extensive areas. 4. Distribution. The distribution of the drift is essentially the same as that of the ice-sheets and glacial waters; but apart from this general fact, several special features may be noted. (a) Within the area of its occurrence, the drift is measurably independent of topography. ‘That is, its vertical range is as great as the relief of the surface itself. Within the state of New York, for example, it CRITERIA OF GLACIATION 619 ranges from sea-level to the tops of the Adirondacks, nearly 5,000 feet above. It is found on hills and in valleys, and on plains, plateaus, and mountains, indiscriminately, though not usually in equal amounts. (b) Locally the drift is so disposed as to make the surface rougher than it would be otherwise, and in other places so as to give it less relief (Figs. 512 and 513). (c) In constitution it is measurably independent of present drainage basins. Thus, mate- Fig. 509. A section of unstratified drift, till or bowlder clay, on bed-rock. Newark, N. J. (N. J. Geol. Surv.) rials from one drainage basin are found in the drift of other drainage basins so commonly as to make it clear that present divides did not constitute divides to the ice. (d) Various sorts of material in the drift at certain points are so related to their sources as to make it clear that they were carried upwards, in some cases hundreds of feet, above their original sites. (e) A considerable area in south- western Wisconsin, and the adjacent parts of Illinois, Iowa, and Minnesota, is without drift. This driftless area is neither notably higher nor lower than its surroundings, and glacial ice seems to be the only agent which could have spared it, while covering its sur- 620 THE PLEISTOCENE PERIOD toundings. (f) Stratified drift extends beyond the unstratified in the direction in which the ice was moving, especially i in valleys and on low land. This is the work of running water. 5. Topography. Among the characteristic features of. ae topography of the drift are: (a) Depressions without outlets, and (b) associated knobs, hills, and ridges, similar in size to the depres- sions (Figs. 168 and 514). Many of the depressions contain ponds Fig. 510. Foliated till. (Photo. by Jefferson.) or lakes. The surface of some parts of the drift, on the other hand, is nearly plane. 6. Thickness. The drift ranges from zero to more than 500 feet in thickness, and the variations may be great within short distances. ‘The drift may be thick on hills and thin in valleys, or, more commonly, the reverse. No agent besides glaciers habitually leaves its deposits so unequally distributed, and in such disregard of pre-existing topography. 7. Contact with underlying rock. The plane of contact be- tween the drift and the rock beneath is generally, though not always, CRITERIA OF GLACIATION 621 Fig. 511. A section of stratified drift. Fig. 512. Diagram to show how drift may be so disposed as to increase the re- lief of the surface. This should be compared with the following figure. Fig. 513. Diagram to illustrate how drift may decrease ‘elief. 622 THE PLEISTOCENE PERIOD sharply defined, and the surface of the rock is likely to be fresh and firm (Fig. 145). This relation is in contrast with that between mantle rock and the underlying formations where there is no drift (Fig. 152). 8. Striation and planation.! The rock surface beneath the drift, and especially beneath the unstratified drift, is in many places polished, planed, striated (Fig. 145), and grooved. ‘These features are widespread throughout the drift-covered area, and they appear Fig. 514. Terminal moraine topography near Oconomowoc, Wis. at all elevations where there is drift. The strie on the bed rock beneath the drift are generally parallel in any given locality, and tolerably constant in direction over considerable areas; but when large areas are considered, the striz are in some places far from par- allel. Their direction corresponds with the direction in which the drift was transported. 9. Shapes of rock hills. Many rock knolls which were left bare when the ice retreated show peculiarities of form and surface which are distinctive. ‘They were worn more on the side from which the ice approached (the stoss side) than on the other (Fig. 153). Bosses of rock which do not show notably unequal wear show dis- tinct smoothing. Projecting glaciated knolls of rock which show the characters seen in Fig. 167, p. 162, are known as roches moutonnées. 1 Seventh Ann. Rept., U. S. Geol. Surv., has a full discussion of this topic. CRITERIA OF GLACIATION Fig. 515. The radiation of striz in the area of the Green Bay glacial lobe and in the west part of the Lake Michigan lobe, during the last glacial epoch. The true theory of the drift must explain all the foregoing facts and relations. Any hypothesis which fails to explain them all must be incomplete, and any hypothesis with which these facts and rela- tions are inconsistent must be false. Geologists are now agreed that glacier ice, supplemented by the agencies which it calls into being, is the only agent which could have produced the drift. This does not preclude the belief that at various times and places in the course of the ice period, icebergs were formed, or that locally and 624 THE PLEISTOCENE PERIOD Fig. 516. Small protuberances of rock showing the effect of ice wear. Glacial knobs and trails. Movement of ice from left to right. The projections consist of chert in limestone. Near Darlington, Ind. (U.S. Geol. Surv.) temporarily they played an important réle. It does not preclude the idea that, contemporaneously with the production of the great body of the drift by glacier ice, the sea may have been working on some parts of the present land area, modifying the deposits made by ice and ice drainage. The glacial theory does not deny that rivers produced by the melting of the ice were an important factor in trans- porting and depositing drift, both within and without the ice- covered territory. It does not deny that lakes, formed in one way and another through the influence of ice, were locally important in determining the character and disposition of the drift. Not only does the glacier theory deny none of these things, but it distinctly affirms that rivers, lakes, the sea, and icebergs must have co-operat- ed with glacier ice in the production of the drift, each in its appro- > poo eo Fig. 517. Diagram to show the effect of ice wear on slight depressions in the surface of rock. priate way and measure, and that after the disappearance of the ice and the ice-water, the wind had some effect on the drift before it was clothed with vegetation. Development and Thickness of the Ice-sheets The development of glaciers from snow-fields has been dis- cussed (pp. 124-7). If the expansion of the ice-sheets was due prin- THICKNESS OF ICE-SHEETS 625 cipally to movement from a center or centers, the ice at these centers must have been prodigiously thick, for in the course of its progress it encountered and passed over hills, and even mountains, of con- siderable height.. In the vicinity of elevations which it covered, its thickness must have been at least as great as the height of these elevations above their bases. If the centers of the North American ice-sheets remained the centers of movement throughout the glacial period, and if the degree of surface slope necessary for movement were known, the maximum thickness of the ice could be calculated. But it is probable that the centers of the ice-sheet did not remain the effective centers of movement. If the fall of snow toward the margin of the ice-sheet greatly exceeded that at its center, as it probably did, a belt near the Fig. 518. Diagram to illustrate the surface configuration of a great ice-sheet, according to the conception here presented. The central part is relatively flat, and the margins have steep slopes. margin, rather than the geographic center of the field, may have controlled the marginal movement of the ice. With excess of ac- cumulation near the border, the slope of the surface near the edge might be relatively great, while it was slight in the center of the field, as shown by Fig. 518. Under these conditions, the maximum thickness of the ice-sheets might be notably less than if the geo- graphic center remained the effective dynamic center. No sufficient data are at hand for determining with accuracy the average slope of such an ice-sheet as that which covered our con- tinent, but something is known of its slope at certain points. Near Baraboo, Wisconsin,! the edge of the ice at the time of its maximum extension in that region lay along the side of a bold ridge, the axis of which was nearly parallel to the direction of ice movement. The position of the upper edge of the ice against the slope of the ridge is sharply defined. For the last 134 miles, its average slope was about 320 feet per mile. This was at the extreme edge of the ice, where the slope was greatest. In Montana, the slope of the upper surface of the ice for the 25 miles back from its edge has been estimated at 50 feet per mile.’ 1 Jour. Geol., Vol. III, p. 655. 2 Calhoun, Jour. Geol., Vol. IX, p. 718. 6206 THE PLEISTOCENE PERIOD The southern limit of drift in Illinois is not less than 1,500 or 1,600 miles from the center of movement. An average slope of even 25 feet per mile for 1,600 miles would give the ice a thickness of 40,000 feet at the center, the slope of the surface on which the ice rested being disregarded. ‘This thickness seems incredible. Even an average slope of 10 feet per mile would give a thickness of about three miles at the center. If by reason of relatively great precipita- tion near its margins, the only part of the ice-cap which had con- siderable slope was its outer border (Fig. 518), a lesser maximum thickness would suffice. Stages in the history of an ice-sheet. The history of an ice- sheet which no longer exists involves at least two distinct stages. These are (1) the period of growth, and (2) the period of decadence. If the latter did not begin as soon as the former was completed, an intervening stage, representing the period of maximum ice ex- tension, is to be recognized. In the ice-sheets of the glacial period, each of these stages was probably more or less complex. The general period of growth was doubtless interrupted by short inter- vals of decadence, and the general period of decadence by brief intervals of growth. In the study of the work accomplished by an ice-sheet, it is of importance to distinguish between these main stages. Work of Ice-sheets Erosion and deposition were the two great phases of ice work (p. 147 et seq.). The surface over which the ice-sheets moved prob- ably had an erosion topography, and was covered by a layer of man- tle rock. ‘The ice removed the mantle of decayed material, and cut deeply into the undecayed rock beneath. By its erosion, the ice modified the topography to some extent, for weaker formations were eroded more than resistant ones, and topography favored more forcible abrasion at some points than at others. On the whole, the topographic effect of glacial erosion was probably to soften the sur- face contours, without diminishing the relief. The second great result of the ice-sheets was the deposition of the drift. Some of it was deposited while the ice-sheets were grow- ing, some of it after they had attained their growth, and some of it while they were declining. Some of it was deposited beneath the body of the ice, and some at its edge. Where it was thick, the drift altered the topography notably, especially where the relief of the underlying rock was slight. THE WORK OF ICE-SHEETS 627 Formations made by ice-sheets.1 The drift formations fall chiefly into three categories, (1) those made directly by the ice (un- stratified), (2) those made by ice and water conjointly (stratified, but stratification more or less disturbed), and (3) those made by water emanating from the ice (stratified; cross-bedding common). Ground moraine (p. 159) is nearly co-extensive with the ice-sheets themselves, though it failed of deposition in some places, and has le iF "Ne he = ;——4 —* = oe = ra su au a Ie 1 1/2 0 1 MILE Fig. 519. One phase of ground moraine topography; elongated hills of drift of the type shown, are called drumlins; southeastern Wisconsin. (U. S. Geol. Surv.) been removed in others. The ground moraine (till) of the North American ice-sheets is thickest in a broad belt a little within the margin of the drift (Fig. 504), extending from central New York through Ohio, Indiana, Illinois, Iowa, Minnesota, and the Dakotas, and thence northwestward. The topography of the ground moraine varies within wide limits. It is commonly undulatory, involving gentle swells and sags. In some places the swells take on rather defi- nite elongate shapes, with their longer axes in the direction of ice movement. They are then called drumlins (Fig. 519). Drumlins have pronounced development in eastern Wisconsin, where they are numbered by the thousand, in central and western New York, 1 Jour. Geol., Vol. II, pp. 517-538, and Internat. Geol. Congr., 1893. 628 THE PLEISTOCENE PERIOD in some parts of New England, and in some other places. The drumlins of New York are, in general, longer and narrower than those of Wisconsin. | The origin of drumlins has been much much discussed. Opinion is divided chiefly between the views (1) that they were accumulated beneath the ice under special conditions, and (2) that they were — developed by the erosion (by the ice) of earlier aggregations of drift.! A terminal moraine (p. 149) may be very like the adjacent ground moraine in constitution, though in many places there is more strati- _ Fig. 520. A Wisconsin drumlin seen from the side; two miles north of Sullivan. (Alden, U. S. Geol. Surv.) fied drift associated with it. It commonly constitutes something of a ridge, but it is more accurately characterized as a belt of thick drift. Its most distinctive feature does not le in its importance as a topographic feature, but in the details of its own topography. Its surface is, as a rule, characterized by hillocks and hollows, or by interrupted ridges and troughs (Figs. 168 and 514). Many of the hollows and troughs contain marshes, ponds, and lakes. The shape and abundance of round and roundish hills, and of short and more or less serpentine ridges closely huddled together, have given rise locally to such descriptive names as ‘‘knobs,” “‘short hills,” etc.; but it is the association of ‘‘knobs” or ‘‘short hills” 1Some of the more important papers on drumlins are: Upham, Proc. Bos. Soc. Nat. Hist., 1879, pp. 220-234, ibid., Vol. XXIV (1889), pp. 228-242; Chamber- lin, Third Ann. Rept., U. S. Geol. Surv., 1883, p. 306, and Jour. Geol., Vol. I, pp. 255-267; Davis, Am. Jour. Sci., Vol. XXVIII (1884), pp. 407-416; Salisbury, Glacial Geology of New Jersey, 1902; Lincoln, Am. Jour. Sci., Vol. XLIV (1892), pp. 293-296; Tyrrell, Bull. Geol. Soc. Am., Vol. I (1890), p. 402; Leverett, Monogrs. XXXVIII and XLI, U. S. Geol. Surv., and Russell, Amer. Geol., Vol. XXXV_ (1905), Pp. 177. , TYPES OF DRIFT 629 with ‘‘kettles,” and not either feature alone, which is characteristic of terminal moraine topography. The ‘‘knobs” vary in size, from low mounds but a few feet across, to hills half a mile or more in diameter, and a hundred feet or more in height. Not rarely they are about as steep as the mate- rial of which they are composed will lie. The ‘‘kettles” are the counterparts of the elevations. They may be a few feet, or many 630 THE PLEISTOCENE PERIOD rods, or even furlongs in diameter. They may be so shallow that the sagging at the center is hardly seen, or they may be scores of feet in depth. Where steep-sided depressions are closely associated with abrupt hillocks, the topography is notably rough. The topog- raphy of the terminal moraine may be well developed, even where the moraine as a whole does not constitute much of a ridge. The surface of the terminal moraine, where well developed, is generally rougher than that of the ground moraine, but because the g 8 - vi S way (Oy in w ‘ANS \' } 2 kA \ . > ’ S \\ SS} 43 RS YW J SAL <5) ah. | & gear ~\ Wh Ne Fig. 522. ‘Topography of drift shown in contours; an area near Minneapolis, Minn. Scale about one inch to the mile. (U.S. Geol. Surv.) sags and swells are of smaller area and steeper slopes, rather than because the relief is notably more. It is not to bé understood, however, that the topography described affects all terminal mo- raines, or that it is confined strictly to them. The elevations and depressions of terminal moraines grade from strength to weakness, and locally even disappear, while the features characteristic of ter- minal moraines are found, now and then, in other parts of the drift. 1 References to papers on terminal moraines: Chamberlin, Third Ann. Rept., U.S. Geol. Surv., 1881-2, pp. 291-402, and Amer. Jour. Sci., Vol. XXIV (1882), — pp. 93-97; Salisbury, Glacial Geology of New Jersey, pp. 92-100 and 231-260. TYPES OF DRIFT 631 Where an_ice- sheet halted in its retreat, its edge remaining in a constant or nearly constant position for a sufficiently long period, a ter- minal moraine (called a recessional moraine) was de- veloped. The not uncommon impres- sion that a terminal moraine necessarily marks the terminus of the drift is erro- neous. The word terminal refers to the terminus of the ice at the time when it formed the moraine. Fluvio-glacial de- posits have been re- ferred to in earlier Fig. 523. A group of kames near Connecticut Farms, pages(p.164). They N.J. (N. J. Geol. Surv.) are made (1) at the edge of the ice (kames and various ill-defined accumulations of gravel and sand); (2) beyond the edge of the ice (valley trains, Ord 1, 00 be Ts Nt ES es es Fig. 524. Diagram to illustrate kame terraces. ABC represents the stratified drift of the kame terraces which are underlain by ground moraine. Till also covers the valley bottom. outwash plains, deltas and various ill-defined bodies of stratified drift); and beneath the ice (eskers, etc.). 632 THE PLEISTOCENE PERIOD Changes in Drainage Effected by Glaciation One result of the unequal erosion and unequal deposition by the ice-sheets was the derangement of drainage. This is seen in the thousands of lakes, ponds, and marshes which affect the surface of the drift. The basins of the lakes or ponds arose in various ways. There are (1) rock basins produced by glacial erosion; (2) basins due to the obstruction of river valleys by drift; (3) depressions in the surface of the drift itself; and (4) basins produced by a combi- nation of two or more of the foregoing. Besides the lakes and ponds now in existence, others have become extinct by the filling of their basins or by the lowering of their outlets. Glaciation also changed the courses of many streams. In many cases, pre-existing valleys were filled with drift in some places, so that when the ice melted, the drainage followed courses which were partly new. In other cases, the ice forced streams to flow around its edge, and some of the drainage channels thus established were held after the ice melted. There are few streams of great length in the area covered by the ice which were not turned from their old courses for greater or less distances by the ice or the drift which the ice left. The Mississippi, the Ohio, and the Missouri, the master streams of the United States within the glaciated area, and a host of their tributaries, suffered in this way.! Succession of Ice Invasions The glaciation of North America was accomplished by a se- ries of ice-sheets separated from one another by long intervals of time. Some of the interglacial intervals were much longer than the time since the last ice-sheet disappeared, and there is also good evidence that in some of them the climate was at least as mild as to-day. The proofs of the interglacial intervals and the evidences of 1 For changes in the Mississippi and in the rivers of Illinois, see Leverett, Monogr. XXXIII, U. S. Geol. Surv., p. 120. For changes in the Upper Ohio, see Chamberlin and Leverett, Am. Jour. Sci., Vol. XLVII, 1894 (contains refer- ences to earlier work in this region). For changes in the Erie and Ohio Basin, see Leverett, Monogr. XLI, U. S. Geol. Surv., Chap. III, and Tight, Professional Paper No. 13, U. S. Geol. Surv. For changes in the course of the Upper Missouri and its tributaries, see Todd, Science, Vol. XIX, p. 148 (1892), Geol. of S. Dak., pp. 128 and 130 (1899), and Bull. 144, U. S. Geol. Surv. Changes in drainage in New York have been summarized by Tarr, Phys. Geog. of New York, 1902, with references to earlier literature. SUCCESSION OF ICE-SHEETS 633 their duration are found (1) in the erosion effected by streams after the deposition of one sheet of drift and before the deposition of the next, (2) in the depths to which earlier sheets of drift were leached and oxidized by weathering before the deposition of later ones upon them, (3) in the accumulations of peat, soil, etc., now ‘found between different sheets of drift, and (4) in the changes of topographic attitude which intervened between the deployment of successive ice-sheets.! The following are the stages of the glacial period recognized in North America numbered in the order of their age: VIII. The Glacio-lacustrine (including the Champlain). VII. The Wisconsin or Wisconian, the last important invasion. VI. The Sangamon-Peorian, or third interglacial interval. VB. The Iowan, the third invasion in the Keewatin field. (VA. The Illinoian, the third invasion from the Labradorean field. IV. The Yarmouth or Buchanan,’ the second interglacial interval. III. The Kansan, or second ice invasion. II. The Aftonian, or first interglacial interval. I. The Jerseyan or sub-Aftonian ice invasion, the earliest recog- Peenized. I. Jerseyan or Sub-Aftonian glacial stage. The oldest drift which appears in New Jersey is but the frayed edge of a once con- tinuous sheet, and is very old. On the Allegheny and upper Ohio rivers, the great age of the oldest drift is shown by the deep erosion of the valleys since the first ice invasion turned the streams into new channels. Farther westward the corresponding old drift is covered by later drift. In the Keewatin area in Iowa, a very old drift (sub- Aftonian), probably the equivalent of the Jerseyan, lies below the Aftonian and Kansan. Very old mountain drift has recently been found (Atwood) high on the mesas near the San Juan Mountains in Colorado and also on the high mesas in front of the Rocky Moun- tains in Montana (Alden). The evidences of age of all these seem to be of the same order, and they are thought to represent the earliest ice invasions in the Labradorean, Keewatin, and Cordil- leran fields. II. Aftonian interglacial interval. Overlying the oldest till in 1Distinct glacial epochs and criteria for their recognition, Jour. Geol., Vol. I, pp. 61-84. 2 The Buchanan gravels lie between the Kansan and Iowan drift-sheets where the Illinoian is not present, and hence their age is not quite certain. 634 THE PLEISTOCENE PERIOD Iowa is an irregular sheet of sand and gravel with remnants of old soil, muck, and peat, with stumps and branches of trees. The sur- face of the drift beneath shows much weathering and erosion. The fossils in these interglacial beds imply a cool-temperate climate; but as a cool-temperate stage must be passed through twice between successive glacial epochs, once as the ice retreats, and a second time as it advances again, fossils indicating a cool climate do not necessarily show how warm the interglacial epoch may have become. ~ III. Kansan glacial stage. The Kansan stage is represented by a sheet of till occupying a large surface area in Kansas, Missouri, Iowa, and Nebraska. ‘Theoretically it extends under the later gla- cial formations to the northward, as far back as the Keewatin center of radiation. Much of this sheet of drift, as originally devel- oped, probably was rubbed away by later glaciations. Presumably a similar sheet was formed by a contemporaneous ice-sheet spread- ing from the Labrador center, but it has not been certainly identi- fied. The Kansan till is clayey and there is little stratified drift associated with it. IV. The Yarmouth interglacial stage.! Where the Illinois till overlaps the Kansan (eastern Iowa), an old soil, with deep subsoil weathering, lies on the surface of the latter. V. Illinoian and Iowan glacial stages or Iowa-Illinoian stage. On the borders of the Labradorean field near the Mississippi River, the Illinoian drift sheet overlies the Kansan sheet with the Yarmouth beds between. In the Keewatin field in eastern Iowa, the Iowan drift lies over the Kansan, with the Buchanan beds between. Some geologists now think that the Iowan represents the same stage in the Keewatin field that the Illinoian does in the Labradorean field; i. e., the third ice invasion. The earlier view was. that the Illinoian drift was the older. V. A. Illinoian drift sheet (Labradorean field). The exposed por- tion of this drift occupies the surface in the southern and western parts of Illinois. It runs back under the later drift to the north- east toward the Labradorean center. ‘To the eastward, it is traced as far as Ohio, where it is covered by later drift. To the northward its margin is covered in southern Wisconsin, but in central Wis- consin it seems to re-appear and is traced westward on the north side of the driftless area, beyond which, in Minnesota, it seems to connect with the Iowan drift. The Illinoian till is clayey, with little 1 Leverett, Mono. XXXVIII, U. S. Geol. Surv. SUCCESSION OF ICE-SHEETS 635 assorted drift associated. The west edge of the Illinoian ice-lobe pushed out into Iowa a score of miles, forcing the Mississippi in front of it. Ice of the Kansan epoch had earlier invaded Illinois from the west, and probably forced the Mississippi east of its present course, if such an easterly course had not been taken be- fore the Kansan epoch. V. B. The Iowan drift (Keewatin field). In northeastern Iowa the ice of this stage left a thin sheet of till marked by a profusion of large granitoid bowlders most of which lie on the surface. To the northward in Minnesota these bowlders are less abundant, and the formation passes beneath later drift. To the northeast it appears to be connected with the third drift of Wisconsin. VI. Sangamon interglacial stage. In central Illinois a sheet of sandy material marked by remnants of old soil, muck, peat, weathering and erosion overlies the Illinoian glacial drift. Above this lies a mantle of loess and the Peorian peaty beds. According to the older view, the Iowan was placed between the Sangamon and the Peorian, now regarded, tentatively, as equivalents. VII. Wisconsin or Wisconian stage. The ice radiated from the Labradorean, Keewatin, Cordilleran, and from many mountain centers. It had probably done this at each of the preceding glacial stages, but the record is much obscured by erosion and concealment. The margin of the Wisconian ice was pronouncedly lobate, and the drift which it left is characterized by stout terminal moraines, numerous kames, eskers, drumlins, outwash aprons, valley trains, and other features distinctive of glacial action and glacio-fluvial coéperation. This drift-sheet, more than any of the others, bears the stamp of the great agency of the period. The distinctive topog- raphy of the various phases of this formation is in contrast with the relatively expressionless surfaces of the older sheets of drift. Part of this difference is due to the fact that the Wisconsin formation has been eroded less than the older drifts; but the larger part, apparently, is assignable to a stronger original expression. Unlike the earlier sheets of drift, the Later Wisconsin drift was not overriden by later sheets of ice, and its original development is therefore better shown at the surface. It bas nearly a score of concentric terminal moraines in some places. Some of them repre- sent re-advances of the ice in the course of its general retreat, while others mark halts in the retreat sufficient to permit an exceptional accumulation of drift at the border of the ice, 636 THE PLEISTOCENE PERIOD Not all of these several sheets of drift have been seen in super- position, and the history sketched above is based on the relations of the sheets of drift at different points.1 Theoretically, the several sheets of drift are imbricated as suggested by Fig. 525, but each sheet of drift is discontinuous beneath the overlying one, and this —_ SS eee eee eT ll OTC OTC — comer que oe ow es ons Oe oe oS ee Fig. 525. Diagram illustrating the imbrication of the successive sheets of drift. The full lines represent the portion of the drift-sheets not overspread by later ice-sheets; the broken lines represent the portions of the successive drift- sheets which were covered by ice at a later time. 1 corresponds to Jerseyan or sub-Aftonian, which in general is less extensive than the Kansan, though locally, as in New Jersey, it extended farther south than any other; 2 repre- sents the Kansan drift, the southern margin of which is not covered by younger drift; 3 and 4, respectively, represent the Illinois-Iowan, and Wisconsin sheets of drift. discontinuity goes so far that beneath the Wisconsin drift, for ex- ample, the several sheets are more commonly wanting than present. VIII. Glacio-lacustrine stage. In the course of the retreat of the ice of the Wisconsin epoch, a complex series of lakes arose be- tween the ice border on the one hand, and the higher land fronting it on the other. Many of these lakes were temporary and shifting, and had shifting outlets. Their history cannot be given here; but a brief sketch of the history of the Great Lakes will indicate the na- ture of the changes which took place. When the end of the Lake Michigan ice-lobe (Fig. 526) with- drew a little from the southern end of the Lake Michigan basin, a lake formed there, and discharged its waters into the Illinois valley — southwest of Chicago. The channel followed by the outflowing waters has since become the site of the Chicago drainage canal. The glacial lake (Lake Chicago) thus initiated was gradually extend- ed northward (Fig..527) as the ice-lobe was melted. A similar lake was formed about the head of the Lake Superior ice-lobé. Lake Maumee developed about the end of the Erie ice- lobe, and its waters flowed to the Wabash. A later stage of Lake Chicago and Lake Maumee is shown in Fig. 527, when, finding a lower outlet as the ice melted back, Lake Maumee sent its outflow across southern Michigan to Lake Chicago. Later, the whole Erie basin, and a portion of that of Ontario, 1 Jour. Geol., Vol. I, pp. 61-84. GLACIO-LACUSTRINE STAGE 63% ~Nyhaee? ate aes -e., ‘a, x Ps ras - eat ate « ‘ SNS — — Fig. 526. The beginning of the Great Lakes. The ice still occupied the larger parts of the present lake basins. (U.S. Geol. Surv.) ris aaa “ Eero. Ne pone nn nner en 2° 8S r a us ecreeeor” River ae Pe mnoe ~weeeceererrrs Fig. 549. The same at a late stage, when the slope has become nearly stable. however, the larger the number of animal types not known to have lived this side the last glacial stage whose remains are commingled with human relics, the stronger the presumption of man’s presence before the close of the glacial period. From this point of view, the European case seems to be strong. There is one further feature in the European case that is, at least, suggestive. Two climatic groups of animals are associated LIFE 675 with the human relics—a subarctic and a subtropical. In the sub- arctic group there were reindeer, mammoths, woolly rhinoceroses, musk-oxen, and other boreal forms; in the subtropical group, lions, leopards, hippopotamuses, hyenas, southern rhinoceroses, and other African types. These contrasted groups, as interpreted by James Geikie and others, imply migrations of the kind already sketched as characteristic of the glacial period. These seem to indicate, therefore, that man lived in Europe before the close of the glacial period. The relics thus associated with extinct animals have been assigned to paleolithic man, and to a primitive stage of culture. This interpretation is based on the crudeness of the stone artefacs, Fig. 550. Etching of an aurochs on a slab of slate, from the bone cave of Les Eyzies, Dordogne, France (% size). This sketch may be instructively compared with the similar work of the ancient Assyrians and Egyptians. (Prestwick.) rather than upon the evidence of a higher order of art which the record presents. If, however, the rude stone artefacs can be inter- preted as the waste incidental to the making of good stone imple- ments, a more favorable judgment of the art of these ancient peoples would be reached. Associated with the ruder artefacs (or paleo- liths) there are implements of bone, such as needles, awls, harpoons or spears with barbs, etc., implying some advance in art; there are carvings that show not a little skill, and drawings in which the ele- ments of perspective and shading, as well as skill in delineation, are indicated (Fig. 550). These seem to imply a higher stage of art development than is consistent with the exclusive use of paleolithic 676 THE PLEISTOCENE PERIOD stone implements. On the whole, present evidence seems to justify the conclusion of most European archeological geologists that man was present in southern and central Europe during the later part of the glacial period, and perhaps even early in the period. A recent discovery in Switzerland would seem to place the beginning as far back as the interglacial’ epoch which may correspond with the Noe American Yarmouth (p. 634). A few references relative to the antiquity of man: Chamberlin, T. C., Jour. Geol., Vol. X; Geikie, James, The Great Ice Age, pp. 616-6090; and Prehistoric Europe, pp. 568 et seq.; Gilbert, G. K., Sci. Am. Supp., Vol. XXIII, 1887; Lyell, Sir Charles, Antiquity of Man; McGee, W. J., Am. Geol., Vol. XXII, pp. 96-126; Sci., new ser., Vol. IX, pp. 104-105; Upham, Warren, Science, Vol. XVI, pp. 355-6; Am. Geol., Vols. XXX and XXXI; Whitney, J. D., Am. Jour. Sci., 2d ser., Vol. 43, pp. 265- 267, 1867; The Airitecnia Gravels of the Sierra Nevada of California, Cambridge, 1879. 1 Science, Vol. XXIX, 1909, P. 359. . CHAPTER XXX THE HUMAN OR PRESENT PERIOD The end of the glacial period. The close of the glacial period is usually placed at the time when the ice-sheets disappeared from the lowlands in the middle latitudes of Europe and North America. Notwithstanding this usage, the ice-sheets had not then disappeared completely, and have not even now, for about 10% of the recently glaciated area of North America (chiefly in Greenland) is still buried inice. ‘These relics of the last glacial epoch show that the continent has not yet emerged completely from the glacial period. Indeed it is not absolutely clear that there may not be another increase of ice before the long series of glacial epochs closes, but the probabilities seem to be against it. It is not wholly clear that the deformative period which began in the late Tertiary, and extended through the Pleistocene, is yet completed. We are accustomed to regard it so, and perhaps this position is justified; but the movements of post-glacial times are not to be ignored (p. 661). A recent movement in the region of the Great Plains seems to be suggested by certain physiographic features. Many phenomena suggest that the western side of the Great Plains was lower than now, relatively, until about the close of the glacial period. On the western side of the continent there is much evidence of recent movement, some of which appears to have taken place since the close of the glacial period, as usually defined. Similar phenomena are found in other continents. It is not wholly clear, therefore, whether the present is to be regarded as a part of that period of deformation which had its climax in the Pliocene, or whether it is rather the initial stage of a period of quiescence now being entered upon. FORMATIONS The formations which have been making since the end of the glacial period are similar to those of that period, except that gla- cial drift is now being made in limited areas only. Most marine 677 678 THE HUMAN PERIOD post-glacial formations remain beneath the sea, and are not avail- able for study. The general character of the formations being made will be readily inferred from what has been said in earlier chapters. LIFE In the seas, and on the land in the tropics, the life of the Pleis- tocene period appears to have passed by imperceptible gradations into that of the present. In the higher latitudes the transition was marked by two exceptional features, the re-peopling of the - lands depopulated by the ice, and the invasion of the human race. _ Re-peopling the glaciated areas. The re-peopling of the north- western half of North America by plants and animals after the re- treat of the last ice-sheet was a great event of its kind. Certain plants that abounded in Europe before the glacial period were forced across the Mediterranean, or southeastward into Asia, and did not recross the barriers of water and desert when the climate of Europe became mild again. No such barrier intervened in North America. ‘There was, however, an ill-defined climatic barrier be- — tween the arid plain region of the southwest and the humid forest region of the southeast. There is abundant evidence that open plains and arid climates had developed in the middle latitudes of the west by the later part of the Tertiary, and that these have persisted, perhaps with brief interruptions, till now. The pre- glacial arid tracts of the west seem to have been distributed much as now, while the eastern half of the continent was more moist, and covered with forests. As the floras and faunas of the western mountain region were driven south by the ice, they were hemmed in by mountain barriers at the sides, and resisted by arid lands in front. As the trend of the mountains was mainly north and south, they defined a series of meridional tracts which directed the life migrations. In the eastern half of the continent, forests and forest-life were driven southward in a more unrestrained way, but for the most part they kept within the eastern humid tract. Following the last ice-retreat, the life of each of these sections moved northward, expanding as it went. ‘The arctic or tundra flora and fauna that had probably been crowded into a narrow zone fringing the ice-sheet, moved northward through about 20°, and expanded to a breadth of 600 or 700 miles in the northern part of the continent, and occupied the arctic islands not covered by DYNASTY OF MAN | 679 perennial ice and snow. The zone of this arctic flora and fauna now lies mostly north of 60°. The subarctic zone of stunted conifers moved northward about 12°, and expanded to a width of 400 to 600 miles. The cold-temperate belt of deciduous and evergreen trees moved a less distance, but expanded almost equally, while the warm-temperate flora spread over the territory abandoned by the last. With each of these vegetal zones went the appropriate fauna. The musk-ox, whose remains have been found skirting the glaciated area in Pennsylvania, West Virginia, Ohio, Kentucky, Oklahoma, Missouri, and Jowa,! has since retired to the extreme arctic regions. The reindeer, which had a similar distribution about the edge of the ice, occupies the barrens of the northern border of the conti- nent, while fur-bearing animals distributed themselves through the three northerly zones. At the south the floras and faunas of the southeast spread west- ward but little, but the arid and prairie floras and faunas of the southwest spread eastward at the expense of the southeastern group. This does not seem to be equally true in the higher latitudes, where the trees of the eastern group are distributed far to the northwest. The arid and semi-arid floras and faunas of the southwest seem to have pushed the more boreal and arboreous forms to the north- ward, or forced them to ascend the mountains; but the movement was less sweeping and more complicated than that of the east, because of topographic interference and the effect of the lingering mountain glaciation. The Dynasty of Man Human dispersal. As yet there is little geologic evidence relative to the place of man’s origin, or to the earliest stages of his development. Various considerations connected with his physical nature and his distribution seem to point to the warm zone of the eastern hemisphere, perhaps southern Asia or northern Africa, as the place of his appearance. There are some grounds for the in- ference that the earliest dévelopments of those qualities that gave him dominance were associated with the open tracts of the sub- tropical zone, rather than with the forests of the equatorial belt. Subsequent history, as vell as the nature of the case, teaches us that 1. Hay’s Catalogue of Fossil Vertebrates in North America, Bull. 179, U. S. Geol. Surv., 1902: A 680 THE HUMAN PERIOD extreme desert conditions and excessive heights are prohibitive, that semi-arid conditions of varying and precarious intensities lead to nomadic habits, sparse distribution, and limited social and civic evolution, while well-watered plains and fertile valleys, under congenial skies, invite fixed habitation and the development of stable civil and social institutions. Excessive humidity, dense forests, and extreme ruggedness of surface tend to limitation and repression among primitive peoples. Early in the history of the race, it is presumed that a warm climate was more favorable than a severe one. From these considerations and from historical evidence arises the presumption that the primitive centers of evolu- tion of the race were somewhere in the open or diversified parts of the warm tract of the largest of the continents. From this, or from some analogous tract in that quarter of the globe, there seem to have been divergent movements to all habitable lands. A basal factor in the early evolution of civilization was the productiveness of the soil. The advance from hunting and fishing and herding was dependent essentially on agriculture, and was therefore influenced largely by the fertility of the soil and suitable climatic conditions. Loss of soil-fertility has been one cause which has forced the migration of centers of civilization. In lower lati- tudes the upland soils are mostly the residue produced by the decomposition of the underlying rocks and not removed by erosion. With cultivation, wash and wind-drift are accelerated, and unless protective measures are employed, as has not been the case usually, the soils are carried away, and barrenness succeeds fertility. There are areas in the Orient, once well settled, where nothing grows ex- cept such plants as find a foothold in the crevices of the rock. In some places, soils underlain by sandy subsoils have been washed away, leaving barren wastes. Sands from the exposed subsoil have then been driven by the wind over adjacent fertile tracts, making them barren. The explanation of much of the former richness and present poverty of Oriental peoples no doubt lies in this simple proc- ess. Impoverishment of soil threatens many peoples to-day, and is in process of actual realization. This is one of the fields in which conservation is most important. : In glaciated lands the soil-factor has a character quite its own. 1. Near the centers of ice radiation the old soils were worn away, and new soils have not developed in equal amount in their stead. Reduced fertility is the result. These areas lie chiefly in high DYNASTY OF MAN 681 latitudes where other factors do not favor human development. 2. In regions of heavy glacial deposition, which fortunately include the greater and the more southerly parts of the glaciated area, a deep sheet of comminuted rock-material, ready for easy conversion into soil by weathering and organic action, covers great plains. Furthermore, the drift has a gentle relief that does not favor rapid erosion. North of theborder of theglaciated area in North America, in a belt 400 or 500 miles wide, the subsoil of glacial flour and old soil, glacially mixed, has an average thickness of about 100 feet. A similar statement may be made of a large area in north-central Europe. The average thickness of the residuary soils of unglaciated regions similarly situated is about 5 feet. The twenty-fold provision for permanent fertility thus arising from glaciation seems likely to be a factor of importance in the localization of the basal industry (agriculture) of mankind, and of the phases of civilization that are dependent on it. With the evolution of the industrial arts, resources which were neglected at first have come to play important parts in the distri- bution and in the activities of the race, among which are the long and growing lists of mineral resources. Chief among these are the metallic ores, the fossil fuels, the mineral fertilizers, and the struc- tural and ornamental materials of stone and clay. These now influence man’s distribution and activities far more than formerly, and they are quite certain to be more influential still in the future. The distribution and activities of men recently have come to be affected by the distribution of the water-power that arose from the deformations of the late Tertiary periods, and the stream- diversions of the glacial period. With little doubt, such sources of power are to play an increasingly large part in human affairs as time goes on and the stored fuels are exhausted. With the increasing complexity of human activities, the locali- zation of the race will more and more depend on combinations of resources and conditions; but it is difficult to see the time when persistent fertility of the soil, under favorable climatic conditions, co-ordinated with great supplies of fuels, ores, and structural ma- terials, will not constitute a decisive and controlling advantage. Provincialism giving place to cosmopolitanism. The early history of human dispersal was marked by pronounced provincial- ism. Early peoples were much isolated by distance and by natural barriers, and they often interposed artificial barriers against free 682 THE HUMAN PERIOD intercommunication, and hence against the development of a common cosmopolitan type. So long as. hunting and fishing were the dominant pursuits, a wider and wider dispersion into small tribes was a necessary tendency. That such artificial sources of provincialism were more effective than natural ones seems to be implied by the fact that while physiological differences sufficiently marked readily to characterize varieties are numbered by hundreds, dialects sufficiently different to prevent free intercourse are num- bered by thousands. Provincial sentiment to-day manifests itself more conspicuously in language than in most other ways. When efficient water-transportation was developed and the control of the sea attained, a period of cosmopolitan tendency was inaugurated. This has been greatly accelerated in the last few decades, supplemented by rapid land-transportation and electric communication, and is rapidly involving the whole race in a cosmo- politan movement. Almost the whole world is already in daily communication, and most races are more or less habitually inter- mingling by travel and trade. That this is to become more and more habitual until the whole race shall be in constant inter- communication, is not to be questioned. There will then have been inaugurated the most marked period of cosmopolitanism, in all senses of the term, which the world has ever witnessed. What all this will ultimately mean for the race we do not venture to predict. Man as a geological agency. The earlier geologists were in- clined to regard man’s agency in geological progress as rather trivial, perhaps because physiographic geology, in which his influ- ence is felt chiefly, was then less studied than other phases with which he has little to do. The fact probably is that no previous agent, in an equal period of time, has so greatly influenced the life of the land, or the rate of land-degradation, as man has since the agricultural epoch was well established. That this influence will be increased during coming centuries seems clear. The flora is rapidly passing from that which had been evolved by natural agencies through the ages, to that which man selects for cultivation or preservation. With the further progress of this movement, native floras seem destined to early extinction. The same may be said of native faunas. Favored animals, under man’s care, flourish beyond precedent, while others, so far as they are within his reach, are suffering rapid declines that look toward extinction. LIFE 633 Life in the sea is less profoundly affected than that on the land, but even that does not escape modification. The most pronounced exceptions to man’s dominance, and those that bid fair to contest his supremacy longest, are found in organisms too minute to be controlled easily by him, and in organisms that, quite against his will, flourish on the conditions he furnishes. But even the acceler- ated evolution of these organisms is a part of the profound biological revolution which attends man’s dominance. Man’s control has not thus far been characterized by much recognition of the complicated interrelations of organisms and of the consequences of disturbing the balance in the organic kingdom, and he is reaping, and is certain to reap more abundantly, the unfortunate fruits of ignorant and careless action. For the most part, man has been guided by immediate considerations, and even these not always controlled by much intelligence. Thus great wantonness has attended his destruction of both plant and animal life. But a more intelligent as well as a more sympathetic attitude is developing, and will doubtless soon become dominant. A new era in control and selection is dawning. New varieties and races are being produced that not only depart widely from the parent stock, but diverge in lines chosen to meet given conditions, or to produce desired products. How far this may yet go it is impossible now to predict. Prognostic geology. The long perspective of the past should afford at least some suggestions of the future, but it must be con- fessed that the most important conjectures as to the future are dependent on interpretations of the past that are not yet certain. A word has been said relative to a possible return of a glacial epoch, but no sure prediction can be made. Question has been raised as to whether the deformations of recent times are over, but the answer remains uncertain. The duration of the earth as a habitable globe has been a common theme of prognosis. A final refrigeration as the result of the cooling of a once molten globe has been the usual forecast, and the final doom of the race has been a favorite theme for pseudo-scientific romances. But this all hangs on the doctrine of a former molten earth, if not on the doctrine of its origin from a gase- ous nebula. Under the alternative conception of a slow-grown earth conserving its energies, conjoined with a more generous conception of the energies resident in the sun and the stellar system, no narrow limit need be assigned to the habitability of the earth. A Psycho- 684 THE HUMAN PERIOD zoic era, as long as the Cenozoic or the Paleozoic, or an eon as long as the cosmic and the biotic ones, may quite as well be predicted as anything less. ‘The forecast is at best speculative, but an optimistic outlook seerns more likely to prove true than a pessimistic one. An immeasurably higher evolution than that now reached, with attain- ments beyond present comprehension, is a reasonable hope. The forecast of an eon of intellectual and spiritual development comparable in magnitude to the prolonged physical and biotic evolutions lends to the total view of earth-history great moral satisfaction, and the thought that individual contributions to the higher welfare of the race may realize their fullest fruits by con- tinued influence through scarcely limited ages, gives value to life and inspiration to personal endeavor. 7 APPENDIX REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS. Alge and algoid forms THALLOPHYTES (Thallus plants) Fungi and fungoid forms Lichens BRYOPHYTES Hepatice, liverworts. (Moss plants) | Musci, mosses. Lycopodiales. Daernoeyrrs Sphenophyllales. (Fern plants) Equisetales Filicales Gymnosperme (naked seed) SPERMATOPHYTES (Seed plants) Angiospermze (covered seed) (Flowering plants) 685 Diatomacee, diatoms. Coccospheres \ p ee Rhabdospheres { ~ “*5!° *'8®- Cyanophycee, blue-green alge. Chlorophycee, green alge, including stoneworts. Rhodophycee, alge. Pheophycee, brown alge. Myxomyeetes, slime-molds. Schizomycetes, bacteria. Phycomycetes, molds. Ascomycetes, dews. Basidiomycetes, basidium-fungi, smuts, rusts, mushrooms. Symbiont alge and fungi. red >; True alge. ‘animal fungi,” *fission-fungi,” algze-fungi, water- ascus-fungi, mil- Lepidodendra, _ sigillarias, club- mosses. ( Calamites. { Equisete, | — tails. Filices, true ferns. Cycadofilicales. Bennettitales. Cycadales. Cordaitales. Ginkgoales. Coniferales. Gnetales. Dicotyledones. Most common forest trees (except conifers), most shrubs and most netted- veined leaved herbs. Monocotyledonez, cereals, grasses, eLc. scouring-rushes, horse- 686 APPENDIX REFERENCE TABLE OF THE PRINCIPAL GROUPS OF ANIMALS! PROTOZOA (the simplest animals) COELENTERATA Porifera (Sponges, corals, jellyfishes) Cnidaria Pelmatozoa ECHINODERMATA (Crinoids, star- Asterozoa fishes, sea-urchins) Echinozoa VeERMES (Worms) MOLLUSCOIDEA (Mollusc-like forms) MOoOLuLusca (Molluscs) Branchiata ARTHROPODA (The articulates) Tracheata VERTEBRATA 1 After Zittel in the main. | | | | { Rhizopoda Foraminifera. Radiolaria. rarliniay Unknown in Gregarina fossil state. Spongize { Calcareous sponges. \ Siliceous sponges. Anthozoa, coral polyps. Hydrozoa, hydroids and medusz. Cystoidea, cystids. | Crinoidea, stone lilies. Blastoidea, blastoids, Ophiuroidea, brittle-stars. Asteroidea, starfishes. Echinoidea, sea-urchins. Holothuroidea, sea-cucumbers. Platyhelminthes Rotifera Nemathelminthes Gephyrea Annelida, sea-worms. Bryozoa, sea-mosses. Brachiopoda, lamp-shells. Pelecypoda, lamellibranches, bivaives. Scaphopoda, tusk-shells. Amphineura, chiton. Gastropoda, univalves, snails, etc. Cephalopoda, nautilus, cuttlefish. Crustacea. Trilobita, trilobites. Gigantostraca, horseshoe crabs. Entomostraca, ostracoids, barnacles. Malacostraca, lobsters, crabs. Myriapoda, centipedes. Arachnoidea, spiders, scorpions. Insecta, insects. Cyclostomata, lampreys. Selachii, sharks. Holocephali, spook-fishes. Dipnoi, lung-fishes. Teleostomi, ganoids and | teleosts(common fishes). Amphibia, amphibians, batrachians. Reptilia, reptiles. . Aves, birds. Rare as fossils. Pisces (fishes) Wintnnehn Prototheria, monotremes. (mammals) Metatheria, marsupials. Eutheria, placentals. INDEX Abrasion by wind, 22 Acadian epoch, 344, 346 Accidents to streams, 101-107 Acidic rocks, 252 Acondylacanthus gracilis, 433 Acrocrinus amphora, 436 Acrotreta gemma, 360 Actzon shilohensis, 597 Actinocrinus lobatus, 434 Actinocrinus senectus, 433 Actinolite, 255 Actinopteria textilis, 414 Adams, F. D., cited, 240 Adirondacks, Proterozoic rocks of, 340 Adjustment of streams, 97, 98 Africa, Cretaceous in, 544 Devonian in, 412 Miocene of, 590 Pennsylvanian in, 457 Permian in, 474 Triassic in, 493 Aftonian epoch, 633 Agassiz, A., cited, 167 Agassizocrinus dactyliformis, 436 Agates, 254, 286 Age of earth, 168 Agglomerate, 250, 258, 263 Aggradation by streams, 107 Agnostus interstrictus, 359 Alabaster, 257 Algonkian. See Proterozoic. Allegheny series, 443 Alluvial cones, 108, 109 deposits, 108, 660 fans, 109, LIO plains, 111 terraces, 119 Alpine glaciers, 127 Amazonstone, 254 Amber, 577 ~ Amberleya dilleri, 531 Amethyst, 254 Ammonites, 499 Jurassic, 509 Amphibians, Eocene, 572 Pennsylvanian, 464 Amphibians — continuea Permian, 477 rise of, 464 Triassic, 494 Amphibole, 253, 255 Amygdaloid, 259 Anatina austinensis, 530 Anchippus, 579, 592 Anchisaurus colurus, 495 Anderson, R., cited, 583 Angiosperms, Comanchean, 528 place of origin, 528 Animals, classification of, 586 Animikean, 331 Annelids. See worms. Annularia sphenophylloides, 461 Anomalina ammonoides, 572 Anomalocrinus incurvus, 384 Antecedent drainage, 105 Anthracite coal, 453 Anthracite coal field, 443 Anthrapalemon gracilis, 466 Anticlinal fold, 274 Anticlinoria, 220, 277 Antiquity of man, 670-676 Apennines, Pliocene in, 608 Aperiodic movements, 219 Aphanite, 248, 259, 262 Aphorrhais prolabiata, 554 Appalachian coal field, 443 Appalachian drainage, 105 Appalachian Mountains, age of, 475 Aqueous metamorphism, 286 Arabellites cornutus, 386 ovalis, 386 Arapahoe formation, 539 Arca staminea, 595 tehamaensis, 531 Archeopteris bochsiana, 460 Archeopteryx macrura, 519 Archean, bearing on origin of earth, 323 delimitations, 317 distribution of, 320, 322 general characteristics, 317 granites, 318 origin of, 318, 320 687 688 INDEX Archean — continued Aucella crassicollis, 531 rocks, 316, 317 mosquensis, 510 schists, 318 piochii var. orata, 531 Archecyathus minganensis, 386 Augite, 255 rensselericus, 362 Augitite, 260 Archeocrinus desideratus, 384 Augusta series, 426, 427 Archeozoic eon, 308 Aurochs, 675 Archeozoic era, 314 Australia, Cambrian glacial beds in, climate of, 324 357 duration of, 324 Carboniferous in, 457 life of, 324 coal in, 457 Archimedes, 438 Cretaceous in, 527, 545 swallovanus, 437 glacial Permian in, 474 Archinacella cingulata, 381 Miocene of, 590 Argillite, 292 Autoclastic rocks, 295 Arid regions, erosion in, 83 Aviculopecten occidentalis, 469, 481 Aridity, Permian, 470 Azoic eon, 307 Silurian, 392, 393 Arietide, 508 Baculites, Cretaceous, 555 Arikaree formation, 588 grandis, 554 Aristozoe rotundata, 359 Bad lands, 83, 85, 576 Arkose, 266 Bain, H. F., cited, 420 Arnold, R., cited, 558, 574, 583, 604, | Ball, Sir Robt., cited, 649 611 Barker, A. S., cited, 167 Artefac, 670 Barnacles, 418 Artesian wells, 52 Barrier, 184 Arthracantha punctobrachiata, 419 Bars, 186, 187 Arthrodirans, 418, 422 Barus, Carl, cited, 33 Arthrolycosa antiqua, 466 Barycrinus hoveyi, 434 Artiodactyls, 568 Basal conglomerate, 328 Ashley, G. H., cited, 607, 661 Basalt, 260, 262 Asia, Cambrian in, 356 Base-level, 66, 68, 69 Carboniferous in, 457 Base-leveled plain, 68 Cretaceous in, 527, 544 Basement complex, 319 Devonian in, 412 Basic rocks, 252 Jurassic of, 507 Bastin, E. S., cited, 642, 666 Miocene of, 590 Batholiths, 228 Ordovician in, 376 Batocrinus, 434 Pennsylvanian in, 457 Bays sandstone, 369 Permian in, 474 Beach, 184 Triassic in, 492 Beadnell, H. J. L., cited, 19 Astarte californica, 531 Becraft limestone, 402 thomasii, 595 Beekmantown limestone, 368 Asteroids, 2 Belemnites, 509 Astral eon, 307 densus, 512 Astresius liratus, 531 Bellerophon clausus, 381 Athyris lamellosa, 435 percarinatus, 469 Atlantic coast, submergence of, 171 sublevis, 437 Atmosphere, 4 Berry, E. W., cited, 611 beginning of, 310 Betulites westi, var. subintegrifolius, composition of, 4 547 thermal effects of, 28 Bifidaria armifera, 644 under nebular hypothesis, 309 corticaria, 644 work of, 12-29 muscorum, 644 Atrypina imbricata, 414 pentodon, 644 Billingsella coloradoensis, 360 transversa, 360 Bilobites varicus, 414 Biotite, 255 Birds, Cretaceous, 550, 5 51 Eocene, 572 Jurassic, 510 Bison, 666 Black Hills, 340 Black River limestone, 368 Blastoids, Pennsylvanian, 468 Blue mud, 198 Bonney, T. G., cited, 245 Botriopygus alabamensis, 554 Bowlder-clay, 618 Bowlders in drift, 617 Brachiopods, Cambrian, 360 Devonian, 418 Jurassic, 512 Mississippian, 414, 415, 419 Ordovician, 378, 382 Pennsylvanian, 468 Silurian, 396, 397 Triassic, 499 Brachiosaurus, 517 Brachiospongia digitata, 386 Branner, J. C., cited, 373, 564 Brazil, coal in, 457 Breccia, 267 Brontosaurus (Apatosaurus), 517 Bronzite, 255 Brooks, A. H., cited, 447 Brooksella alternata, 362 Broom, R., cited, 479 Brule formation, 575 Bryozoans, Devonian, 414, 419 Mississippian, 435 Ordovician, 383 Pennsylvanian, 468 Silurian, 398 Buchanan epoch, 633 Buckley, E. R., cited, 34, 122, 374 Bumastus trentonensis, 378 Bunter formation, 491 Buttes, 93, 94 Bysmaliths, 228 Cairngorm, 254 Calamites, 424, 459, 476 Calamites cistii, 461 Calcareous tufa, 660 Calcite, 256 Calhoun, F. H. H., cited, 625 California earthquake, 208, 209, 217 rift, 583 INDEX 689 Calliostoma philanthropus, 597 Callipteridium membranaceum, 466 Callopora pulchella, 383 Caloosahatchie beds, 604 Calvert, W. R., cited, 539 Calvin, S., cited, 645, 665 Calymene callicephala, 378 niagarensis, 399 Camarotoechia barrandei, 415 Cambrian faunas, origin of, 366 succession of, 364 Cambrian glaciation, 356, 357 Cambrian life, 358 advancement of, 363 Cambrian of Europe, 356 Cambrian period, 344-366 close of, 355 duration of, 357 Cambrian rocks, distribution, 345, 347 metamorphism of, 354, 355 outcrops, 352 Cambrian sedimentation, 351 submergence, 349 Campbell, M. R., cited, 97 Campeloma harlowtonensis, 530 Camptonectes bellistriatus, 512 Canada, Archean rocks of, 321 Proterozoic rocks of, 340 Canadian epoch, 368 Cancellaria alternata, 597 Canoe-shaped valleys, 95 Canyon of the Yellowstone, 70 Canyons, 70, 87 Carabocrinus vancortlandti, 384 Carbon formation, 539 Carbonaceous slates in Huronian, 332, ak glee Carbonation, 23 of igneous rocks, 264 Carboniferous. See Pennsylvanian. Cardioceras cordiformis, 512 ‘Cardium leptopleurum, 595 Carnivores, Eocene, 570 Miocene, 594 Caryocrinus ornatus, 395 Cascade formation, 525 Cascade Mountains, age of, 588 Cassidulus subquadratus, 554 Catazyga headi, 382 Cat’s-eye, 254 Catskill formation, 402, 406 Caverns, 38, 390 Cavity filling, 286 Cayugan epoch, 388 series, 391 690 Cepnalaspis, 421 Cephalopods; Cambrian, 361 Cretaceous, 554 Devonian, 417 Jurassic, 512 Mississippian, 433, 437 Ordovician, 378-380 Pennsylvanian, 468 Permian, 480, 481 Silurian, 397 Triassic, 498, 490 Ceratites nodosus, 498 whitneyi, 500 Ceratops beds, 539 Ceratosaurus nasicornis, 516 Ceraurus pleurexanthemus, 378 Cerithium paskentensis, 531 Cerithium (?) texanum, 530 Cetaceans, 571 Chadron formation, 575 Chalk, 267 origin of, 537 Chamberlin, R. T., cited, 226, 241, 310, 451 Chamberlin, T. C., ‘cited, 43, 52, 131, 226, 337, 374, 376, 628, 630, 632, 645 Champlain epoch, 633 Champsosaurus, 551 Changes of level, effect on streams, ror during Pleistocene, 661, 662 Charleston earthquake, 210 Chautauquan series, 402 Chazy limestone, 368 Chemical deposits in sea, 193, 199 sediments, 268 Chemung fauna, 420 formation, 402, 406 Chert, 268, 288 Chesapeake Bay, 106 Chesapeake formation, 581 Chester series, 426, 427 Chickamauga limestone, 360 Chico series, 526, 540 Chief Mountain, 542 Chimney-rocks, 179 China, Cambrian glacial beds in. 357 coal in, 457 Chloritic rock, 291 Chlorite, 256 Chonetes cornutus, 396 coronatus, 419 granulifera, 460 Choristoceras marshi, 4098 Chrysolite, 255 Cidaris coronata, 511 INDEX Cincinnati Arch, 373° Cincinnatian epoch, 36% Cinder-cones, 236 Cladoselache fyleri, 439 Cladodus springeri, 433 _ Claosaurus annectens, 544: Clark, W. B., cited, 532 Classification of rocks, 297 Clastic rocks, 267, 269 Clear Fork limestone, 471 ‘ Cleland, H. F., cited, ror a) Cliff glaciers, 130 = Cliffs in arid regions, 83, 94 Climacograptus bicornis, 385 Climate, Cambrian, 356 Comanchean, 528 Cretaceous, 545 Devonian, 413 effect on erosion, 81 Eocene, 564 Jurassic, 507 Miocene, 591, 598 Ordovician, 376 Pennsylvanian period, 463 Permian, 476 Pleistocene, 613, 648 Pliocene, 611 Proterozoic, 342 Quaternary, 613, 648 Silurian, 393 Triassic, 490 Climates of past, criteria of, 195 Clinton beds, 388, 380 iron ore, 389 Coal, 269, 447,448, 486, 507, 538, 5590, 560 Eocene, 559 Jurassic, 506, 507 Laramie, 538 Pennsylvanian, 443 et seq. varieties of, 453 Coal-beds, extent of, 453 Coal fields, 443-445 Coal measures, 441 Coastal plain, structure of, 524 Coast lines. See shore lines. Cobalt in Huronian rocks, 334, 337 Cobleskill limestone, 388 Coccosteus decipiens, 423: Coeymans limestone, 402 Coleman, A. P., cited, 342, 665 Coleopters, 467 Colorado Canyon, 88 age of, 606 Colorado River, delta of, 116, 118 © Colorado series, 535, 536 INDEX Columbia River, 107 Columbia series, 652 Columnar structure, 247, 248 Columnaria alveolata, 385 Comanchean angiosperms, 528 Comanchean, distinct from Cretaceous, 521 Comanchean fossils, 528 Texan, 530 Comanchean of Mexico, 525 Comanchean period, 521-531 close of, 526 Comanchean system, distribution of, 522 Comarocystis punctatus, 384 Compound alluvial fan, 110 Concretions, 287, 288 in Cretaceous, 536 Condylarthra, 566, 567 Conemaugh series, 443 Conglomerate, 267, 271 great thicknesses of, 191 Conifers, Jurassic, 515 Conocardium prattenanum, 437 trigonale, 416 Consequent streams, 61 Constellaria polystomella, 383 Continental creep, 350 glaciers, 127 shelves, 171 Continent-forming muvements, 221 Contour maps, 17 Conularia trentonensis, 381 Conus diluvianus, 597 Cook, G. H., cited; 523 Copper in Michigan, 337 Coral reefs, oldest, 391 Corals, Cambrian, 362 Cretaceous, 555 Devonian, 414, 417 Jurassic, 509 Mississippian, 415, 420 Ordovician, 384, 385 Pennsylvanian, 468 Silurian, 398 Triassic, 500 Corbula aldrichi, 573 blakei, 500 idonea, 595 Corbula (?) persulcata, 531 Cordilleran ice sheet, 613 Cordilleran Mountains, age of, 541 region, Proterozoic rocks of, 340 Cordaites, 424, 458, 459, 462, 476 Coroniceras bisulcatum, 508 691 Corniferous formation, 402 Corrasion, 73, 78 Correlation, basis of, 346-349 Corrosion, 73, 80 Cotylosauria, 478 Cowles, H. C., cited, 20 Crag and tail, 156 Crania loelia, 382 Crassatellites alaeformis, 574 marylandicus, 595 Craters, 230 Crazy Mountains, 543 Creep, 40, 75 continental, 350 Creodonta, 566, 567, 570 Crepidula fornicata, 597 Crepipora hemispherica, 383 Cretaceous fossils, 554 Cretaceous life, 546 Cretaceous period, 521, 532-555 climate of, 545 close of, 541, 542 Cretaceous plants, 546, 547 saurlans, 546 Cretaceous system, 533 structure of, 532, 535, 540 Crevasses in glaciers, 135 Crinoids, Cretaceous, 555 Devonian, 418 Jurassic, 509 Mississippian, 437 Ordovician, 383 Silurian, 396 Cristellaria gibba, 572 radiata, 572 Criteria of glaciation, 616 Critical level, 170, 455 Crocodiles, Cretaceous, 540 Crocodilians, 514, 519 Croll, James, cited, 649 Cross, Whitman, cited, 40, 489, 504, 538, 539 Cross-bedding, 192, 270, 271 Crustacea, Cambrian, 359 Jurassic, 510 Crustal movements, amount of, 224 cause of, 224 Crustal shortening, 223 Cryphzus boothi, 419 Crystals, growth in lava, 251 Crytina hamiltonensis, 416 Ctenodonta nasuta, 381 pectunculoides, 381 recurva, 381 | Culm, 431 692 Currents in streams, 77 Currents, ocean, 188 Cushetunk Mountain, 94 Cut-offs, 114 Cycadeoidea dakotensis, 529 Cycads, Comanchean, 529 Jurassic, 515 Cycle of erosion, 66, 103 stages in, 69 Cyclomena bilix, 381 Cyphaspis christyi, 399 Cypricardella bellistriatus, 419 Cyrtoceras neleus, 380 Cyrtolites ornatus, 381 Cystids, 362 Ordovician, 383 Pennsylvanian, 468 Silurian, 396 Dedicurus clavicaudatus, 669 Dakota formation, 535, 536 Dall, W. H., cited, 556,574, 581, 506, 611 Dalmanella testudinaria, 382 Daly, R. A., cited, 103 Dana, J. D., cited, 179, 226, 245, 307 Darton, N. H., cited, 575 David, T. W. E., cited, 357, 413 Davis, B. M., cited, 37 Davis, C. A., cited, 202 Davis, W. M., cited, 13, 68, 118, 487, 560, 606, 628, 645 Dawson, G. M., cited, 429, 526, 641 Dawson, J. W., cited, 641 Dead Sea, 204 Debris in ice, +53 Deccan lava flows, 544 Deep-sea deposits, 189, 196 Deer, earliest, 592 Degradation, rate of, 84 Dehydration, 291, 292 Deiphon forbesi, 399 Delaware River, 105 Delaware Water-Gap, 92 Delta fingers, 117 Delta of the Mississippi, 115 Deltas, 108, 116, 117 Dendrocrinus polydactylus, 384 Dentalium attenuatum, 597 Denver formation, 539 Deposition by shore-currents, 184 by streams, 107 by undertow, 184 by waves, 184 of drift, 156 of mineral matter from solution, 37 INDEX Deposits in sea, 172, 189 Deposits by springs, 652 Derbyia crassa, 469 Deroceras subarmatum, 508 Desert sandstone, 545 Des Moines series, 443 Devonian, climate of, 413 Devonian fauna in Great Basin, 420 fishes, 416, 418 floras, 423 igneous rocks, 410 Devonian land life, 421, 423 life, 413 ? Devonian oil and gas, 410 Devonian period, 402-425 close of, 410 Devonian phosphates, 411 system, outcrops of, 410 Diabase, 261 Diallage, 260 Diastrophic movements, periodic, 219, 220 Diastrophism, 2, 170, 206 of Archeozoic era, 318 of Jurassic, 505 of Middle Miocene, 584 of Permian, 475 of Quaternary, 661 Diatom 00ze, 199 Dichocrinus inornatus, 433 Dichograptus octobrachiatus, 385 Dicranurus hamatus, 414 Dictyopteris rubelia, 460 Didymograptus nitidus, 385 Dielasma bovidens, 469 Dikellocephalus fauna, 349 Dikellocephalus pepinensis, 348 Dikes, 228 Diller, J. S., cited, 586, 606, 663 Dinichthys herzeri, 418 Dinoceras mirabile, 569 Dinocerata, 568 Dinosaurs, 494 Cretaceous, 546 Jurassic, 515 Dinotherium, 610 Diorite, 260, 261, 262 Dip, 275 hee Diplograptus pristis, 385 Diplopodia texanum, 530 Dipnoi, 422 Dipterus valenciennesi, 423 Discorbina turbo, 572 Divides, permanent, 63 Dolerites, 262 INDEX Dolomite, 196, 257, 267, 491 Dolomization, 267 Don River beds, 665 Dosiniopis lenticularis, 573 Double Mountain formation, 471 Drainage, affected by glaciation, 632 Drake, N. F., cited, 561 Drepanochilus nebrascensis, 554 Drift, 8, 151 constitution of, 616 contact with rock, 620 deposition of, 160 distribution of, 618 structure of, 618 thickness of, 620 topography of, 620 transportation of, 153 Drowning of valleys, 106 Drumlins, 627, 629 Dumble, E. T., cited, 600 Dunes, 15, 655 distribution of, 21 migration of, 19 topography of, 16 Dust, eolian, 12 volcanic, 13 Dutton, C.E., cited, 83, 206, 226, 245,604 Dwyka conglomerate, 474 Dynamic metamorphism, 291 Earth, constitition of, 315 form of, 6 mass of, 3 origin of, 299 size of, 6 specific gravity, 11 Earth history, stages of, 313 Earthquake fissures, 212 vibrations, 208 waves, 210 Earthquakes and changes of level, 216 and faults, 214 Earthquakes, distribution of, 211 effect on life, 215, 216 geologic effects, 207, 211 Eastern Interior coal field, 444 Eatonia medialis, 414 Eccyliomphalus triangulus, 381 Echinocaris punctata, 419 Echinoderms, Cretaceous, 554, 555 Jurassic, 509-511 Mississippian, 436 Ordovician, 383, 384 Silurian, 395 Triassic, 499 eeaees 2 Echinoids, Silurian, 306 Ecphora quadricostata, 597 Ectonocrinus grandis, 384 Edentates, Eocene, 571 Eldridge, G. H., cited, 583 Electricity, effects of, 29 Elephants, Miocene, 592 Elephas antiquus, 669 Eleutherocrinus cassedayi, 419 Elevated barrier beach, 185 Ellensburg formation, 607 Elotheres, 579 Empire beds, 585 Enchanted Mesa, 93 Endothyra baileyi, 437 Englacial drift, 147, 153 Ensis directus, 595 Enteletes hemiplicata, 469 Entrenched meanders, 102 Eocene carnivores, 570 Eocene, close of, 564 geography of, 564 Eocene coal, 559 Eocene flora, 566 Eocene in South America, 564 in West Indies, 564 Eocene life, 565 mammals, 565, 566 mollusks, 573 Eocene of western interior, 559 Eocene period, 556-580 Eocene system, 556 composition of, 558 in the west, 558 thickness of, 558 Eolian deposits, 652 sand, I5 Eoscorpius carbonarius, 466 Eotrocus concavus, 437 Epeirogenic movements, 218 Epicontinental seas, 5 Equisetales, 458 Eretmocrinus remibrachiatus, 434 Erian series, 402 Erosion, affected by climate, 81 analysis of, 73-80 conditions affecting, 80 cycle of, 66 Erosion by glaciers, 147 by running water, 59 by waves, 176 Erosion in arid regions, 83 in Mississippi basin, 84 Erosion of folds, 95 Eskers, 164, 165 693 604 Esopus grit, 402 Eucalyptocrinus crassus, 395 Euconulus fulvus, 644 Kugnathus athostamus, 513 [Eumetria marcyi, 437 Eunicites gracilis, 386 varians, 386 Eupachycrinus magister, 469 Euphoberia armigera, 466 Europe, Cambrian of, 356 Carboniferous of, 430, 456 Cretaceous of, 527, 542 Devonian of, 411 Eocene of, 562 Jurassic of, 506 Miocene of, 588 Oligocene of, 576 Ordovician of, 375 Permian of, 472 Pliocene of, 608 Silurian of, 394 Trias of, 401 European ice sheet, 615 Eurypterus, 400 Eurypterus fischeri, 401 mansfieldi, 467 Eutaw formation, 534 Evanston formation, 539 Evaporation, 28 Exfoliation, 265 Exogyra (Ostrea) virgula, 510 Extinct lakes, 202 Extra-terrestrial deposits in sea, 198 Falls, 89 Fanfold, 276 Faulting in western mountains, 541 Faulting, Quaternary, 659 Faults, 281 and earthquakes, 214 and folds, 282 distributive, 283 hade of, 282 heave of, 282 normal, 281 significance of, 283 throw of, 282 thrust, 282 Fault-scarp, 282 Fauna defined, 346 Favosites occidens, 3098 Feldspar, 253, 254 Felsites, 262 Fenestella emaciata, 419 parvulipora, 398 INDEX Fenneman, N. M., cited, £73 Ferns, Devonian, 423 ~ Pennsylvanian, 450, 450 Ficus inequalis, 547 Fiords, 152, 153 Fisher, O., cited, 226 Fishes, Cretaceous, 554 Devonian, 418 Jurassic, 510, 512, 513 Mississippian, 438-449 Onondagan, 415 Ordovician, 386 Silurian, 399, 401 Fissure eruptions, 229 Fissuridea alticosta, 597 griscomi, 597 Flaxseed iron ore, 289 Flints, 268 Flood plain deposits, .111 Flood plains, material of, 113 Floods, 57 Florissant beds, 575 Flow structure, 247, 248 Flowing wells, 54 Fluviatile deposits, 652 — Fluvio-glacial deposits, 631 Folding, 220 Folds, erosion of, 95 Foliation, 294 Foliation of ice, 126 Foraminifers, Cretaceous, 555 Eocene, 572 Forbesiocrinus wortheni, 434 Fordilla troyensis, 361 Formation, defined, 269 Fort Union series, 539 Fossils, 267 Fraas, E., cited, 575 Fredericksburg formation, 524 French Broad River, 105 Fulgar spiniger, 507 Fuller, M. L., cited, 34 Fusulina limestone, 457 Fusulina secalicus, 468, 469 Fusus (?) interstriatus, 573 texanus, 530 Gabbroids, 263 Gabbros, 260, 262 Galena limestone, 369 Gangamopteris cyclopteroides, 478 Gangamopteris flora, 477 Ganoids, 422 Garnet, 257 Gas and oil in Ohio, 374 INDEX Gas and oil, Ordovician, 374 Gases in igneous rocks, 241 Gases of volcanoes, 236 Gastropods, Cambrian, 361 Cretaceous, 554, 555 Devonian, 417 Miocene, 597 Ordovician, 380, 381 Silurian, 397 Triassic, 499 Geanticlines, 277 Geikie, Sir A., cited, 226, 245, 356 Geikie, J., cited, 160, 171, 668, 676 Genesee shale, 402 Geodes, 286 Geology defined, 1 Geology, subdivisions of, 1 time divisions, 323 Georgian epoch, 344 Geosaurus suevicus, 515 Geosynclines, 277 Geysers, 49 (ilbette 5 2 J itete re sett te atste crises: stat tislete S20 Deve * . . + eae ee Oe cele ee ve te +e a ave om Staeetetetet *sT.cie es seen stele $itste2 Meats? ° Saas toa esentie . ise : , : ; f nants he = . oo tet. petal? 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