ELEMENTS OF GEOLOGY BY ELIOT BLACKWELDER ASSOCIATE PROFESSOR OF GEOLOGY, UNIVERSITY OF WISCONSIN AND HARLAN H. BARROWS ASSOCIATE PROFESSOR OF GENERAL GEOLOGY AND GEOGRAPHY UNIVERSITY OF CHICAGO NEW YORK : CINCINNATI : CHICAGO AMERICAN BOOK COMPANY COPYRIGHT, 1911, BY ELIOT BLACKWELDER AND HARLAN H. BARROWS. ENTERED AT STATIONERS' HALL, I ONDON. B. A B. GEOLOGY. w. i>. 3 PREFACE THIS is an elementary textbook, not a manual or reference book. The authors have sought to give the student (1) an understanding of the general principles and processes of the science, (2) a few of its fundamental facts, (3) an interest in the subject, and especially (4) training in clear thinking. Many of the mere facts of the science will be forgotten presently by most students. The power and habit of reasoning logically and of thinking clearly will be of service to the student in meeting every problem of later life. The constant aim has accordingly been to make the text explanatory rather than merely descriptive, and to appeal to the judgment rather than to the memory. The book has been written with the belief that, while it is the duty of the teacher to develop in the student the power to reason, it is the business equally of the text. This has determined even the nature of the questions asked at the ends of most of the Chapters, and elsewhere. They are in general not questions the answers to which may be found in the text, but questions which the student may reason out for himself, provided the text has been read with under- standing. The book departs from current practice more or less in the arrangement of its material, and particularly in the omission of separate chapters on volcanoes and earthquakes. Though very interesting and from some standpoints important, vol- canoes, and especially earthquakes, have been minor factors in the development of the earth, so much so as not to merit, in the opinion of the authors, the space commonly allotted to them in textbooks. In the historical chapters "standard sections" have been omitted because in general they are of relatively local appli- 903869 6 CONTENTS PAOE V. THE WORK OF STREAMS . ., . . . . 125 THE PROCESSES OF EROSION . . . . . .125 FEATURES DEVELOPED BY RIVER EROSION . . 137 STREAM DEPOSITS 171 SUMMARY .188 REFERENCES 189 VI. GLACIERS 191 CHARACTERISTICS OF GLACIKRS 191 THE GEOLOGICAL WORK OF GLACIERS .... 208 THE WORK OF WATERS ASSOCIATED WITH GLACIERS . 225 SUMMARY 230 QUESTIONS 230 REFERENCES 231 VII. OCEANS AND LAKES , 233 THE SHORES OF THE OCEAN 237 OCEAN DEPOSITS 255 LAKES 261 QUESTIONS 271 REFERENCES . . . . . . . . . . 272 VIII. THE GREAT RELIEF FEATURES OF THE LAND 274 MOUNTAINS 274 PLATEAUS 284 PLAINS 285 REFERENCES 287 PART IT HISTORICAL GEOLOGY IX. HISTORY OF THE EARTH 289 GROUPS OF ANIMALS AND PLANTS ..... 292 FOSSILS AND THEIR USES 302 QUESTIONS 307 REFERENCES 307 X. ORIGIN AND DEVELOPMENT OF THE EARTH . 308 THEORIES OF ORIGIN 309 QUESTIONS 316 REFERENCES 316 CONTENTS 7 PAGE XT. THE ARCHEOZOIC ERA . . * . ." . 317 XII. THE PROTEROZOIC ERA . '. . .. . . . . 322 PROTEROZOIC ROCKS OF THE LAKE SUPERIOR REGION . 322 PROTEROZOIC ROCKS IN OTHER REGIONS .... 325 GENERAL CHARACTERISTICS OF THE PROTEROZOIC GROUP 327 LIFE IN THE PROTEROZOIC ERA ...... 329 QUESTIONS 330 XIII. THE CAMBRIAN PERIOD 331 XIV. THE ORDOVICIAN PERIOD 339 XV. THE SILURIAN PERIOD . . ... 349 XVI. THE DEVONIAN PERIOD . . . . , .358 DEVONIAN IN THE WEST 359 DEVONIAN IN THE EAST 359 MIGRATIONS AND CHANGES OF THE SKA LIFE . . . 361 LIFE ON LAND 367 QUESTIONS .......... 368 XVII. THE MISSISSIPPIAN P0RIOD 369 LIFE OF THE MISSISSIPPIAN SEA 372 QUESTIONS 375 XVIII. THE PENNSYLVANIAN PERIOD .... 376 LIFE OF THE COAL SWAMPS 382 QUESTIONS .......... 387 XIX. THE PERMIAN PERIOD 389 SUMMARY OF THE PALEOZOIC ERA . . . . . 394 QUESTIONS .396 XX. THE TRIASSIC PERIOD 397 LIFE OF THE TRIASSIC ....... 399 QUESTIONS 404 XXI. THE JURASSIC PERIOD 405 LIKE OF THE JURASSIC ....... 409 QUESTIONS , , , f , , , , , 413 8 CONTENTS XXII. THE COM ANCHEAN PERIOD. . . . . 414 XXIII. THE CRETACEOUS PERIOD. . . . r . 418 THE MESOZOIC ERA IN NORTH AMERICA . ... . 430 QUESTIONS 432 XXIV. THE TERTIARY PERIOD ... ,433 LIFE OF THE TERTIARY PERIOD 442 QUESTIONS 446 XXV. THE QUATERNARY PERIOD 448 THE GLACIAL EPOCH OUTSIDE OF THE ICE SHEETS . 457 ANIMALS OF THE GLACIAL EPOCH ..... 460 THE RECENT EPOCH ........ 463 QUESTIONS 465 INDEX 467 ELEMENTS OF GEOLOGY INTRODUCTION The meaning and scope of geology. Geology has to do with the history of the earth and of its inhabitants. Its field is so broad that for the sake of convenience- and specialized study it has been divided into numerous branches. Geology is concerned with the different members of the solar system and with other heavenly bodies in so far as they yield evidence as to the origin of the earth, or affect the activities now in progress upon it. This division of the general subject is some- times called Astronomic Geology, and is related closely to the science of Astronomy. The processes and agents at work changing the earth must be studied carefully by the geologist, for they are shaping the present chapter in the history of the earth, and an understanding of them affords also a key by which much of its earlier history, recorded in the rocks, may be read. This phase of the subject is Dynamic Geology, and it has common ground with the special science of Physi- ography or Physical Geography, with Meteorology, the science of the atmosphere, and with other sciences. The study of the remains and impressions of the plants and animals of past ages that are found in the rocks is Paleontology; it is really the historical side of Botany and Zoology. Structural Geol- ogy is concerned with the arrangement of the materials of the earth. That branch of geology which deals with minerals is Mineralogy, that which studies rocks is Petrology; both are connected closely with Chemistry. There are still other divisions of geology, but the ones mentioned are chief, and enough have been enumerated to show that geology is a very 9 10 ELEMENTS OF GEOLOGY brqad sci^hc l a&il that it is related closely to various sister sciences^ The Jimits of these many branches are more or less 4?tif&>&L/ and* of necessity they overlap. A thoroughgoing study of any one of them requires more or less knowledge of some or all of the rest. Geology is indeed one great unified subject, and its branches are really leading phases of the sub- ject, and not distinct divisions. Little or no attention is paid to them in this introductory survey of the science. In Part I the materials of the earth and their arrangement, together with the processes and agents which affect them, and the changes which these processes and agents are bringing about upon and within the earth, are discussed. This may be called Physical Geology. In Part II the history of the earth is out- lined briefly in the light of the principles developed in the earlier chapters, and the progress of plant and animal life through past ages is sketched. This is Historical Geology. Geologic processes and agents. Throughout the earth in- cessant changes are going on, often so slowly, however, that centuries are required to make their effects visible. Rocks are broken, or are bent into folds, some of which appear on the surface as mountain ridges. These highlands are attacked in turn by wind, rain, ice, and other destructive agencies; their crumbled substance is carried off by streams, winds, and glaciers, only to be deposited elsewhere. Much of the detritus comes to rest finally in the oceans. There, other pro- cesses are at work to bind the loose grains into firm rocks, which may later be elevated above the sea and even be folded into more mountains. The processes of change are most conspicuous where air, water, and rocks are in contact with one another. It is at the contact of air and sea that waves are made, and these in turn help to wear the land and to assort the sand and mud brought down by many streams. Where air and land meet, winds blow dust from one place to another, rains wash the soil, streams wear their channels, and mountain crags are riven by the expansion and contraction of the rocks and by the expansion INTRODUCTION 11 of water freezing in cracks. Beneath the surface, where the rocks are partly or wholly filled with water, changes are taking place slowly, as in a great chemical laboratory. Some parts of the rock are dissolved out, leaving a spongy, crumbling mass ; other parts are cemented tightly by min- erals left in the pores and cracks among the grains. Still deeper, where great pressure and heat are ever present, the rock is mashed, welded, squeezed into sheets, and molded like plastic clay. When such rock is resurrected through the wearing away of the cover, it is found so changed as to bear little resemblance to its original state. The many processes of change may be grouped under four general headings. They are diastrophism, vulcanism, metamor- phism, and gradation. (1) Diastrophism includes all move- ments of the earth's crust of whatever sort. Some are ex- tremely slow and continue for long periods, while others are rapid and of brief duration. Some affect vast areas, and others are local. (2) Vulcanism comprises all processes by which lava and other volcanic products are forced to the surface from below, and by which lava is moved from lower to higher levels, even though it does not reach the surface. (3) The processes by which rocks are changed, whether that change results in decay or in consolidation, are included under meta- morphism. (4) Gradation covers all processes which tend to reduce the irregularities of the solid part of the earth. An uneven surface may be made level by wearing down the high places, or by building up the low ones, and so gradational pro- cesses are divided into two classes. Those which seek to ac- complish their end by leveling down the surface are called degradational processes, in contrast to those which tend to level it up, called aggradational processes. Both phases of grada- tional work are done by the atmosphere, underground waters, streams, glaciers, and by the waves and currents of the ocean and of lakes and seas. These processes and agents are dis- cussed in subsequent Chapters. PART I PHYSICAL GEOLOGY CHAPTER I THE COMPOSITION OF THE EARTH THE GREAT DIVISIONS OF THE EARTH THE great divisions of the earth are the atmosphere or air, the hydrosphere or water portion, and the lithosphere or solid part (Fig. 1). The atmosphere. The atmosphere is a mixture of several gases. While nitrogen. predominates, the three most impor- tant things in the atmosphere, geologically, are oxygen, carbon dioxide, and water vapor. They combine chemically with many substances of the lithosphere to form new compounds, and are especially important in decomposing surface rocks (p. 103). The condensation of the water vapor leads to the precipitation of rain or snow, and makes possible the work of running water and of ice. The work of the atmosphere in conditioning the rainfall is perhaps its greatest function, geologically. So far as mere volume is concerned, however, these gases are of minor importance. The water vapor, regarded frequently, like dust, as a foreign substance in the air, rather than a constituent of it, varies greatly in amount at different times and places. The carbon dioxide makes about .03 per cent and the oxygen about 21 per cent of the air, or approximately one fifth by volume. The remaining four fifths consists chiefly of nitro- gen, an inactive gas chemically, whose importance geologi- cally is confined largely to its mechanical effects. 13 14 PHYSICAL GEOLOGY The air when in motion performs mechanical work of great importance, especially in dry regions, transporting dust and sand, often for great distances, and wearing exposed rock surfaces (pp. 86-91). Wind-formed waves bring about important changes along ocean coasts and lake shores. The FIG. 1. Diagram showing the general relations of the lithosphere, hydro- sphere, and lower atmosphere*. atmosphere also acts as a blanket, protecting the rest of the earth from the fierce heat of the sun and preventing it from cooling off rapidly by radiation of heat. Winds distribute heat and tend to equalize temperatures. Although the atmosphere is known to extend more than one hundred miles above 'sea level and probably continues very much higher, yet three fourths of the air lies below the tops of the highest mountains, and its geological activity is confined largely to its bottom portion, where it is in contact with the land and the water. THE COMPOSITION OF THE EARTH 15 The hydrosphere. The hydrosphere includes all the waters of the earth, the oceans, seas, lakes, streams, and the water underground. The oceans occupy nearly three fourths of the earth's sur- face, and contain water sufficient to cover the solid part of the earth nearly two miles deep, were the latter a perfect sphere. The oceans are all connected. If the level of the water in one is changed, all are affected. .Streams wear the rocks over and against which they flow, and move loose material to lower levels, much of it to the sea. Together with material worn by waves from the shore, or brought to the sea by other agents, the stream- borne waste of the land is spread out on the floor of the ocean as layers of sediment. The general effect of the work of the hydro- sphere is therefore to wear down the surface of the land, and to build up the bottom of the ocean. The work of the waters beneath the surface of the land is chiefly chemical. Near the surface the general result is to bring about the de- FIG. composition of the rocks ; at greater depths, the general effect is to strengthen them by depositing material in their pores and cracks. In the waters of the hydrosphere the same gases which make the air are dissolved, together with many solid substances. Common salt is dissolved in greatest abundance in the ocean, but the lime carbonate (p. 22) and silica (p. 19) in solution 2. Rock containing several kinds of fossils. (Photograph by Jessup.) 16 PHYSICAL GEOLOGY arc more important from the geological standpoint, since they are used by various forms of ocean life for the construc- tion of their shells. The shells of marine organisms have frequently been embedded in the sediments derived from the land, and their remains or the impressions they made (fossils, Fig. 2) constitute an important, though imperfect, record of the life which existed at the time and place the sediments were accumulated. The lithosphere. The lithosphere, as the name implies, is composed of rock so far as known ; it is the solid portion of the earth. As the science of geology deals very largely with rocks in one aspect or another, it is essential to study them and their arrangement in some detail. THE MATERIALS OF THE LITHOSPHERE The mantle rock. Loose, earthy material covers most of the land. When capa- ble of supporting plant life, this is called soil. The earthy matter of soil is usually mixed with partly decayed vegetable matter, and then is often dark-colored, even black. Soils are generally com- posed of sandy, clayey, or limy particles, or of combinations of these in any proportion. In ex- cavations for cellars, in railroad cuts, or in other exposures, it may often be seen that the soil gives place below to material which, though loose, is commonly coarser, more compact, and of different color. This is the sub- Fu. 3. Decaying granite and resulting rock waste. The granite is cut by a dike (p. 49). Southeastern Wyoming. THE COMPOSITION OF THE EARTH 17 soil. The soil and subsoil have been called mantle rock, since they form a covering or mantle for the underlying rock, which is usually solid. Since the loose mantle rock is formed by the decay and breaking up of solid rock, it is also called rock waste (Fig. 3). Soil which remains above the solid rock from which it was derived is residual soil, in contrast to transported soil, which has been brought from its place of origin to its present situation by some of. the agents which transport materials on the sur- ,_ . . face of the earth. Such soils when de- posited by rivers are alluvial soils, and when accumulated by the wind, eolian soils. Much of the mantle rock of Can- ada and of the north- ern part of the United States was brought to its present posi- tion by the continen- tal glaciers which once covered the re- gion. This ice-trans- ported material is called drift. The mantle rock ranges in thickness from inches to scores and, in exceptional cases, hundreds of feet. Classes of rocks. Any considerable amount of mineral matter that has been brought together by natural means constitutes rock. A rock may contain material of one kind, or of several kinds, and may be loose, like sand, or solid, like granite. Popularly, one does not speak of sand or clay as rock, but thinks only of the solid rocks as such. B. & B. GEOL. 2 FIG. 4, Igneous rock. El Capitan, Yosemite Valley. 18 PHYSICAL GEOLOGY Although solid rocks are exposed only occasionally in the interior of the United States, as in quarries, mines, along the FIG. 5. Horizontal stratified rocks and bedding planes. courses of certain streams, and in a few other situations, they outcrop (come to the surface) over large areas in eastern FIG. 6. Metamorphic rock. Contorted gneiss. (Young, Can. Geol Surv.) Ontario, Canada. THE COMPOSITION OF THE EARTH 19 Canada, among the western mountains, and elsewhere. They are found to differ among themselves in many ways. Their particles are of different kinds, sizes, and shapes; some of them are held together weakly, others firmly. Some rocks are arranged in distinct layers, while others are not. Since these and other differences are largely the result of the different ways in which the rocks were formed, they have been classi- fied in the first instance on the basis of origin. Rocks formed by the solidification of lavas are Igneous Rocks (Fig. 4). Rocks formed by the consolidation of sediments are Sedimentary Rocks. Because the latter are usually arranged in layers or strata, they are often called Stratified Rocks (Fig. 5). If the character of an igneous or a sedimentary rock is radically altered, it becomes a Metamorphic Rock (Fig. 6). The more common rocks, and the minerals of which they are composed, are discussed below. MINERALS The igneous rock shown in Figure 7 is made up of many angu- lar particles of several distinct kinds, each of which has its own constant characteristics. These particles can be separated, and, when treated in the proper manner, may be divided chemically into simpler things. Some of them, for example, may be divided into oxygen and silicon. Although chemists have been working with oxygen and silicon since their dis- covery, it has been impossible to get any still simpler things from them. They are accordingly called chemical elements. While some 70 elements are found in rocks , only 8 are impor- tant quantitatively. These are, in the order of abundance : oxygen (0), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K). The first two make up three fourths of the earth's crust ; the eight, 98.95 per cent. The oxygen unites with the other seven elements to form the following oxides: silica (SiO 2 ), alumina (A1 2 O 3 ), the iron oxides (FeO, Fe 2 O 3 , and Fe 3 O 4 ), lime (CaO), 20 PHYSICAL GEOLOGY magnesia (MgO), soda (Na 2 O), and potash (K 2 O). The union of silica with the other oxides named, forms silicates. In similar manner, very many other combinations of elements, both simple and complex, occur in nature. The few elements which exist free as constituents of rocks, together with many definite compounds of elements which naturally take the solid form, are minerals. Ice is truly a mineral in this sense, but is not popularly so considered. Thus combinations of elements give rise to minerals, and aggregations of minerals form rocks. When studied by themselves in larger pieces, minerals are found to have certain definite characteristics in addition to their nearly constant chemical composition. When formed under favorable conditions, most of them assume geometrical forms that are constant for each mineral. Common salt, for example, forms cubes. The form of any given crystal is determined by its internal structure, probably by the ar- rangement of the particles (molecules) of which it is com- posed. Even though the external form be marred or de- stroyed, this internal crystalline structure remains the same for each kind of mineral, and may be observed with the aid of a microscope in even the most irregular pieces of the sub- stance. Other convenient differences which may be used to tell one mineral from another are found in their color, manner of breaking, hardness, and luster. Some minerals break with a shelly fracture, like glass; others leave ragged surfaces, while many split more or less perfectly along certain planes, and thus leave shiny, flat surfaces. This relatively easy splitting in certain planes is called cleavage. It is one of the most distinctive features of many minerals. Of the more than 800 varieties of minerals that have been named and classified, fewer than 50 are important either geologically or commercially, while about 8 make nearly all of the common rocks, Most of the metals are derived from a few more. THE COMPOSITION OF THE EARTH 21 Quartz 1 (silica, SiOs), familiar as the chief constituent of sand, is generally light-colored and glassy in appearance. When pure it is transparent, but various impurities give it different colors and special names. Quartz sometimes forms crystals, usually six-sided prisms capped with pyramids. In most rocks, it occurs as grains without definite shape. It is a very stable compound and is the hardest of the common minerals. Quartz will scratch glass and cannot be scratched with a knife. The cleavage of quartz is very poor; indeed, for all practical purposes, it may be regarded as with- out cleavage. It has a glasslike fracture, which is often a great help in distinguishing it in igneous rocks. Igneous rocks decay when exposed to the weather, and the loose products make mantle rock. In this process the more complex minerals are broken up, their elements entering into new and simpler combinations ; but the quartz remains unaltered. This loose material may then be washed or blown away, the hard quartz particles becoming efficient agents in wearing the rock surfaces with which they come in contact. Feldspars. There are several kinds of feldspar, composed of silica and alumina, together with potassium, calcium, or sodium. The most common variety is orthoclase (KAlSisOs), or potash feld- spar, in which potassium is the distinguishing constituent. Feld- spar is not quite so hard as quartz, but too hard to be scratched readily with a knife. The color of feldspar is variable, pale yellow, pink, and especially white and red varieties being common. The general color of many igneous rocks is determined by that of their feldspars. The excellent cleavage of feldspar leaves flat, glisten- ing faces, which often afford the readiest means of distinguishing feldspar from quartz in rock. Feldspars are important constituents of most igneous rocks. Certain clays result from their rather ready decomposition. Augite is a silicate of lime, magnesia, iron, and alumina. It is dark green or black in color, and crystallizes in oblique rhombic prisms. Augite crystals are short and stubby. Hornblende is very similar to augite in chemical composition. Since the two minerals are also much alike in color and hardness, they are easily confused, and when they occur in small grains are often not distinguishable. Hornblende has two perfect cleavages, the surfaces meeting at angles of 125 and 55, while in augite the cleavage planes meet nearly at right angles. This difference helps 1 It is hardly necessary to say that the study and identification of actual specimens of minerals and rocks are absolutely essential to an understanding of them. 22 PHYSICAL GEOLOGY greatly in distinguishing the minerals when they occur in large crystals. Mica is a complex silicate, and may be identified readily by the fact that it is the only common mineral that splits or cleaves into very thin (paperlike), elastic leaves. It is soft, and ranges in color from white to green and black. A light-colored variety of mica, called Muscovite, a silicate of alumina and potash, is often used (under the name of isinglass) in stove doors and lanterns. A dark- colored variety, an iron-magnesia-silica compound, is called biotite. Hornblende, augite, and biotite are all iron-magnesia-silica com- pounds, and are known as ferromagnesian minerals. Calcite (calcium carbonate, CaCO 3 ) is the principal constituent of limestone. It is scratched easily with a knife, and effervesces when touched with acid ; by these tests it may be distinguished readily from feldspar, which it frequently resembles in general ap- pearance. Calcite cleaves readily along three planes, so arranged as to make a rhombic pattern. Gypsum (hydrous calcium sulphate, CaSO 4 + 2 H 2 O) is a white mineral, softer than calcite. It usually occurs in masses of small grains or fibers in which the crystal form is not visible, and is fre- quently stained brown or gray by the impurities it contains. Olivine (a silicate of magnesia and iron) is a hard, glassy mineral, which may often be recognized by its grass-green or bottle-green color. It rarely forms good crystals in rocks, but occurs as grains and small masses without definite shape. The fracture is uneven. Olivine is rarely found in the presence of quartz. Kaolin (a hydrous silicate of alumina) forms the basis of clay. It is very soft, and the individual particles are not visible. Pure kaolin is white and is known as " porcelain clay " because of its use in the manufacture of chinaware. Hematite (iron oxide, Fe2Os) is steel-gray and hard when in the pure crystalline form, but soft, red varieties are common, and con- stitute the most important sources of iron ore in the Lake Superior district, at Birmingham, Alabama, and in many other places. This mineral occurs in many igneous rocks and is the red coloring matter of many soils and of bricks. It gives a red streak when rubbed with a harder surface, a fact by which it may be distin- guished from other iron minerals. Magnetite (iron oxide, FesO^ is black, and acts as a magnet. Its crystals are often cubes or double pyramids. Like hematite, it is an important ore of iron. It gives a black streak. Limonite (hydrous iron oxide, 2 Fe 2 O 3 + 3 H 2 O) is often called brown hematite. It is found frequently in marshes, THE COMPOSITION OF THE EARTH 23 and is then also called bog iron ore. It gives a yellowish brown streak. Of the minerals described above, quartz, feldspar, hornblende, augite, and mica make up" the bulk of most igneous rocks. Few igneous rocks have a very large amount of any other mineral. IGNEOUS ROCKS As noted above, igneous rocks are the product of the con- solidation of lavas. The character of an igneous rock is determined by (1) the chemical composition of the parent lava, and (2) the conditions under which that lava solidified. FIG. 7. Granite, about % natural size. The light parts represent crystals of two kinds of minerals, and the dark spots represent crystals of other minerals. (Photograph by Baker.) The kinds and proportions of chemical elements in a lava deter- mine the kinds of minerals and their relative abundance in the rocks derived from it. Under different conditions of solidi- fication, lava of a given chemical composition produces rocks of very different appearance. The mineral grains, for ex- 24 PHYSICAL GEOLOGY ample, may be large and easily distinguished (Fig. 7), or minute and unrecognizable. They may be of the same size, or of very unequal sizes (Fig. 8). These are matters of tex- ture, rock texture having to do with the size, shape, and ar- rangement of the particles of a rock. FIG. 8. Porphyritic texture. About % natural size. (Photograph by Baker.) Chemical classes of igneous rocks. Those which con- tain a large proportion of silica (more than 65 per cent) are called acidic rocks, because silica is an acid-forming oxide (uniting with water to form silicic acid). Of the other lead- ing oxides (p. 19) none commonly form acids. Similarly, those igneous rocks which contain much less silica (less than 55 per cent) and a larger proportion of the bases (lime, soda, magnesia, potash, etc.) are called basic rocks. Most acidic rocks are light-colored if crystalline, while basic rocks are commonly dark-colored. An intermediate or neutral group is sometimes recognized, including rocks that contain 55 to 65 per cent of silica, THE COMPOSITION OF THE EARTH 25 Factors influencing the physical character of igneous rocks. The circumstances under which lavas solidify vary greatly, and many factors influence the texture of igneous rocks. Among the leading ones are (1) the rate at which the lava cools, (2) the fluidity of the lava, and (3) the pressure under which it consolidates. The texture is influenced also by (4) the chemical composition of the lava. (1) Lava is liquid rock, a solution in which certain minerals are dissolved in others. The high temperature of the lava appears to make it possible for the minerals to form a mutual solution. As lava cools, the point of saturation of some mineral present is reached, and it be- gins to take the solid form. The molecules of this mineral tend to collect and arrange themselves in reg- ular order, build- ing up crystals having a definite geometrical form. As cooling proceeds, the point of satu- ration of other minerals is reached, and crystals of other kinds begin to form. With continued cooling, the entire mass may become crystalline. Lava is probably never so fluid as water, and is commonly rather stiff (viscous). It clearly re- quires some time for molecules scattered throughout such a liquid to come together and form crystals. Slow, regular cool- ing therefore favors the development of large crystals. The resulting coarse-grained rocks are sometimes called granitoids (granitelike rocks). If cooling is rapid, the mass is likely to become solid before crystals have formed, or while they are still very minute. In the former case the rocks may FIG. 9. Obsidian, or natural glass. Shows the glassy luster and fracture. About % natural size. (Photograph by Baker.) 26 PHYSICAL GEOLOGY have the structure and luster of glass, and so are called glassy rocks (Fig. 9). When composed of very small crystals, the rock may have a dense, stony appearance, rather than a glassy luster. If, after certain minerals have crystallized out, the still liquid mass in which they float be cooled suddenly, a rock may result which is partly glassy or stony and partly crystalline (Fig. 8). The rate of cooling, then, is a chief factor in the crystalliza- tion of igneous rocks. The rate is influenced by several con- ditions, of which the following are chief, (a) Large bodies of lava cool less rapidly than small ones, (b) Masses deep within the earth's crust cool more slowly than those at or near the surface, (c) The rate at which a body of lava cools is affected also by its shape. Thus a globular mass containing the same quantity as a thin sheet would cool far less rapidly. (What combination of these conditions would most favor the formation of coarse-grained rocks? Fine-grained?) (2) The more fluid the lava, the easier and the farther may the molecules move, and the larger, other things being equal, may crystals grow. The mobility of the lava is determined largely by its temperature and composition, but partly by the amount of water vapor it contains, and to less extent by the presence of other volatile substances, such as carbon dioxide and fluorine. In addition to hindering the lava from becom- ing stiffly viscous, (a) the waters, etc., lower the temperature at which solidification occurs and so prolong the period of crystal growth, and (6) some common minerals do not form save in their presence. These substances, especially water, influence the process of crystallization so greatly that they are appropriately called mineralizers. That many lavas contain large quantities of water vapor is familiarly illustrated by the heavy rains which frequently attend volcanic eruptions, due to the condensation of escaping steam. (3) The direct effect of pressure upon texture is probably not great. Most rocks contract on becoming solid, and, were other things equal, lava would therefore solidify more THE COMPOSITION OF THE EARTH 27 quickly under pressure than otherwise. Accordingly, pressure tends to oppose slow crystallization and the development of coarse textures. Indirectly, pressure affects texture greatly through its in- fluence upon the gases and vapors included in the lava. As lava rises in the pipe or chimney which leads downward from the crater of a volcano, the weight of the overlying column of lava becomes less. This relief of pressure is likely to per- mit the explosive expansion of the steam, by which the lava is sometimes blown into fine bits and hurled high into the air. Very fine material was thrown to an estimated elevation of some 17 miles during the great eruption of Krakatoa (near Java) in 1883. Lava blocks a number of feet in diameter and weighing tons are also sometimes ejected, together with much material of intermediate size. In other cases the lava, possibly because not so heavily charged with steam, flows quietly from the volcanic vent. As the lava at and near the surface of a flow solidifies, the gases expand readily without violent explosion, and the many steam bubbles frequently give the rock an open, spongy tex- ture (Fig. 11). Since it cooled promptly, such material is often glassy. If the lava becomes solid under great pressure, as at the bottom of a thick flow or when intruded into the rocks deep below the surface, the included gases cannot expand freely, and the resulting rock has a compact texture. Cooling very slowly, such rock is apt also to be coarsely crystalline. The gases may be confined by even a thin covering which is relatively impervious to them, a solid texture resulting. (4) It has already been pointed out that the chemical com- position of lava determines the kinds and proportions of min- erals in the rock formed from it. It may now be noted that this influences the texture of the rock, for different minerals form crystals of different shapes, so far as their interference with one another while growing will permit. Furthermore, lavas poor in silica, and particularly those rich in iron and magnesia, retain their fluidity to much lower temperatures 28 PHYSICAL GEOLOGY than do those containing much silica. Hence the former lavas may produce coarse-grained rocks under conditions where the latter would give fine-grained ones. We are now prepared to describe a few of the more impor- tant igneous rocks. The different kinds grade into each other without hard and fast lines. 1 Distinctly grained rocks. These rocks have a solid tex- ture, are wholly crystalline, and the grains can be distinguished with the unaided eye. The grains may be of uniform size (large, medium, or small), or large crystals may be scattered through a ground mass of smaller ones. In the latter case, the rock is called porphyry, and is said to have a porphyritic texture. These terms are applied also to rocks in which distinct crys- tals are scattered through a glassy or stony ground mass (Fig. 8). One way in which porphyritic texture develops has been explained (p. 26). The distinctly grained rocks, whether porphyritic or nonporphyritic, may be further classi- fied on the basis of the minerals they contain. While there are a great many kinds, only a very few of the more important ones can be described here. All the different varieties shade gradually into one another. Granite (Fig. 7) is perhaps the most common of the dis- tinctly grained rocks. It always contains feldspar (as the predominant mineral) and quartz, and frequently has subordi- nate amounts of other minerals, especially mica. Descriptive names are often employed, which indicate the leading sec- ondary minerals ; thus one may speak of a mica-hornblende granite. Granites have different colors, depending largely on that of the feldspar, and on the abundance of dark minerals. Gray and red varieties are especially common. Granite is an acidic rock. Large crystals of feldspar (less often quartz) may be scattered through a granitic ground mass of smaller (but distinguishable) grains, giving a granite-porphyry. 1 For this reason there is no general agreement concerning the classifica- tion of igneous rocks. The classification used here differs from that employed in many other books. THE COMPOSITION OF THE EARTH 29 Syenite (B, Fig. 10) is a rock composed chiefly of feldspar, with smaller amounts of the ferromagnesian minerals, partic- ularly hornblende, and little or no quartz. It is usually gray or reddish, and often closely resembles granite both in color and texture. Syenite is a neutral rock. Like granite, it may have a porphyritic texture. Granite and syenite are called fe Idspathic rocks (Fig. 10), because feldspar predominates in both. Syenite is a much less common rock than granite. Diorite is made up chiefly of hornblende and subordinately of feldspar (C, Fig. 10) . Gabbro consists mainly of augite, with a subordinate amount of feldspar. Diorite and gabbro are gen- erally dark-colored. They are sometimes not distinguishable, because it is not apparent to the naked eye whether the dom- inant mineral is hornblende or augite. The rock may then be called dolerite (meaning deceptive). Some diorites are neutral rocks, while others are basic. Gabbro is basic. Diorite and gabbro are widely distributed and common rocks. Peridotite (D, Fig. 10) is a basic rock composed entirely of the dark-colored minerals olivine, hornblende, or augite. These may occur alone or in mixtures. Both feldspar and quartz are absent. The rock is black or dark green, and is much less common than the preceding ones. Dense rocks. Most or all of the grains in the rocks of this class are too minute to be distinguished by the naked eye. When nonporphyritic, many of these rocks have a rather uni- form, stony appearance. Such a rock, when dark-colored, may be called basalt; when light-colored, felsite. (The stu- dent must guard against confusing felsite with certain fine- grained sandstones.) Similar names are used when the tex- ture is porphyritic ; if the ground mass is light-colored, the rock is felsite-porphyry, if dark-colored, basalt-porphyry. Fur- ther subdivisions of the porphyries may be made in terms of the minerals which form the visible crystals. Thus there is quartz-felsite-porphyry, feldspar-basalt-porphyry, etc. The light rocks of this class are chiefly feldspathic, while the dark ones are mainly ferromagnesian. 30 PHYSICAL GEOLOGY A. Anorthosite, all feldspar. B. Syenite, mostly feldspar. C. Diorite, some feldspar. D. Peridotite, no feldspar. FIG. 10. Contrast of f eldspathic and f erromagnesian rocks. (Pirsson, Rocks and Rock Minerals.) THE COMPOSITION OF THE EARTH 31 Glassy rocks. This class includes rocks composed wholly or in large part of glass. It has already been seen that rock formed at the surface of a lava flow is apt to be more or less filled with cavities formed by gas bubbles. Such rocks are sometimes said to have a vesicu- lar texture (vesicles = cavities). Pumice (Fig. 11) is a rock in - miim FIG. 11. Pumice, about \ natural size. (Photograph by Baker.) which such cavities take up much of the space, and are divided by very thin partitions of glassy material. Bits of pumice are found distributed widely over the ocean floor, for they are often floated long distances before their small pores become filled with water, thus causing them to sink. As the walls of the cavities become thicker and the material stony, pumice grades into scoria (Fig. 12). The cavities of scoriaceous lavas are sometimes partly or wholly filled at a later time by depo- sition of minerals from solution in ground water (Fig. 13). This, for example, is one mode of occurrence of copper in some of the mines of northern Michigan. Obsidian or volcanic glass (Fig. 9) is a solid, glassy rock, 32 PHYSICAL GEOLOGY generally black in color. Its glassy condition signifies rapid cooling, while its compact texture means that gas bubbles FIG. 12. Scoriaceous texture. About % natural size. (Photograph by Baker.) were not forming as it solidified. Obsidian usually has a com- position much like that of granite. Pitchstone is a glassy rock FIG. 13. Amygdules in lava. About natural size. The material of the amygdules was deposited from solution in ground water in the cavities of a scoriaceous lava. (Photograph by Baker.) THE COMPOSITION OF THE EARTH 33 which has a resinous surface, and is thought to resemble pitch in appearance. It is variable in color, red, brown, and green varieties being common. Either obsidian or pitchstone may contain scattered crystals which can be recognized, giv- ing rise to obsidian-porphyry and pitchstone-porphyry : Fragmental volcanic rocks. Volcanoes of the explosive type throw out material which falls in solid fragments. These are classified on the basis of size, shape, and texture. Volcanic ash is composed of very small, glassy fragments. It sometimes forms thick deposits about volcanoes. Still finer material constitutes volcanic dust. This is scattered widely by the winds, some slight amount probably having been carried from certain volcanoes to all parts of the world. Dust and fine ashes from Iceland vol- canoes settled in 1783 on cer- tain farm lands in northern Scotland in such quantity as to destroy crops. Such ma- terial from Krakatoa was carried several times around the earth in 1883. If the material is about the size of hickory nuts or medium coarse gravel, it is called lapilli. Cinders are made up of angu- lar pieces of open texture, and, together with lapilli and similar fragments, form many steep-sided volcanic cones (Fig. 20) . Masses of lava which have become more or less rounded because of rapid rotation in the air are bombs (Fig. 14) . They vary from the size of one's fist or less, to a diameter of several feet. Volcanic breccia is a general term applied to the beds of coarser material (bombs, lapilli, coarse ashes, etc.), which ac- cumulate around the vent. The dust and lighter ashes settle farther away to form beds of tuff. FIG. 14. Volcanic bombs, Cinder Buttes, Idaho. (Russell, U.S. GeoL Surv.) B. & B. GEOL. 34 PHYSICAL GEOLOGY Summary. The more important points concerning ig- neous rocks may be summarized as follows : (1) Igneous rocks are formed by the solidification of lavas. (2) Although they contain many minerals, a few minerals make up the great mass of the igneous rocks. The most important are (a) quartz, (6) feldspar, (c) the ferromagnesian minerals, and (d) the iron oxides. (3) Chemically, igneous rocks may be divided into three great classes acidic, neutral, and basic. (4) The physical character of igneous rocks is determined by (a) the character of the parent lava, and (6) the conditions under which it solidified. (5) Since both the composition of lavas and the circumstances attending their solidification vary widely, many kinds of igneous rocks result. Of these the few that have been mentioned are most important. They are classified in the accompanying table. CLASSIFICATION OF IGNEOUS ROCKS 1 A. GRAINED, CONSTITUENT GRAINS RECOGNIZABLE. MOSTLY INTRUSIVE a. Feldspathic rocks, usually light in color 6. Ferromagnesian rocks, gener- ally dark to black With quartz With little or no quartz With subordi- nate feldspar Without feld- spar Nonporphyritic GRANITE SYENITE DIORITE GABBRO DOLERITE PERIDOTITE . Porphyritic GRANITE-POR- PHYRY SYENITE-POR- PHYRY DlORITE-PoR- PHYRY B. DENSE, CONSTITUENTS PARTLY OR WHOLLY UNRECOGNIZABLE. INTRUSIVE AND EXTRUSIVE a. Light-colored, usually feld- spathic 6. Dark-colored to black, usually ferromagnesian Nonporphyritic FELSITE BASALT Porphyritic FELSITE-PORPHYRY BASALT-PORPHYRY C. ROCKS COMPOSED WHOLLY OR IN PART OF GLASS. EXTRUSIVE Nonporphyritic OBSIDIAN, PITCHSTONE, PUMICE, ETC. Porphyritic OBSIDIAN-PORPHYRY, PITCHSTONE-PORPHYRY D. FRAGMENTAL IGNEOUS MATERIAL. EXTRUSIVE TUFF, VOLCANIC BRECCIA 1 After Pirsson, with slight modification. THE COMPOSITION OF THE EARTH 35 The oldest known rocks are igneous rocks, or metamorphic rocks which have been produced from them. Since all other rocks have been formed directly or indirectly from igneous rocks, the latter have been called the mother rocks. Igneous rocks, or their altered products, are thought to un- derlie all other kinds of rocks, and to make up a very large proportion of the earth's mass. SEDIMENTAR^ ROCKS The formation of sediments. It has been seen (p. 16) that the greater part of the land surface is covered with loose rock material formed from the solid rock. This rock waste varies greatly in size, ranging from fine clay, through sand and gravel, to large pieces of rock. It is a matter of common ob- servation that the finer material is shifted frequently from place to place. Winds blow dust in quantity from roadways and the bare surfaces of fields. Rain sometimes washes large amounts of earth down the sides of freshly plowed hills. Streams are commonly made muddy in rainy weather by the fine silt which they carry, and they drag and roll coarser ma- terial, such as sand and gravel, along their channels. Since water always flows down slope, the material it carries is also moving to lower levels. And because all the water which does not sink into the ground, evaporate, or stop in some lake runs to the sea, it follows that much of the rock waste it moves is carried into the ocean. If water from any stream is evaporated, a mineral residue remains. This means that rivers are carrying mineral matter to the sea in solution as well as in solid pieces. The Thames River of England carries over a ton of dissolved matter to the sea each minute on the average. It has, indeed, been declared that the one great mission of running water is to get the land into the sea. The dissolved material is likely to remain in solution in the sea water for a longer or shorter period, some of it indefinitely. Nearly all of the sediment which is carried to the sea in the solid form soon settles to the bottom, 36 PHYSICAL GEOLOGY the larger and heavier pieces first, the smaller and lighter later. Offshore waters are frequently agitated down to the bottom by winds and tides, the undertow (a from-shore movement of the water which has come in with the -waves), and by various currents. The sediment on the bottom is rolled and dragged about by these movements of the water, and is often shifted long distances before reaching a final resting place. Normally the bottom water moves most, close to shore where it is shal- low, and frequently only coarse material, such as gravel, comes to rest there, all sand and mud being swept away. Farther out the quieter bottom water is able to move only mud par- ticles, and drops any sand it may have had. Still farther from shore ^he bottom waters become so quiet with increasing depth that even the finest mud comes to rest upon the floor. Thus FIG. 15. Diagram showing the relations to one another and to the land, of beds of gravel, sand, and mud. the stream-borne waste from the land tends to accumulate in belts of gravel, sand, and mud, which merge gradually into one another (Fig. 15). Since the depth of the water and the strength of the waves and currents vary at points equally dis- tant from the shore, different material is likely to be accumu- lating at these different places; traced alongshore, gravel may give way to sand and sand to mud. Furthermore, the agitation and depth of the water vary from time to time at a given place, because of alternating storms and calms, high and low tides, etc. Hence the character of sediments is subject to changes vertically as well as horizontally, and alternate layers THE COMPOSITION OF THE EARTH 37 of gravel, sand, and mud are formed in the same place. This division into layers, or stratification, as it is called, is the most universal and important characteristic of water-laid beds, though not confined to them. Countless numbers of minute organisms live in the clear and relatively quiet waters beyond the reach of abundant land- derived sediment. Many of these organisms take calcium carbonate or silica from solution in sea water, and build it into their shells and other hard parts. When they die, these shells, etc., sink, and over millions of square miles of the ocean floor form a deposit, called ooze (Fig. 277, p. 259). Near the shores, these organic deposits are usually less important than the gravels, sands, and muds brought down from the land. The consolidation of sediments. Material in solution in sea water is sometimes deposited among the particles of the sediment, binding them together to form firm, solid rocks. Furthermore, the bottom sediment is under the weight of the overlying material deposited later, and this may become effec- tive in pressing the particles closer together, though it prob- ably does not aid greatly in making the mass coherent. Sea-laid sediments may be exposed by an elevation of the ocean bottom, or by a lowering of the sea surface, and ground waters containing minerals in solution may subsequently deposit material in their pores, further cementing the rocks. Rock cementation is often a very slow process, and coastal plains that have emerged from the sea recently (as geology measures time) are apt to be underlain by beds of loose material rather than of solid rock. This is generally true in the Atlantic Coastal Plain of the United States (Figs. 43 and 44, pp. 62, 63). Certain sediments (particularly lime carbonate oozes) are consolidated not only by cementation, but also, and in some cases chiefly, by the formation of minute interlocking crystals. Chief kinds of sedimentary rocks. Sedimentary rocks are formed from loose sediments by (1) cementation, (2) crystal- lization (in some cases), and (3) pressure (to slight extent), 38 PHYSICAL GEOLOGY as indicated above. Cemented gravel is conglomerate (Fig. 16), while if the pieces are angular instead of roundish, the FIG. 16. Conglomerate. About % natural size. (Photograph by Baker.) rock is known as breccia (Fig. 17). Cemented sand is sand- stone. The common sandstone cements are lime carbonate, the iron oxides, and silica. The nature of the cement influences the color and strength of the sandstone. Quartzite is a dense and very hard rock, pro- duced when the pores of a sandstone are completely filled with quartz. The sand grains of the sandstone are worn fragments of quartz crystals, and the quartz molecules FIG. 17. -Quartzitic Breccia. (Neal.) deposited about them arrange themselves in accordance with the internal structure of quartz crystals (p. 21), and seek to develop again the THE COMPOSITION OF THE EARTH 39 six-sided prisms. Cemented and compacted clay forms shale. Conglomerate, breccia, sandstone, and shale, since they are made up of fragments of older rocks, are often called fragmen- tal rocks. The remains of organisms that take calcium car- bonate from solution in the waters to form their shells become limestone when cemented or crystallized. Some limestones have been formed by chemical precipitation of calcium car- bonate. Chalk is a very soft limestone of fine texture. Dolo- mite (magnesian limestone) is developed when some considerable proportion of the calcium of a limestone is replaced by mag- nesium. This replacement may take place long after the formation of the limestone, or while the material of the lime- stone is accumulating. Flint is a very compact, dark gray, siliceous rock. Chert is an impure flint, usually of light color. These rocks do not in most cases form extended independent beds, but occur chiefly in limestones in the form of irregular masses and thin layers. Both contain fossils of the siliceous parts of various sea animals, particularly sponges and protozoans (p. 294). The silica was taken from sea water by such animals, and at their death formed deposits, often scattered through other sediments. Subsequently, some of it was dissolved by ground water, and redeposited in certain places where conditions favored. (See Concretions, p. 121.) While this seems quite certainly to be the origin of some flints and cherts, that of others is uncertain. The larger part of the land surface is covered with sedimen- tary rocks. Most of these rocks are in layers and contain marine fossils. For these and other reasons, it is concluded that such rocks are consolidated sediments that were deposited beneath the sea in the same manner that offshore sediments are now forming. This conclusion carries with it the inference that at some time in the past the ocean waters have covered large areas which are now land. Since the beds of sediment now forming are nearly horizontal, we conclude further that all sedimentary beds originally had that position, and that 40 PHYSICAL GEOLOGY great departure from horizontality indicates later disturb- ance. Nonmarine fragmental rocks. While the ultimate goal of running water and of the waste it carries is the sea, much material is deposited in lakes, along valley bottoms, and in other situations on the land. These sediments, like marine beds, may become firm rock by cementation. Beds formed in lakes that have since been destroyed usually betray their origin by their form and attitude, and by the fossils which they contain (p. 267). River deposits also have distinguishing characteristics, some of which are suggested by Figure 185. Long, relatively narrow strips of coarse material indicate former positions of the shifting stream channel, while the broader layers of fine material were spread upon the flood plain by the quieter waters of the overflow. Cross-bedding (p. 54) and great irregularity of stratification are among the most characteristic features of stream deposits. Occasionally, they contain river and land shells. River deposits will be considered in greater detail in Chapter V. Other sedimentary rocks. Certain special classes of sed- imentary rocks, some of them very important, may best receive attention in later connections. These include gypsum and rock salt, precipitated from solution under special condi- tions (p. 268), the iron ores (p. 323), and a few rocks formed by organisms, or themselves organic, like coal (p. 379), to- gether with deposits made by winds (p. 98) and by glaciers (pp. 204, 212). Summary. The more important points concerning the origin of sedimentary rocks are the following : (1) Loose sur- face material is being formed constantly by the decay and breaking up of solid rock. (2) Various agents which transport material on the land, particularly rivers, are shifting this rock waste to new situations, especially to the sea. (3) In the pro- cess of transportation and deposition it is more or less per- fectly sorted, and beds of gravel, sand, and mud result, THE COMPOSITION OF THE EARTH 41 (4) These sediments are cemented to form conglomerate, sandstone, and shale, the principal classes of fragmental rocks. (5) Limestones are formed from organic remains, and sometimes by precipitation from solution. (6) These sedimentary rocks may themselves be exposed and may decay, and the resulting waste may be carried to the sea, or other lodgment areas, to form new sediments and rocks, which may in turn experience a similar fate. In this manner many gen- erations of sedimentary rocks have been formed, and later more or less wholly destroyed. Since all sedimentary rocks are formed from still older rocks, they are sometimes called secondary rocks. METAMORPHIC ROCKS Metamorphic means changed, and metamorphic rocks are those which, originally igneous or sedimentary, have been altered in composition, or in texture, or in both, since they were made. Metamorphism may result in the weakening and decay of rocks, or it may strengthen and consolidate them. When rocks formed at or near the surface are buried deeply beneath later beds, they encounter conditions very different from those under which they were made. They are under the great pressure of the rocks above, and may also be subjected to lateral compression. They are affected by higher temper- atures, and are acted upon by ground waters that are made powerful chemically by heat and pressure. In consequence of these things, their composition may be altered, their minerals changing into other minerals whose chemical composition is more stable under the new conditions. They may become more thoroughly consolidated, harder, and more crystalline. They may develop also a sheeted or banded structure (Fig. 18), which is distinct from the stratification of sedimentary rocks. Similarly, when igneous rocks that were formed by the solidi- fication of lavas at great depths are exposed at the surface through erosion, they find entirely new conditions. They are subjected to the influences of temperature changes, of the 42 PHYSICAL GEOLOGY gases of the atmosphere, of wind and water, of plants and animals, and of other agents. They commence at once to break up and decay, their constituents forming new combina- tions suited to the new conditions. As in the cases suggested, metamorphic changes in general are in the nature of adapta- tions to a new environment. Although metamorphism, strictly speaking, includes all changes in all rocks, and may be destructive in its effects, as well as constructive, yet in common usage it implies radical changes of the latter type, in consolidated rocks. Such changes take place within the earth, especially at great depths. The processes of metamorphism are treated in later pages (78-83). In working out its physical history, it is frequently impor- tant to determine whether the metamorphic rocks of a given region were derived from igneous or from sedimentary rocks. The answer is sometimes given by bodies of unaltered or little changed rocks within the metamorphic rocks. In some cases, more or less distorted pebbles of various kinds indicate that the parent rock was a conglomerate whose finer material has been changed greatly, while the larger pebbles were merely flattened and lengthened. Or again, it may be possible to trace the gradation from the metamorphic rocks, through less and less changed rocks, into the unaltered rocks of a neighboring area. The origin of many metamorphic rocks may be detected, too, by microscopic examination or by chemical analysis. Gneiss (pronounced "nice") is a crystalline rock, in many cases containing the same minerals and having the same general appearance as granite, except that it is distinctly banded (Fig. 18), due to the partial arrangement of unlike minerals in separate layers. The bands may be bent and twisted in a way that suggests intense crumpling (Fig. 6). Granitic rocks usually become gneisses when metamorphosed. Gneiss may be made, however, from various other kinds of rock. Schist is in general more closely and regularly banded than THE COMPOSITION OP THE EARTH 43 gneiss, and exhibits a strong tendency to split into uneven leaves or plates. These plates are often spangled with glisten- FIG. 18. Banded gneiss. (Pirsson, Rocks and Rock Minerals.) ing flakes of mica, or with needles of hornblende. Indeed, the splitting habit characteristic of schists is due largely to the presence of cleavable minerals in parallel arrangement. Many varieties of schist are recognized. Mica schist is most common, and consists chiefly of quartz and mica, usually with a sub- ordinate amount of feldspar. It is often formed from slates and feldspathic sandstones. In hornblende schist, leaves of imperfect hornblende crystals are separated by other minerals, in many cases by feldspar, quartz, and mica. Basic igneous rocks, when metamorphosed, often become hornblende schists. The latter may be formed also from sedimentary rocks. Slate is formed from shale by compression. It is a hard, very fine-grained rock, not obviously crystalline, usually dark- colored, and characterized by a remarkable cleavage, often so perfect that the rock is quarried extensively in parts of New England and in other regions for roofing purposes (Fig. 19). The formation of slate involves far less change than the develop- ment of gneiss or schist. 44 PHYSICAL GEOLOGY Marble is metamorphic limestone. It represents an ad- vanced stage in the crystallization of calcareous sediments (p. 37). In some marbles the fine grains of calcite have re- formed as interlocking crys- tals larger than those in most granites, while in other cases the texture is so fine that in- dividual grains cannot be distinguished. Marble is white if formed from pure limestone, but because of im- purities may be of any color. Carbon and other impurities often form streaks or bands of varying color, producing beau- tiful and odd effects on pol- ished surfaces. Marble is much used as an ornamental building stone. There are extensive quarries at various points in the East, particu- larly in Vermont. Unlike most metamorphic rocks, pure mar- ble is without cleavage. It may be scratched easily with a knife, and thus distinguished readily from sandstone and quartzite, which it may resemble in appearance. FIG. 19. Fossiliferous slate near Townsend, Mont. (Walcott, U.S. Geol. Surv.) A. META-SEDIMENTARY SERIES a. SEDIMENTS b. SEDIMENTARY ROCKS c. METAMORPHIC ROCKS Gravel Conglomerate Gneiss, and schists of vari- ous kinds Sand Sandstone and quartzite Various schists (especially quartz schist) from quartz- ite. Mica schist from cer- tain sandstones Clay Shale Slate, and various schists (especially mica schist) Calcareous deposits (shells, etc.) Limestone Marble THE COMPOSITION OF THE EARTH 45 B. META-IGNEOUS SERIES a. IGNEOUS ROCKS b. METAMORPHIC ROCKS Granite, syenite, and other grained felds- pathic rocks Gneiss Felsite and acidic tuffs Various schists Diorite, gabbro, basalt, and other ferro- magnesian rocks Various schists (especially hornblende schist) Only the leading varieties of metamorphic rocks have been described. There are many other kinds which cannot be considered here. The general relations of those discussed to the rocks from which they are commonly derived are shown in the preceding table. The relation of rocks to one another. At the very outset the student is likely to encounter rocks that cannot be iden- tified readily with any of the kinds enumerated in the pre- ceding pages. Thus, a rock may be found which contains both sand and calcite in perhaps nearly equal proportions, and which therefore combines the features of a sandstone and a limestone. Varieties of rocks are, in fact, not definite species, as are most kinds of animals and plants. Rather, they grade into each other by imperceptible stages. By a gradual de- crease in quartz, granite verges toward syenite. By an in- crease in hornblende and a decrease in feldspar, syenite passes into diorite. By a decrease in the size of its pebbles, con- glomerate approaches sandstone. Similar transitions occur between all related varieties of rocks. Furthermore, rocks change after they have been made, and this produces further gradations from one kind to another. Thus, as noted above, granite may be slowly altered into gneiss, and shale into slate, and slate, in turn, to schist. Shale and schist are distinct in appearance and constitution, yet all possible gradations may be found between them. It is evident, then, that rock names must be used loosely, and that there are few sharp dividing lines anywhere in the classifica- tion, 46 PHYSICAL GEOLOGY ORIGINAL STRUCTURES OF ROCKS By rock structure is meant the mode of occurrence of rocks, the shapes of rock bodies, and the position or attitude of those bodies. Thus, to say that certain rocks are stratified and that the beds are horizontal, or tilted, or folded, is to state phases of their structure. The principal original structures of rocks are discussed below, while some structures developed by the changes which take place in the outer part of the earth are described in the next Chapter. SURFACE STRUCTURES OF IGNEOUS ROCKS Volcanic cones. The greater part of the material extruded by volcanoes accumulates near the vents, forming conical ele- vations. These cones vary in size and shape. They range in height from a comparatively few feet up to high mountains like Mauna Loa in Hawaii, whose summit is some 14,000 feet above the neighboring sea and about 30,000 feet above the sea floor. The slope of a cone is determined by its com- V 2 a.ie7, ders stand in steep piles (Figs. 20 and 21), while the more liquid lavas spread freely and build cones whose sides in exceptional cases form angles of only two or three degrees with a horizontal plane (Fig. 22). Stiffer lavas form cones of intermediate steepness. Lava cones consist of many solidified streams of lava which flowed from the vent in different directions at different times, producing a sort of radial structure. Most cones, like Vesuvius, are built of both lava and fragmental material, and for various reasons they are often irregular in form and structure. A majority of the existing volcanic mountains are near the THE COMPOSITION OF THE EARTH 47 edges of the continents and in the sea, though some are far in- land. With some notable exceptions, the active and recently FIG. 21. Lava flow (in right foreground) and cinder cones near Flagstaff, Ariz. (R. T. Chamberlin.) extinct volcanoes are arranged in lines or belts where the earth's crust has recently undergone severe movement. Lava plateaus. When lavas issue from long cracks (fis- sures) in the surface, or from numerous more restricted vents, they may spread widely over the surrounding country before solidifying. The distance to which the lava flows depends upon how fluid it is, upon the amount, and upon the character ffavna Loa FIG. 22. Profile of the cone of Mauna Loa, Hawaii. Vertical scale same as horizontal. (U.S. Geol. Surv.) of the surface over which it moves. (What combination of these conditions would enable the lava to flow farthest ?) The lava congeals first at the top, forming a crust whose surface is comparatively smooth (Fig. 23), unless it is repeatedly broken by the continued movement of the still liquid mass beneath. In the latter case the surface is extremely jagged 48 PHYSICAL GEOLOGY and irregular (Fig. 24). Successive fissure eruptions may build up great plateaus. Such a lava plateau, with an area FIG. 23. Surface of a comparatively smooth lava flow. Jordan Craters, Oregon. (Russell, U.S. GeoL Surv.) How may the fissures be explained ? of some 200,000 square miles, occurs in Washington, Oregon, and Idaho (Fig. 25). The lava is locally 4000 feet in thick- FIG. 24. Margin of a lava flow, Cinder Buttes, Idaho. The broken condi- tion of the lava is due to movement after the outside had hardened. Note the steep edge of the flow. (Russell, U.S. GeoL Surv.) ness along the Snake River, which flows through the plateau in a deep canon. The canon walls show that the elevations THE COMPOSITION OF THE EARTH 49 buried by the lava floods were in some cases mountains of considerable elevation. Lava flows built an even larger pla- teau in the peninsula of India. FIG. 25. Lava flows of the Northwest. UNDERGROUND STRUCTURES OF IGNEOUS ROCKS Most of the lava forced upwards from great depths fails to reach the surface and solidifies underground. Igneous rocks formed from lavas deep below the surface are called plutonic rocks. Such rocks may be exposed at the surface through the wearing away of the rocks which overlay them. Indeed, much of the igneous rock at the surface is intrusive rock. Intrusions of lava vary in shape and in their relations to the inclosing rock. These differences have given rise to special names. Lava hardens in cracks and fissures in the rock to form dikes (Fig. 26). Dikes vary in thickness from B. & B. GEOL. 4 FIG. 26. Diagram of dikes and sills, d, dike ; s, sill. 50 PHYSICAL GEOLOGY a few inches to two or three hundred feet, and in exceptional cases have a length of scores of miles. In many cases dike rock is more resistant than the adjacent country rock, and hence many dikes form low, narrow ridges (Fig. 27). If softer than the rock it penetrates, the line of outcrop of the dike rock becomes a depression. Lava intruded be- tween rock layers in wedge-shaped sheets forms sills (Fig. 26). Sheets of lava ex- truded upon the surface may later be covered by other rocks. Such sheets then have the po- sition of sills, though of different origin. In working out the geological history of a region, it is sometimes important to determine whether a given lava sheet which lies between sed- imentary beds is intrusive or extrusive. If the bottom of the bed resting on the lava sheet has been baked by the hot lava, which origin may be inferred? If the top of the lava sheet is glassy and has a vesicular texture? If tongues of igneous rock extend from the lava sheet into the overlying rock? By these and other observations, the problem may usually be solved. Sills merge into dome-shaped intrusions. If these merely arched the overlying beds, they are called laccoliths (Figs. 28 and 29). The Henry Mountains of Utah are notable ex- FIG. 27. Porphyry dike cutting tuff. South- western Colorado. (Howe, U.S. Geol. Surv.) THE COMPOSITION OF THE EARTH 51 amples. If the sedimentary beds were broken and the broken edges displaced (faulted), the intrusion is a bysmalith. The Spanish Peaks of southeastern Colorado are an illustration. Deep-seated intrusions of very great size (often many miles across) are known as batholiths. Usually batholiths are of ir- regular form. . Unlike laccoliths, they do not simply bulge the FIG. 28. Diagram of a laccolith cover, but occupy vast spaces with associated dikes and sills - which have been actually hollowed out of the preexisting rocks. Whether this was accomplished by melting and as- FIG. 29. Two Buttes, Prowers County, Colo. Sandstone beds uplifted by a laccolithic intrusion. The slopes have been modified by erosion. (Dar- ton, U.S. Geol. Surv.) similating the previous rocks, or otherwise, is not known definitely. Such intrusions are of rather common occurrence in eastern Canada, in asso- ciation with very ancient rocks. Erosion has removed their original covering, ex- posing the igneous cores. Granite batholiths form the FIG. 30. Diagram of a stock. > f ,-, central cores ot many 01 the great mountain ranges. Certain bodies of intrusive rock, ex- posed by erosion and rudely circular or elliptical in ground plan, are called stocks (Fig. 30). They vary in diameter at the 52 PHYSICAL GEOLOGY FIG. 31. Diagram of a young volcanic mountain. surface from a few hundred yards to a number of miles, and in many cases increase in size downwards, their sides cutting ir- regularly across the surrounding rocks. In New England, eastern Canada, and other regions, many gran- ite stocks form hills be- cause of the more rapid erosion of the less-re- sistant inclosing rocks. Stocks differ from batho- liths chiefly in being very much smaller, and from laccoliths and bysmaliths particularly in their rela- tions to the surrounding rocks. Igneous rock oc- curring as laccoliths, bys- maliths, batholiths, and stocks is not n beds, has no cleavage, and its crys- tals are without system- atic arrangement. Accordingly, it is said to have a massive structure. Volcanoes are, geologically speaking, short lived. When the volcanic forces die away or find relief through other vents, no further additions are made to the cone of a volcano (Fig. 31), which in time is worn away by the agents of erosion. Long after it has disap- FIG. 32. Diagram showing a volcanic neck and several mesas (p. 167) resulting from the long continued erosion of a vol- canic mountain similar to that shown in Figure 31. FIG. 33. Volcanic neck near Adair, south- eastern Colorado. A cylindrical mass of basalt occupies the throat of an extinct volcano, and is surrounded by an accumu- lation of talus. (U.S. Geol. Surv.) peared, the resistant rock formed by the slow solidification of the lava which re- mained in the tube leading down from the crater may remain THE COMPOSITION OF THE EARTH 53 as an abrupt, steep-sided hill. These elevations, known as vol- canic necks or plugs (Figs. 32 and 33), range in diameter from a few yards to a mile or more. They may be regarded as mon- uments, marking the sites of volcanoes which died ages ago. Volcanic necks are known at various points in the West, espe- cially New Mexico, in Scotland, and in many other places. No matter how resistant their rocks, volcanic necks are them- selves finally destroyed as topographic features, leaving as perhaps the only record of the ancient volcanoes the igneous rock occupy- ing the old tubes lead- ing to unknown depths below. There are many examples of this stage in the West. Columnar structure. The cracking of fine mud as it contracts on drying is a familiar phenomenon. In a similar way, some lava cracks on cooling, sometimes forming regu- lar columns (Fig. 34). F IG. 34. -The Devil's Post Pile in the These are six-sided in Sierra Nevada Mountains. Basalt which t -, has split into columns. (Nat. Geog. many cases, and stand at Mag.) right angles to the cool- ing surfaces. In horizontal sills and lava flows, therefore, the columns are vertical, while in vertical dikes they are horizon- tal. They occur, among other places, in the Palisades of the Hudson, and in Mount Holyoke, Massachusetts. The cracks which separate the columns are joints. Joints are not pecu- liar to igneous rocks. They affect rocks of all kinds, dividing them into blocks of various sizes and shapes. PHYSICAL GEOLOGY ORIGINAL STRUCTURES OF SEDIMENTARY ROCKS Stratification. It has been noted (pp. 36-37) that sedi- ments are commonly arranged in distinct layers, and that this stratification is the most important structural feature of sedimentary rocks. A layer may be called a bed or a stratum (plural strata). A group of consecutive layers composed of the same kind of rock is often called a formation. Layers are separated by more or less pronounced division planes, known as bedding planes (Fig. 5). An individual layer im- plies essentially uniform conditions of sedimentation. A notable pause in deposition, a change in the kind of sediment, or a marked change in the texture of the material is indicated by a new layer. The longer conditions remain constant, therefore, the thicker a given layer becomes. Thickness of beds is, however, only a very rough measure of time, for the same material gathers at unequal rates at different times and places. Very thin beds, such as those in shales, are termed lamince. Lamination is absent or inconspicuous in pure limestones, and usually pronounced in shales. Cross-bedding. If a current (of water or air) moves material along a surface which terminates in an abrupt slope, most of the material will roll down the slope and come to rest. Coarse material will rest at a steep angle, and fine material at a gentler angle. If the coarseness of the material moved forward to the slope varies frequently, numerous inclined laminae will be formed. If, in addition, the direction and strength of the FIG. 35. Cross-bedded sandstone, canon of Virgin River, southern Utah. (Fairbanks.) THE COMPOSITION OF THE EARTH 55 currents change frequently, the inclined laminae will slope in different directions, and meet each other at various angles. This structure is called cross-bedding or oblique lamination (Fig. 35). It is especially characteristic of deposits made by streams, and is found in many wind deposits (Fig. 83, p. 94). It is developed also off ocean and lake shores, where the water is shallow enough to be agitated frequently at the bottom. Conglomerates and sandstones are cross-bedded more often than other kinds of sedimentary rock (Why?). Ripple marks. The rhythmical movement of shallow waters often develops on the bottom miniature ridges, com- monly an inch or two from crest to crest. Such ridges are known as ripple marks (Fig. 36). Often they may be ob- FIG. 36. Ripple marks upon a sandy beach, at low tide. (Greger.) From which direction did the waves which formed the ripple marks come ? served on the sandy beds of clear and shallow streams. Here the rudely parallel ridges extend crosswise of the current, each having a relatively long and gentle slope upstream, and a shorter and steeper slope on the downstream side. Sand grains are rolled by the current up the gentle slope to the crest, whence they fall down the steep slope into the trough. By a continuation of this process, the ridges shift slowly in the direction of the current. Ripple marks are produced, too, along lake shores and seacoasts, particularly by undu- latory movements of the undertow, out to depths of twenty to thirty, or even more feet. (What things determine how far from shore they may be formed ?) Ripple marks may be preserved in the consolidated sediments, and are especially 56 PHYSICAL GEOLOGY common in sandstones. Ripple marks are also formed in sand by wind (Fig. 89, p. 97). Sun cracks. When the water in roadside pools evaporates, the bottom mud shrinks and cracks, forming the familiar mud cracks or sun cracks (Fig. 37). If the clay particles were of uniform size, and drying equal everywhere, the shrinkage cracks would probably be arranged in regular FIG. 37. Mud cracks. (Fairbanks.) figures, after the man- ner of certain cooling lavas (p. 53). As these conditions seldom hold, the cracking is usually irregular. Sun cracks may form extensively in sediments that are exposed along seashores during low tide, in dry interior basins on the smooth mud floors of shallow and temporary lakes (play as), and about lake borders and along stream courses when the water is low. If the sun-cracked surface is exposed for a sufficient time,it will harden enough so that the cracks will not be washed out readily by the returning waters, which may fill them with FIG. 38. Cast of sun cracks in sand- other material and so preserve them permanently (Fig. 38) . (In which of the above situations are the chances for preservation best? Why?) Shales contain sun cracks more often than do other rocks. THE COMPOSITION OF THE EARTH 57 Above features aid in determining geology of past times. One may infer from the composition and structure of sea-laid rocks the character of the waters in which they formed and something of the nature of the lands which furnished the sediments. A conglomerate or sandstone formation of ma- rine origin tells of shallow, rather rough waters, and of relatively high lands whose vigorous streams were able to carry coarse material. Shallow water origin may be indicated further by frequent alternation in the degree of coarseness, by cross-bedding, ripple marks, or sun cracks. If the forma- tion contains fossils, they are likely to be the remains of ani- mals which inhabit water of slight depth. No one formation would be apt to show all these features, but many formations show several of them. A sea-laid shale formation implies bottom waters too quiet to carry away mud. The presence of ripple marks would, however, record some agitation of the bottom water, while sun-cracked shales must have gathered close inshore where wave and current action was weak and streams did not furnish coarse sediment. If muds accumu- lated extensively along the ancient shore, the adjacent land must have been so low that its streams were sluggish and therefore unable to carry coarse material. The fossils of many limestones represent organisms which live only in clear, quiet waters. Such limestones may have formed close to shore if the land was sufficiently low, and protected from wash by vegetation. In a similar way the composition and structure of non- marine formations throw light on the conditions which existed when the rocks were forming. The principles indicated here will be applied frequently in the historical chapters, in determining the geography of North America at the several stages of its development. QUESTIONS 1. Acidic lavas are in general stiffer than basic lavas. Which should you expect to be the leading type in (1) lava flows, (2) sills, (3) laccoliths ? 58 PHYSICAL GEOLOGY 2. Which of the two dikes in Figure 39 is the older ? Reasons. 3. What is the age of the lava sheet L (Fig. 40) in comparison with the age of the sedimentary beds S and Si, (1) if the lava sheet is intrusive, (2) if it is ex- trusive 4. What is the relative age of the dike and the sedimen- tary beds (S, Si, and S 2 ) in Figure 41 ? FIG. 39. Diagram of dikes. 5. Compare and contrast the texture of the rock in a thin and a very thick dike ; at the sur- face and in the central portion of a massive lava flow. FIG. 40. Diagram of lava FIG. 41. Diagram of dike sheet between sedimentary and associated sedimentary beds. beds. 6. Did the lava of the bombs shown in Figure 14 solidify before or after striking the ground ? Reasons. 7. Might the fact that a given lava plateau had been built up by several distinct flows be told by the texture of the rock ? If so, how? 8. How do igneous rocks come to be at the surface? 9. Coarse-grained gran- ites, schists, and gneisses outcrop at many points in the uplands of southern New England. Where were these rocks formed with reference to the surface? What inference, therefore, may be made concerning the amount of erosion which has occurred in the region ? JO. (1) What is the rela- 1 4 5MILE5 FIG. 42. Generalized map of small area southeast of Port Orford, Ore. Short lines represent igneous rocks ; horizontal lines sedimentary rocks, with some meta- morphics. THE COMPOSITION OF THE EARTH 59 tive age of the igneous and sedimentary rock (Fig. 42) (a) if the former is extrusive, (6) intrusive ? (2) What hypotheses may be advanced to account for the isolated areas of sedimentary rock within the igneous rock area ? How could these theories be tested in the field ? (3) How could one determine in the field whether the igneous rock is intrusive or extrusive ? REFERENCES MINERALS AND ROCKS DANA : Minerals and How to Study Them. 2d ed. (New York, 1902.) GEIKIE, J. : Structural and Field Geology. (New York, 1905.) KEMP : A Handbook of Rocks. 2d ed. (New York, 1900.) PIRSSON : Rocks and Rock Minerals. (New York, 1908.) VOLCANOES, ETC. BONNE Y : Volcanoes; Their Structure and Significance. (New York, 1899.) DILLER : Mi. Shasta, a Typical Volcano, in Physiography of the United States, pp. 237-268. (New York, 1895.) The Geology and Petrography of Crater Lake National Park; Prof. Paper No. 3, U.S. Geol. Surv. DUTTON : Hawaiian Volcanoes, in 4th Ann. Rept., U.S. Geol. Surv., pp. 81-219. - Mount Taylor and the Zuni Plateau, in 6th Ann. Rept., U.S. Geol. Surv., pp. 105-198. GILBERT : Geology of the Henry Mountains, pp. 18-60 ; U.S. Geog. and Geol. Surv., Rocky Mt. Region. (Washington, 1877.) HEILPRIN : Mont Pelee and the Tragedy of Martinique. (Phila- delphia, 1903.) HILL : Report on the Volcanic Disturbance in the West Indies, in Nat. Geog. Mag., Vol. XIII, pp. 223-267. HOVEY : The Eruption of La Soufricre, St. Vincent, in May, 1902, in Nat. Geog. Mag., Vol. XIII, pp. 444-459. JAGGAR : The Eruption of Mount Vesuvius, April 7-8, 1906, in Nat. Geog. Mag., Vol. XVII, pp. 318-324. JUDD : Volcanoes. (New York, 1893.) RUSSELL : Volcanoes of North America. (New York, 1897.) The Recent Volcanic Eruptions in the West Indies, in Nat. Geog. Mag., Vol. XIII, pp. 267-285. Volcanic Eruptions on Martinique and St. 'Vincent, in Nat. Geog. Mag., Vol. XIII, pp. 415-436. 60 PHYSICAL GEOLOGY SHALER : Aspects of the Earth, pp. 46-97. (New York, 1889.) The topics of this and later chapters are discussed at greater length in larger textbooks and manuals. Among the best of these are : CHAMBERLIN AND SALISBURY : Geology. 3 vols. (New York, 1904, 1906.) College Geology. (New York, 1909.) DANA : Manual of Geology. 4th ed. (New York, 1895.) GEIKIE : Textbook of Geology. 4th ed. (London, 1903.) CHAPTER II PHYSICAL CHANGES OF THE OUTER SHELL The earth's crust. In studying the solid part of the earth, we are necessarily limited to a thin shell near the surface. In the deepest mines and canons we may go down to a depth of a little more than a mile. By the slow denudation of the uplifted lands, rocks which were once buried to a depth of several miles may be uncovered at the surface. This outer shell, which alone is open to investigation, is the subject of the present Chapter. It has often been called the " crust of the earth," in allusion to an older theory that the interior is so hot as to be liquid, but is covered by a thin, solid crust. Although this theory has been largely abandoned, the term is convenient, and we shall use it to mean simply the outer part of the earth, which is partially open to obser- vation. Surface features of the crust. The outside of the earth has an irregular surface. A glance at a model of the globe shows that there are several broad, smooth tracts, which are sunk on an average of about two and one half miles below the surface of the sea. These are the great ocean basins. Be- tween them large plateaus stand out in relief (Fig. 43). The so-called continents are merely the portions of these plateaus that are now out of water, and hence are land. The great surface features of the earth are, then, the oceanic depressions and the continental plateaus. Upon examining these major features in more detail, we find that the surface of the land is notably rougher than that of the sea bottoms. On the former we see mountains, ridges, and minor plateaus, with their complementary valleys, basins, 61 62 PHYSICAL GEOLOGY and lowlands (Figs. 43 and 44). Some of the basins contain lakes or seas, while others do not. In the oceans likewise there are irregularities, such as projecting islands, and the hollows known as " deeps " ; but, on the whole, the ocean FIG. 43. Photograph of a relief model of North America. floors are far less rugged than the lands. This is due partly to the fact that streams, glaciers, and other agencies which roughen the land surface do not operate in the oceans, and partly to the fact that the deposition of sediment upon the sea bottom tends constantly to smooth out such irregulari- ties as may exist. Movements within the crust. As a matter of human experience the earth seems to be firm and stable to the last PHYSICAL CHANGES OF THE OUTER SHELL 63 degree, save for such exceptional and sudden disturbances as landslides and earthquakes. But an examination of the rocks of almost any region discloses evidence of movements which have taken place, and there is even proof that such FIG. 44. Chief topographic divisions of North America. Compare this with the photograph of the model on the opposite page. movements are continually recurring. Indeed, the study of geology can hardly fail to emphasize the fact that the earth is forever undergoing changes of many kinds, which after long lapses of time produce great results. These changes include crustal movements of one kind or another. Some of the 64 PHYSICAL GEOLOGY movements are sudden, like those which produce earthquakes, while others are very slow. The most effective movements are the slow ones, so slow that in comparison the hour hand of a watch is revolving rapidly. We cannot readily detect such changes while they are in progress, but their results, after long periods of time, are obvious. Slow movements of this sort affect every- thing from whole continents to the smallest invisible particles of rock. Some of them may now be considered in more detail. Warping of the surface. On the slopes of Mt. St. Elias, in Alaska, modern sea shells have been found attached to the FIG. 45. Folded beds of limestone on the south coast of Alaska. (Stan ton and Martin, U.S. Geol. Surv.) rocks just as they once grew, but several thousand feet above the sea level. It appears that the coast has been slowly raised above the sea to that extent. Conversely, on the shores of North Carolina, stumps of trees are found stand- ing out in salt water, where they did not grow. From this it becomes evident that either the land has gradually sunk beneath the sea, or the sea has risen upon it. There are many other facts which prove that the surface of the earth is rising in some places and sinking in others, but so slowly that we do not perceive it. Slow upward and downward PHYSICAL CHANGES OF THE OUTER SHELL 65 movements of this sort may be included under the term warping. Local crumpling of the shell. Before they were consoli- dated, the stratified rocks were merely layers of sediment which had been deposited in a horizontal or gently inclined attitude. In many places, however, we now find them crumpled and folded (Fig. 45). The folds in any one area are usually parallel to each other, and are arranged in long, narrow bands. Such folds have evidently been produced by compression from the sides, the part between having wrinkled, just as flat-lying sheets of paper will wrinkle if compressed horizontally. Both the vertical movements mentioned above and these lateral movements change the surface features of the earth. The former produce plateaus, plains, and broad depressions, while the latter make mountain ridges with troughs between. 1 Causes of crustal movements. What are the causes of these movements? This question cannot now be answered satisfactorily. The fact that these movements take place is undeniable, but the causes of them are not yet fully under- stood. 2 EFFECTS OF MOVEMENTS Having now in mind the general nature of these slow move- ments within the crust, we are in a position to study the effects which they produce in the rocks. These effects may be grouped as folds on the one hand and fractures on the other. Fracturing and folding of rocks compared. Rocks in gen- eral are brittle substances. If quickly bent or squeezed, they will break. If, however, the pressure is applied very slowly, and especially if the layer is kept heavily weighted 1 Mountains, plateaus, plains, troughs, and basins are formed, not only by body movements, but in a variety of other ways, which are discussed in later Chapters. 2 The theories relating to crustal movements are discussed at some length in larger textbooks, such as Chamberlin and Salisbury's Geology, Vol. I, 2d ed., Chap. IX. B. & B. GEOL. 5 66 PHYSICAL GEOLOGY clown by thousands of feet of rock lying upon it, a bend may result instead of a break. Since both of these conditions exist in the crust, we actually find the rocks broken in some places and bent or folded in others. In fact, different kinds of rock may show both types of structure in the same place, the stronger rocks being broken, while the weaker are folded. FOLDS Kinds of folds. On examining the layers (or strata) of rock over a large area we may find them flat in one place, wavy or rolling in another, and intricately twisted and crum- pled in a third, with all gradations between. Thus we may describe folding in general as simple or complex; as mild or intense. The individual folds may be classified from a variety of points of view. Simplest of all would be a grouping accord- ing to their attitude. Thus, all folds are either down folds (synclines), up folds (anticlines), or stepfolds (monoclines). Usually anticlines and synclines are combined in a series of undulations, the former making the crests and the latter the troughs of the waves. FIG. 46. Gently folded sedimentary rocks in the central part of the Appa- lachian Moun tains. (U.S. Geol. Surv.) Before going further into the consideration of folds we may stop to examine the parts of a single simple fold: Each consists of two limbs, rising to a crest in the anticline and descending to a trough in the syncline. The inclination of the limb of a fold is called the dip. In field studies the angle and the direction of the dip are of much importance. The dip is always measured downward and from a horizontal plane. Thus a limb having a dip of 5 would be nearly level, while one with a dip of 90 would be vertical. PHYSICAL CHANGES OF THE OUTER SHELL 67 When many anticlines or synclines are compared, it is found that they present a wide variety of forms. Thus there are low, broad folds (Fig. 46), sharp folds (Fig. 47), tightly FIG. 48. Overturned folds. FIG. 47. Closely folded strata in the southern part of the Appalachian Mountains. (U.S. Geol. Surv.) compressed folds, and even inclined or overturned folds (Fig. 48). A layer of rocks may be bent into any of these forms according to the condi- tions under which the pressure was applied. Competent and in- competent folds. It is often advantageous to classify folds according to their competency. In order to form an anticline a layer must have a certain amount of strength. This will be readily apprehended if we imagine several layers of loose sand to be compressed on each side, they would be mashed without definite folding. As compared with loose sand or mud, we can well understand that firm beds of sand- stone or limestone would be likely to buckle up in the form of folds. Beds which are strong enough to hold themselves up in arches have been called competent strata, while materials which may be squeezed and crushed together are incompetent. In considering competency, however, it is necessary to take into account something more than the character of the rock. Sheets of paper lying free upon the table, when com- pressed sidewise, will arch into a fold and, under those con- ditions, are competent. Nevertheless if several books are piled upon the sheets, the latter will not arch when com- pressed, but will merely be crumpled into many little twisted folds. Similarly any layer of rock, however strong, may be so weighted down by overlying beds that it will be complexly 68 PHYSICAL GEOLOGY crumpled instead of being folded in a series of simple waves. We therefore see that a given stratum may be competent if not much weighted, but incompetent if heavily loaded by reason of its burial deep beneath the surface. In accordance with these facts there are two types of folds, one characteristic of the surface layers and the other of the great depths. When a given region is subjected to compressive horizontal forces, the layers at the top may arch and buckle into open folds; while those thousands of feet beneath may be mashed and crum- pled into many little broken parallel crenulations (Fig. 49). Between these there is, of course, a transition zone wherein the weak FIG. 49. Incompetent folds in jasper con- rocks Such as shale will be taming streaks of ron ore. ^ % (3) the rate at IliiQBJHKtA " which the waste already formed is removed. (1) Open-textured rocks with many inirt anrl rvrViPr FlG< 1^. St. Peters Sandstone, eastern Iowa. J01 Showing effects of joints and bedding planes upon cracks absorb weathering. 128 PHYSICAL GEOLOGY much water, and so favor the wedge work of ice, and, where some of the constituents of the rock are soluble, solution. Figures 118 and 119 illus- trate the influence of j oints upon the weathering of stratified rocks, and Fig- ure 120 shows the effect of joints upon the weather- ing of granite. Dark ob- jects heat and cool more rapidly than light ones, and dark rocks accord- ingly favor splitting through changes in tem- perature. (2) No single type of climate favors all the processes of weather- ing. It was seen on page 100 that the wedge work of ice is most important in moist regions where there are frequent changes in temperature which involve the freezing point. The chemical work of the atmosphere and of ground water is, on the other hand, most important in hot, moist climates. An arid climate with great daily range in temperature favors rock splitting, but opposes the work of plants, animals, and ground water. If rocks are not buried too deeply with soil and subsoil, they probably weather fastest, everything considered, in a hot and moist climate; but flat lands in regions having such climates usually have thick accumulations of mantle rock. It is said to reach a thickness of 300 feet or more in parts of Brazil. (3) If the products of weathering remain where formed, they finally cover the bed rock so deeply that it is more or less completely protected from further attack. If, on the other hand, they are removed as fast as FIG. 119. Limestone columns weath- ering away. The openings between the columns are enlarged joints. The sur- rounding rock has been removed by ero- sion. Eastern Iowa. What are the various ways in which these rocks are being weathered ? How may the preservation of these rocks after the removal of the surrounding rocks be explained ? THE WORK OF STREAMS 129 formed, so that bare rock is always exposed, the work of ground water and of plants and animals is reduced greatly. It consequently follows that, other things equal, weathering FIG. 120. Weathered forms in granite, Laramie Hills, Wyo. Three sets of joints may be seen, and their influence upon the weathering of the rock is clearly evident. proceeds most rapidly when its products are rather promptly, but only partially, removed. The fact that over most of the surface of the land there is a mantle of soil and subsoil indicates that, in general, weathering exceeds transportation. Questions 1. Does the absence of soil in any given place mean that weather- ing is not in progress there ? 2. What are the principal agents of weathering in the Sahara? New York ? Louisiana ? The Amazon Valley ? 3. Would a given stone wall stand longer in Labrador or in Florida ? What are the principal agents of weathering by which it would be destroyed ultimately in each place? 4. What differences in weathering might reasonably be expected on the two sides of an east and west valley ? On the two sides of a high north and south mountain range in the latitude of the United States ? Of an east and west range ? 130 PHYSICAL GEOLOGY 5. Flint nodules are of common occurrence in limestone (p. 121). Explain the fact that in certain limestone regions the stream beds contain few limestone bowlders, but many of flint. 6. Why does residual mantle rock in many cases merge gradually into the firm rock beneath? (See Fig. 214.) 7. Describe and explain what you see in Figure 121. FIG. 121. Granite rocks. Laramie Hills, Wyo. TRANSPORTATION BY STREAMS Getting a load. Streams roll and drag material along their beds, and carry it in mechanical suspension and in solution. That which is moved mechanically is obtained in a variety of ways. Streams wear material from their beds and banks, and remove that which is already loose. Sediment is brought in by tributaries. Material loosened by weathering on the tributary slopes is delivered to the stream by gravity or by rain wash. A certain amount of fine material is brought by the wind. Most of the material which is carried in solution is contributed by issuing ground waters; a small part is furnished by the unorganized run- off, and another minor portion is dissolved by streams from the rocks over which they flow. How the load is carried. -A stream pushes sediment along its bottom by the direct impact of the current, and also rolls and drags it by the friction of the bottom water, some- THE WORK OF STREAMS 131 C what as one might move sand grains on a table by dragging the outstretched hand across them. Material is held in suspension chiefly by minor up-moving currents. Since rock material is on the average two and one half to three times as heavy as the water it displaces, it tends, under gravity, to sink to the bottom. In standing water it sinks vertically. In a stream whose water is moving horizontally, two forces act upon it. Gravity, of course, seeks to draw it directly to the bottom (G, Fig. 122), while the current tends to move it in the direction of its flow (C, Fig. 122). The sediment accordingly fol- lows a course (S, Fig. 122), which is a re- sultant of the combined forces. It reaches the bottom in the' same time it would in standing water of the same depth. In na- ture, however, stream water rarely, if ever, moves horizontally for any distance. Bowl- ders and other irregularities on the bottom deflect portions of the main current ob- liquely upwards. Projections of the bank likewise create subordinate currents, some of which move upwards. Sediment settling to the bottom along an oblique path may encounter such up-going currents and be lifted by them. Presently sinking again, it may again be lifted or may reach the bot- tom, perhaps to be presently carried up once more by other upward currents. Material the size of sand, and larger, prob- ably rarely makes extended trips in suspension ; instead, many short trips are interrupted by periods when it rests upon the bottom, or is dragged and rolled along it. On the other hand, mud and silt are often carried long distances before settling. Nevertheless, even fine material probably normally requires thousands of years to make the trip from the sources of the larger rivers to their mouths, for it is dropped on the way many times for long periods, perhaps helping to form bars and FIG. 122. Diagram showing the two forces acting upon a particle at A in the horizontal cur- rent of a stream, and the general course which the particle takes in sinking to the bot- tom. 132 PHYSICAL GEOLOGY islands or being built into flood plains, from which it is re- moved later, to be carried another stage on its journey to the sea. Just as in the atmosphere (p. 87), material sinks more slowly in proportion as the surface it exposes to the friction of the water is great in relation to its weight. Sediment of a given sort settles faster in salt water than in fresh water. The amount of the load. The amount of the load which any given stream is moving depends upon (1) its velocity, (2) its volume, and (3) the amount and nature of the ma- terial to which it has access. Obviously, the swifter and larger a stream, the more and larger the material it is capable of moving. But many swift streams carry little material, because little loose material is available, or because it is in pieces too large to be moved. The Mississippi River carries on the average over 1,000,000 tons of sediment per day into the Gulf of Mexico. It has been calculated that it dis- charges sediment sufficient to fill the basin of Lake Superior, the largest lake in the world, with an area of 32,000 square miles and an average depth of 550 feet, in about 66,000 years. It has been estimated, also, that the work performed each year by the Missouri River in transporting sediment is equivalent to 275,000,000,000 mile tons, or tons carried one mile. The railroads of the United States carried 236,600,000,000 mile tons in 1907. Questions 1. Why are many mountain streams clear? 2. Why in many streams are narrows in the channel floored with coarse material, while broad parts of the channel are lined with fine sediment ? 3. Do two streams of the same velocity and volume necessarily carry the same amount of sediment? Reasons? 4. Can a given stream carry a greater weight of coarse or of fine material ? Why ? 5. (1) Just why do streams carry more sediment after heavy rains ? (2) Would the effect of a given rain in northern United States be likely to be the same in January as in July ? (3) Make a general statement concerning erosion, in keeping with the answer to (2). THE WORK OF STREAMS 133 COBRASION How streams wear rock. Like clear air, clear water can do little in the way of mechanically wearing firm rocks. Per- haps the most striking illustration of this is afforded at Niagara Falls. Seven thousand tons of essentially clear water rush over the brink of the falls each second, and yet certain FIG. 123. The tools of a river. Stream-worn pebbles in the bed of the Potomac River at Barnum, Md. (Md. Geol. 8urv.) tiny plants grow in the water, clinging to the rocks at the very edge. Were erosion actively in progress at the edge, the plants would, of course, be swept away. The St. Lawrence River leaves Lake Ontario as clear as the lake waters them- selves, and for many miles is unable to corrade effectively, even where its current has great velocity and washes the shores of islands whose banks are of clay. Many other streams which flow from lakes illustrate the same thing. They often have mossy channels in spite of swift currents. Streams, like winds, wear rocks by means of the rock fragments which they transport (Fig. 123). Sand grains, pebbles, etc., that are swept along by the main current, rub, rasp, and strike the bed and sides, breaking and wearing pieces from them. Material B. & B. GEOL. 9 134 PHYSICAL GEOLOGY in suspension is also frequently driven vigorously against the bottom by subordinate downward-moving currents, with similar effect. The tools are themselves worn in the process. Stream-swept stones become rounded (Fig. 123), and their surfaces often have many tiny pits, or depressions, made by the blows they have delivered or received. These characteris- tics have helped to prove that the material of certain rock formations was handled by vigorous streams. Rate of wear. The rate at which degrading streams lower their channels depends on several conditions. (1) Weak rocks with soluble cements favor rapid wear, while strong, nonsoluble rocks retard it. Stratified rocks in general prove less resistant than massive rocks. Other things being equal, rocks with numerous joints and cracks are worn faster than others, because these openings are planes of weakness. (2) Rapid streams deal harder blows and more of them than slow ones, and so, other things being equal, wear their Channels faster. The velocity of a stream, in turn, depends upon (a) the slope (gradient} of its channel, (6) its volume, (c) its load, and (d) the shape of its channel. Obviously, the steeper the channel and the larger the stream, the greater its velocity. Energy is expended in moving sediment, which otherwise would express itself in greater velocity ; other things equal, a given stream accordingly flows fastest if clear, and slowest if loaded. A stream is retarded by friction with its bed and sides. Crooked channels, with wide, uneven bottoms, occasion great friction, and tend to produce a sluggish cur- rent ; straight channels with narrow and smooth bottoms de- velop less friction, and promote greater velocity. (3) Since the velocity of a stream is decreased as its load is increased, it follows that the force of its blows is also diminished. In other words, the greater the number of tools carried, the greater the number of blows delivered in a given time, but the weaker each blow is ; while, on the other hand, the fewer the tools carried, the fewer the blows delivered in a given time, but the stronger each blow becomes. Clearly, streams wear THE WORK OF STREAMS 135 fastest, other things being equal, when carrying a partial load, so that many blows are delivered, but not so many that all are weak. (What qualities should render rock fragments most efficient tools for stream corrasion? Would tools possessing these qualities long retain them all? Why?) Graded streams. When the gradient of a stream is just steep enough to give it the velocity necessary to wash forward the sediment brought to it from the tributary slopes, it is said to be at grade. If it is able to transport more than is delivered, it removes material from its bed until it comes to grade at a lower and gentler slope. If it is unable to transport all that is delivered, part of the load is left as a deposit. By this means the channel is raised and the gradient becomes steeper gradually, until in time the stream grows swift enough to carry away the sediment brought to it. Rate of land reduction by stream erosion. Estimates have been made of the rate at which certain river systems are degrading their basins. This may be done as follows : The width, average depth, and mean velocity of the main river at its mouth may be determined at different times by measure- ments, and from these data the average volume of water dis- charged per year may be calculated. The average amount of material contained in a cubic foot of the water, both in solid form and in solution, may also be learned by examination of numerous samples. Knowing the average amount of sedi- ment in each cubic foot of the water, and the average number of cubic feet discharged in a year, the total amount of sediment delivered at the mouth of the river may be computed readily. Finally, the area of the drainage basin being known, one may determine to what uniform depth the sediment removed yearly would cover it. The result indicates the average rate per year at which the drainage basin is being degraded. By this general method it has been estimated, for example, that the Mississippi Basin is being lowered mechanically at the average rate of one foot in about 5000 years, and when the amount removed in solution is considered also, one foot in 3500 136 PHYSICAL GEOLOGY years. The Ganges Basin is being reduced by the removal of material in the solid form alone, at the rate of a foot in less than 2000 years ; and the Danube Basin a foot in approxi- mately 6800 years. In some parts of each basin the rate is far more rapid. Figure 124 shows the results of recent estimates of the rate of land reduction throughout the United States. FIG. 124. Rates of land reduction by stream erosion in the United States. The figures are the number of years required for one inch of denudation. (After National Conservation Commission.) The average elevation of the continents above sea level is about 2300 feet. If they were being degraded by streams at the average rate of the Mississippi Basin, and continued to be cut down at that rate to sea level, and nothing occurred -to off- set the work of the streams, the lands would be destroyed in about 8,000,000 years. Probably, however, the average rate of land reduction is less than that assumed. Nor could the present rate, whatever it may be, continue till the land was at sea level, for as it gets lower, the streams would flow more slowly, and therefore degrade less rapidly. Again, judg- ing by the past, diastrophism and vulcanism would intervene to maintain the land masses. Still other factors would modify THE WORK OF STREAMS 137 the problem, as, for example, the fact that streams cannot re- duce their basins quite to sea level (p. 139) and that an average of 2300 feet would not have to be eroded away to bring the land to the level of the sea, for the surface of the ocean would be raised by the deposition in it of the waste from the land. Nevertheless, such computations are worth while, since they aid one to appreciate the importance of the work being done by running water. Questions 1. Why is the Niagara River practically free from sediment? 2. Other things being equal, would a given stream corrade faster when flowing across the edges of highly tilted beds, or on horizontal beds ? Why ? When the beds dip downstream or upstream ? Why ? 3. Is corrasion favored more by a constant volume, or by great and sudden fluctuations in a stream ? 4. Enumerate all the conditions which might enable one of two streams of equal and constant volume to corrade much faster than the other. 5. Is it possible for a stream to corrade without degrading ? To degrade without corrading ? 6. Will a given stream flow faster when fully loaded with coarse or with fine material ? FEATURES DEVELOPED BY RIVER EROSION VALLEYS Most streams flow in valleys. In general, valleys correspond in size to their streams, and, like the stream it contains, a given valley is smaller than the one it joins, and larger than those which join it. At their union, the bottoms of tributary valleys are normally at the same level as the bottoms of the larger valleys to which they lead. Furthermore, all streams are engaged, with the help of the agents of weathering, in en- larging their valleys. These facts indicate that the valleys were not found ready-made by the streams which occupy them, but that they are a result of the work of the streams aided by weathering agents. Many synclinal troughs (p. 66) form valleylike depressions. Since they are due to the structure 138 PHYSICAL GEOLOGY of the rocks, such valleys are called structural valleys. They usually contain stream valleys in their bottoms. FIG. 125. Sketch of a gully. FIG. 126. Gullies near River- side, Cal. (Fairbanks.) the gully will make it longer ; water coming over the sides will make it wider ; and the water which flows along the bottom will make it deeper. Thus it may become sufficiently long and wide and deep to be called a ravine, and finally a valley. When its bottom is worn below the The beginning of a valley. Figure 125 shows an infant valley or gully, which con- tains running water only during rains. In the future, rain water running down the slope into the head of FIG. 127. A mountain ravine near Marshall, N. C. (U.S. Geol. Surv.) THE WORK OF STREAMS 139 water table, ground water will enter it as seepage and springs from the sides, and flow away as a stream. When the bottom of a valley is below the wet-weather level of the water table, but above the dry-weather level, it contains an intermittent stream, but when the bottom of the valley is eroded below the water table at its lowest level, the stream is permanent, and the enlargement of the valley proceeds without interruption. Figure 126 shows many gullies starting on an unprotected sur- face in a relatively dry region, while Figure 127 shows a mountain ravine in a humid region. Valley deepening. A stream lowers its channel, and so deepens its valley, by removing material loosened by weather- FIG. 128. A stream undercutting its bank and widening its valley. Central Illinois. (Crane.) ing or by its own corrasion. But there is a limit below which a stream cannot degrade its valley flat. This is the level of the lake, sea, or other valley to which it leads. Furthermore, it can cut to this level only at its mouth, from which the valley bottom rises upstream, very gently in the case of large rivers, and more rapidly where the stream is small. For some distance above their mouths, streams may, however, cut their channels slightly below the level of the sea or lake into which they flow. The lowest level to which a stream can cut is 140 PHYSICAL GEOLOGY base level. As a stream approaches base level, it flows on a di- minishing slope, and its current therefore becomes less and less rapid. In other words, a stream approaches .base level more and more slowly as it draws nearer and nearer to it, so that the removal of the last few feet may take longer than all the rest. Valley widening. Valleys are widened in a variety of ways. Relatively sluggish streams are pushed aside by the currents of their tribu- taries or by obstacles. In this way the stream is driven first against one bank, and then against the other, and so undermines each. The points of attack varying from time to time, the valley is opened generally, and a val- ley flat is developed (Figs. 128 and 129). Meanwhile, other agencies assist in widening the valley. Rains wash weathered material down its sides, and if the slopes are sufficiently steep, fragments also roll and fall down them. Material works its way down the sides of the valley also by creep and by slump- ing (p. 115), and is re- moved in other less important ways. If the material re- FIG. 130. The divide between the two valleys mained at the hot- is being T consum ^ by the side cutting of the rivers. It may be cut away entirely, in which tom, the enect WOUld case the two valleys will become one. FIG. 129. Diagrams of a river de- veloping a flat by side cutting. THE WORK OF STREAMS 141 be to narrow the valley there, while widening it above. Us- ually, however, it is carried away by the stream. By the widening of two adjacent valleys, the intervening ridge may be worn out, the two becoming one (Fig. 130). The contin- uation of this process among neighboring valleys would ulti- mately reduce the entire surface of the area affected to the level of the valley bottoms. Valley lengthening. The heads of valleys are usually without permanent streams, for they are commonly above the lowest level of the ground-water surface. The stream, therefore, does not assist in the lengthening of its valley headward, but all the other agencies which widen valleys help also to lengthen them. A valley ceases to grow by headward erosion when a permanent divide (Fig. 131) is established. This is when the wear accomplished by the run-off which a FIG. 131. Diagram of a divide. The crest of the divide (at a) is permanent if the conditions of erosion are the same on the two sides. Rainfall may lower it, but it cannot shift its position horizon- tally. FIG. 132. Bad-land topography near Grand Junction, Colo. Shows many gullies. (Baker.) enters the head of the valley is balanced by the erosion of the water which runs from the divide in the opposite direction. Thus limits are set to the growth of a valley in all three 142 PHYSICAL GEOLOGY dimensions. In depth, the limit is base level ; in width and length it is fixed by neighboring valleys. Struggle among valleys. It is not to be inferred from what has preceded that all gullies become valleys, or even ravines. Quite the opposite is true. Few of the gullies shown in Figure 132, for example, can grow to ravinehood. As they widen, the intervening divides will be worn out, combin- ing adjacent gullies and reducing the number. Many gullies are commonly destroyed in the formation of a single ravine, which in turn is likely to presently find its growth contested by other ravines. Such a conflict is shown in Plate II, among the ravines near Wesley. Little opportunity for growth remains to most of the ravines in the vicinity, and many are doomed to early destruction by their more powerful neighbors. Valleys without gullies. Not all valleys have grown from gullies as described above. In the northern part of the United States and in Canada, for example, thousands of lakes were formed during the Glacial period (p. 214). In the moist cli- mate of this region, the lakes received more water as rain on their surfaces and as run-off from tributary slopes, than they lost by evaporation. Consequently, many overflowed their rims, forming streams. Such streams followed the lowest available lines of descent to other streams or lakes, and by erosion developed valleys. Thus the streams existed before the valleys. In this and other ways, streams and valleys of this class are in contrast with those considered first. Tributary valleys. Most valleys have tributaries, and these in turn branch repeatedly, like the limbs of a tree. A main valley and all its tributary valleys constitute a valley system, whose streams, the main river and all its branches, form a river system. The entire area drained by a river system is a drainage basin. Tributary valleys commonly start as gullies on the sides of their parent valleys. If the slope of the ground back from the sides of a valley is such that more water enters it at some points than at others, the velocity, and hence the erosive power, of the entering water will be THE WORK OF STREAMS 143 greater at such places than elsewhere, and tributary depres- sions (gullies) will result. Even if an equal amount of water entered the parent valley at all points, the same result would follow, provided the rocks of the valley sides were of unequal strength, for erosion would be more rapid where the rocks were weaker, giving rise to tributary FlG depressions. Stages in topographic development. It is apparent from the preceding paragraphs that valleys pass through careers just as men do. Each stage in the career of a valley is characterized by certain features, 133. Cross section of a young valley. FIG. 134. Cross section of a mature valley. FIG. 135. Cross section of an old valley. so that by observ- ing the form of a given valley, one may determine readily what point it has reached in its development. Valleys are young when still narrow and steep - sided (Fig. 133). These features in- dicate that as yet down cutting is keeping ahead of other processes. Most young valleys have few and im- perfectly developed trib- utaries and relatively steep gradients. (What things will determine FIG. 136. Diagram showing changes in whether Or not young the shape of a valley as it advances from vfl ii pvc , flrp r i ppn 178- Tejunga River, southern Cali- fornia, sinking in the sand of its flood- United States IS this plain. (Fairbanks.) 172 PHYSICAL GEOLOGY the case over large areas?) Diminished volume means reduced velocity and carrying power, and hence deposition. (c) Many rivers deposit at their mouths where the current is checked. (d) Deposition is brought about also by changes in the shapes of river chan- nels. If, for example, water charged with sedi- ment leaves a narrow, straight, and smooth sec- tion of the channel to enter a wide, crooked, and irregular one, the friction of the current with the bed and banks is increased, its velocity is therefore decreased, and deposition may result. (2) Tributaries with high gradi- ents often deliver to their sluggish main streams more sedi- ment than the latter can wash forward, resulting in deposits along the floor of the main valley. In many large depositing FIG. 179. An alluvial fan in the Illinois Valley. The velocity of temporary, wet-weather streams is reduced as they leave the gulley in the background, and they are forced' to deposit the sediment which they carry. (III. Geol. Surv.) FIG. 180. Alluvial fan at mouth of Aztec Gulch, Dolores Valley, south- western Colorado. (U.S. Geol. Surv.) Account for the small fan in front of the large one. THE WORK OF STREAMS 173 rivers, like the lower Mississippi, all the above causes, and perhaps other less important ones, are in operation. The principal features produced by stream deposition are described in the following paragraphs. Fans and cones. Alluvial fans are so called because they are half-circular in ground plan when developed typi- cally, and are composed of alluvial material (Figs. 179 and 180). Cones are relatively steep fans. Alluvial fans vary in diameter from a few feet to several miles. Some of the California rivers have built fans some forty miles across. Fans are developed best at the bases of steep slopes in dry regions, where streams of diminishing volume leave the rela- tively high gradients of their mountain valleys to enter low- lands. The deposit in such a situation chokes the channel of the stream, and some of the water spreads around the obstruction. The process being repeated many times, and the stream meanwhile extending the deposits in the direction of its flow, they presently acquire more or less of the " fan " shape which suggested their name. The main water chan- nels of many large fans give off branches that in turn di- vide repeatedly downstream. These branching channels are called distributaries, and their explanation is involved in what has already been said. The deposits in a given chan- nel reduce its size until some of the water breaks over the side and follows a new course to the margin of the fan. The new channel, becoming choked, gives off other distribu- taries, which divide again. The spreading of the water flowing over the fan becomes an important cause of deposi- tion, since it increases the friction of flow, and therefore decreases the velocity. Deposition may be caused also by much or all of the water sinking into the porous material of the fan. Thus the growth of fans is due to deposition brought about by (1) decrease in gradient, (2) increase in friction of flow, and (3) often by decrease in volume. Plate VIII shows a portion of a large fan, together with waste channels, tributaries, and distributaries. PLATE VIII. PORTION OF A LARGE ALLUVIAL FAN IN SOUTHERN CALI- FORNIA. Contour interval, 50 feet. Scale, about 1 mile per inch. (San Bernardino, California, Sheet, U. S. Geological Survey.) THE WORK OF STREAMS 175 The structure of alluvial fans is characteristic, and results from the method of their growth. The coarsest material is dropped at the apex of the fan, where the current is first checked, and the deposit made at any given time becomes progressively finer toward the margin. This does not mean that the material in a vertical section through an alluvial fan, all parts of which are at the same distance from the FIG. 181. Section of an alluvial fan, Owens Valley, Cal. (Trowbridge.) apex, is all of the same degree of coarseness. On the con- trary, the material would probably change frequently, both horizontally and vertically, for the volume (and so the carrying power) of different distributaries would vary at the same time, and that of any given distributary at differ- ent times. Such variations in the tops of fans may often be seen in the sides of the channels which trench them (Fig. 181). The angle of slope of a fan depends upon how suddenly and how much the velocity of the depositing waters was diminished, and upon the kind and amount of material they carried. A sudden and great reduction in the velocity of a stream heavily loaded with coarse material, gives a relatively steep slope; the opposite combination a gentle one. The profile of a fan along any radius, like the profiles of other depositional slopes, is a curve concave upwards 176 PHYSICAL GEOLOGY (Fig. 182). (What would be the character of a curve drawn on the surface of a fan along a line all points in which were equidistant from the apex of the fan?) FIG. 182. Profile of a large alluvial fan near Cucamonga, Cal. section, 6V 2 miles. Length of The growing fans of neighboring streams in arid regions unite in many cases to form extensive alluvial slopes or plains (Fig. 183). Certain rivers have been ponded back by the fans of tributaries, forming broad, lakelike expansions of the river. FIG. 183. A piedmont alluvial plain, Silver Peak Range, Nevada. Waste from the mountain valleys unites to form a compound fan. (Sketch from photograph by Spurr, U.S. Geol. Sitrv.) Lake Pepin in the upper Mississippi, and Lake Peoria in the Illinois River (Plate II), are of this origin. Flood plains. The portion of a valley bottom subject to inundation is called the flood plain (Plate IX). Flood plains, or flats, are usually formed primarily by the side cutting of relatively sluggish streams (p. 140), and subordi- nately by deposits made during overflow. In exceptional cases rivers occupying narrow-bottomed valleys are forced to aggrade their channels, and flood plains rasult that are due entirely to deposition (Fig. 184). The alluvial deposit may cover the underlying rocks thinly or thickly. THE WORK OF STREAMS 177 Normal flood plains are widest in their lower portion, where the gentle gradient favorable to lateral shifting was developed first, and be- come narrower more or less regularly up valley. The lower Mississippi has opened a flood plain from 20 to 60 miles or more wide. The downstream slope of flood plains varies FIG. with the volume of the stream and the character of the material it deposits. Relatively small streams heavily overloaded with 184. Diagram of a flood plain formed by deposition in a narrow valley. What is the age of the rock valley in which the filling has occurred, and how is it shown ? What work was the river do- ing before filling commenced ? The evi- dence? What things may have forced the Driver to cease its earlier work and ag- grade its valley ? coarse material build steep flood plains, sometimes with a descent of 5Q: to 75 feet a mile; large rivers, depositing fine sediment, build nearly level flats. Natural levees. During times of flood a river deposits most actively along the edges of the channel. Here the depth of the overflowing water is diminished suddenly and, in consequence, its velocity and carrying power. Here during the continuance of the overflow the marginal waters of the main current are checked by friction with the slower moving backwaters. Deposition along these lines during many overflows may build low, marginal ridges with a gentle slope away from the river. Such embankments are natural levees (Fig. 185). It is evident that natural levees will not prevent subsequent over- flow, since a stream , . 185. Diagram showing natural levees and can build them only . . the general structure of stream-laid beds. to the level of its 178 PHYSICAL GEOLOGY flood waters. Artificial embankments have been built upon the natural levees of many rivers, in order to reclaim their bottom lands. As deposition continues along the bed of the river, such embankments must be built higher from time to time in order to confine the stream. The low margins of many wide flood plains are marshy. In such marshes, the dead leaves, twigs, and branches of the swamp vegetation gather in the shallow water, along FIG. 186. Meanders of the upper Green River, Wyoming. (Baker.) with minor quantities of silt. This vegetal matter, preserved by the water from complete decay, may be transformed slowly into peat. Some coal beds are thought to represent similar marshes which existed ages ago (pp. 378, 380). Many tributary streams on entering an aggraded valley are prevented by the natural levees from uniting with the main river at once, and flow greater or lesser distances down valley before joining it at some point where it swings to their side of the flood plain (Plate IX). Braided rivers. In some cases the waters of rapidly depositing rivers flow in numerous channels which meet and divide repeatedly. Deposition along the floor of a given channel reduces its capacity. When the channel is presently unable to hold all of its water, a part breaks over the side and follows a new line. The new channel, becoming choked like the old one, gives off branches which in turn divide. The overflowing waters follow the lowest accessible lines of THE WORK OF STREAMS 179 descent, and may reunite only to separate again a little farther down valley. By this process the river is split into many minor streams which shift continually and inclose changing islands of sand and gravel. Such rivers are braided rivers. Stream meanders and flood-plain lakes. Even if nearly straight in the beginning, a river must come to follow a FIG. 187. Meanders of the Jhelum River in the valley of Kashmir, India. serpentine (meandering) course (Figs. 186 and 187) on a flood plain of low slope. This results primarily from the fact that its sluggish current is turned against the banks easily by irregularities of the channel, by the currents of tributary streams, and in other ways. The current cuts into the banks where it strikes them. As it issues from a cut in the bank, it is directed against the opposite bank a little farther downstream, and forms a curve there. The development of this bend leads to the formation of another, and so on. As erosion continues, the cuts tend to become smooth curves, better adapted to the regular movement of the current. At the same time the stream erodes these curves, where the current is relatively swift, it builds up to flood level the opposite side of the channel, where the water is slack. In this way it comes to follow a more or less regu- larly curved course suited to its volume and gradient. As 180 PHYSICAL GEOLOGY the process of cut-and-fill continues, the curves change in outline as suggested by Figure 188. Finally the stream cuts through the narrowing neck of land between the two limbs of a meander (Fig. 189). The cur- rent now abandons the old round- about course because the new route is steeper. The old channel is isolated presently by the shift- ing of the stream to another posi- tion on its flood plain, or by at the ends of the FIG. 188. Diagram showing development of a meander. The current directed against deposition the downstream side of the . , ,. meander is on the average abandoned meander, Whose Stand- stronger than that directed ing 'waters check the edge of the against the upstream side, m, ,,. , , and therefore the growing current. The resulting lake is an meander migrates down the ox-bow lake. The flood plain of a great river, such as the Mississippi or Missouri (Plate IX), may contain numerous lakes, which record recent changes in the position of the river (Fig. 141). The extent to which certain great rivers are shifting thei;- channels is shown by surveys of their courses. Figure 190 shows the changes that occurred in the position of a por- tion of the Missouri River between 1852 and 1879, and between the latter date and 1894. It shows also the tendency of the meanders to work down the valley. Ox-bow lakes, like lakes of other - rrn. FIG. 189. A recently de- origin, are temporary features. They veloped cut-off, are filled gradually (1) by the en- What shows that it is of re- croachment of marsh vegetation upon their shallow borders, (2) by silts deposited in them during ex- ceptional floods, (3) by wind-blown material, and (4) by wash from the surrounding land. Doubtless many generations of lakes are made and destroyed during the formation of great flood plains. 182 PHYSICAL GEOLOGY The materials and structures of flood plains. As already implied, the materials of flood plains range from coarse gravel to finest mud. The coarser material deposited by a river is confined in general to the vicinity of the channel, where the velocity of the overflow is checked first and most. This grades more or less irregularly into the fine muds which gather in the quiet back- waters. When the river changes its position on its valley floor, the coarser deposits along the new channel cover finer de- posits made at a distance from the old channel, whose coarser material is in turn buried with fine. Frequent changes in the position of the aggrading FIG. 190. Map showing changes in the course of a portion of the Missouri River. river result in many ver- tical alternations in coarseness among its sed- iments. Minor variations may be brought about by the unequal strength of the overflow, capable of moving particles of varying size to a given place at different times. Further complexity in the distribution of the materials of a flood plain is introduced by irregular contributions made by wash frojft the bluffs and by tributary streams. Figure 185 shows the general structure of stream-laid beds. The structure described above has made it possible to determine that certain ancient formations were laid down on the land by rivers, and not in lakes or the sea.. THE WORK OF STREAMS 183 FIG. 191. Diagram of high alluvial terraces. Alluvial terraces. Under new conditions, a river which has been depositing may find itself underloaded. The change may be due to a movement of its valley resulting in a steeper gradient, to an increase of volume, to a decrease in the amount of sediment received from its head- waters, or to still other causes. Whatever the cause, the river, if greatly underloaded, sinks its channel rapidly. The remnants of the old flat then stand as alluvial terraces on one or both sides of the valley (Fig. 191). After the river has opened out a new flood plain at a lower level, for some reason it may again degrade actively, leaving a second set of terraces. Indeed, this pro- cess may be repeated a number of times. If a river which has been aggrading is able, under changed conditions, to de- grade, but remains nearly loaded, it may shift from side to side of its valley while it slowly lowers its channel, and by this means form a series of terraces. This is illustrated in Figure 192, where a stream is supposed to have filled its valley to the level A-D-B, and to occupy a position near the left edge of its flood plain, at A. If the stream now shifts toward the op- FIG. 192. Diagram to illustrate the ., >-, f ,, n formation of terraces by a river which is P OSlte Slde f the valley, degrading slowly, and shifting from side meanwhile degrading, it will occupy presently the position C. Should movement to the right stop there, be- cause of contact with a projection of the valley wall, or for some other reason, and the river return toward the left side of the valley, a remnant of the old flood plain, C-D-B, would remain as a terrace. In similar manner, should the river fail to reach the left side of its valley on the re- 184 PHYSICAL GEOLOGY turn swing, a terrace would result, as at E-F-A. Many terraces at successively lower levels might result from a continuation of this process. These terraces might extend a considerable dis- tance along the val- ley, or only a short distance, and their width might vary notably. It is evi- dent that when formed in this way, terraces upon oppo- site sides of a valley will not correspond in elevation. Small ter- races are common even in young val- leys, where they are due in many cases to the fact that, as the streams degraded, they also shifted their positions laterally. Terraces may be destroyed wholly or in part by the widen- ing of the flood plain at a lower level. Indeed, since the goal of stream-borne waste is the sea, the depositional fea- tures discussed in the preceding paragraphs may all be regarded as composed of material which has been dropped only tempo- rarily by overloaded streams, and which sooner or later will resume its journey to the ocean. Many cities are located partly or wholly upon the terraces of great rivers. Peoria, Illinois FIG. 193. Delta of the Mississippi River. FIG. 194. Delta of the Nile River. The dotted area is desert. THE WORK OF STREAMS 185 (Plate II) ; Dubuque, Iowa ; and Hartford, Connecticut, are examples. Miller and Crown City (Plate IV) are examples of hundreds of villages situated similarly. Deltas. Some of the material which rivers bring to the sea or to lakes is carried away by waves and currents ; much of it often accumulates off the mouths of the rivers, especially if they flow into tideless or nearly tideless bodies of water. FIG. 195. The delta of the Alsek River, Alaska. Shows numerous dis- tributaries. (Netland, U.S. Boundary Commission.) Such deposits may form deltas (Fig. 93). Deltas are so called from the Greek letter (A) of that name, whose shape they occasionally resemble (Fig. 194). As the current is checked at the mouth of a river flowing into the sea, the coarsest of the sediment is dropped first, forming slanting beds, whose angle of slope is determined largely by the size and shape of the material. The finer sediment settles less rapidly, and is spread by waves and tides over a larger area in nearly horizontal sheets. As dep- osition continues, the steeper beds of coarser material are built out upon the nearly level beds of fine. When this submarine embankment is built up close to the surface of the water, it becomes in effect an extension of the river bed, across which the projected current rolls and drags material, and upon which it deposits a part of its load. Deposition on the submarine platform is most active along the edges of the river current, because of friction with the relatively B. & B. GEOL. 11 186 PHYSICAL GEOLOGY quiet sea water. Thus levees develop, and the delta is built above the level of the sea. As the original channels across the delta gradually fill with sediment, some of the water breaks over the sides, following new courses to the sea and building up the different parts of the delta in turn. By this means a complicated system of distributaries may be formed (Fig. 195). Portions of the shallow sea covering the submarine platform are sometimes inclosed by the river deposits or between them and the old shore line, forming delta lakes. This was the origin of Lakes Borgne and Pontchartrain, on the delta of the Mississippi near New Orleans (Fig. 193). Extensive deposits of peat may accu- mulate in delta lakes and swamps. Apart from the shallow basins of their lakes and marshes, and the low ridges along their distributaries, the land surfaces of great deltas are nearly level, FIG. 196. - Profile and section of a delta. continuing the glope of the flood plain farther up river. The upper beds of a delta, de- posited by the river upon the submarine flat, are nearly hori- zontal. Deltas are accordingly characterized by three sets of beds (Fig. 196). The bottom and top beds are nearly horizontal, while the middle beds are inclined more or less steeply. Deltas grow at very unequal rates. The ratio between the volume of sediment brought by the river and the strength of waves and currents off the river mouth is a chief determinant. The Mississippi brings down about 7,500,000,000 cubic feet of sediment a year ; and as the tides of the Gulf of Mexico are weak, the delta is being extended seaward off the mouths of the main distributaries at the rate of about a mile in sixteen years. It appears to have grown at about this rate for many years. An English writer reported in 1770 that the Balize, a small fort built by the French on a little island which was at the mouth of the river in 1734, was then two miles up. The depth of the water into which a delta is being built also in- THE WORK OF STREAMS 187 fluences the rate of its forward growth. Furthermore, great deltas are as a rule sinking slowly, and the relation of up- building to subsidence varies greatly. In some cases, for example the Mississippi and Ganges, rivers have built up their deltas faster than the region has subsided. In other cases, subsidence is so rapid as to prevent the building of deltas above the sea. In the Chesapeake Bay region recent subsi- dence has formed great estuaries, and the rivers are now building marshy bay-head deltas. The delta of the Mississippi has an area of over 12,000 square miles, and the compound delta of the Ganges and Brahmaputra rivers is between 50,000 and 60,000 square miles in extent (about as large as the state of Illinois) . As a result of long-continued subsidence and up-building, delta deposits may attain great thickness. Ancient delta beds of great thickness, their origin revealed by their structure, occur in certain localities, for example, in the vicinity of Puget Sound. They afford a record of the physical geography of the region at the time when the sedi- ments, later changed into firm rocks, were deposited. 1. How could one distinguish in the field between an ancient alluvial fan and an ancient delta ? 2. (1) What occasioned the building of the fan shown in Figure 197 ? (2) Is the front of the fan the same as when built? (3) If not, how has it been changed, and by what ? (4) Account for the trench which crosses the fan. (5) How may the miniature terraces within the trench be ex- plained ? 3. What are the gen- eral conditions which oc- casion the development of distributaries ? 4. What are all the ways in which Plate IX shows that the Missouri River is there a deposit- p IG . 19 7. _ A small fan on the beach of Lake ing stream? Michigan. 188 PHYSICAL GEOLOGY 5. Compare the downstream slope of the higher and lower terraces of a given valley. 6. Interpret Figure 198, indicating (1) the successive steps in the development of the features shown, and (2) how the several changes FIG. 198. Diagram of stream terraces. that are recorded in the work of the river may have been brought about. 7. Why are the materials brought by great rivers to the sea usually fine (though unequally so) ? 8. Interpret the fact that limestones containing marine fossils are sometimes found interbedded with delta deposits. SUMMARY The mission of running water is to wear the land to base level. The material it carries toward and to the sea is pre- pared for transportation largely by the agents of weathering, and in subordinate amount is worn from the rocks by the streams themselves. The irregular reduction of the land produces most of the familiar relief features of the surface, whose characteristics are determined by several factors, especially by the character and structure of the rocks from which they were carved, and the stage of development which they have reached. The waste of the land is often laid aside on its way to the sea by overloaded streams, forming topo- graphic features subject to later destruction by eroding waters or by other agencies. The getting of the land into the sea has been the great task of streams throughout all the geological ages since lands and seas existed, and the materials of the sedimentary rocks of existing lands represent for the most part the stream-borne THE WORK OF STREAMS 189 waste of ancient lands, brought to shallow seas which occupied the areas where the rocks occur. Ancient peneplains and other phenomena show that at various times in different places the streams of past ages have nearly completed their task, only to have it renewed when their basins were rejuvenated by a sinking of the sea or by an uplift of the land. It is evident from the preceding pages that the activities of streams are of prime importance in shaping the present chapter in the history of the earth. It will be seen in sub- sequent pages that the results of stream activity, with which the student is now familiar, are likewise of prime importance in deciphering the earlier chapters of the earth's history. REFERENCES BONNET : Rain and Rivers as Sculptors and Rivers as Transporters, in The Story of our Planet, pp. 103-142. (London, 1893.) DARTON : Examples of Stream-robbing in the Catskill Mountains, in Bull. Geol. Soc. of Am., Vol. VII, pp. 505-507. DAVIS : The Seine, the Meuse, and the Moselle, in Nat. Geog. Mag., Vol. VII, pp. 189-202, 228-238. The Rivers and Valleys of Pennsylvania, in Nat. Geog. Mag., Vol. I, pp. 183-253. Stream Contest along the Blue Ridge, in Bull. Geog. Soc. of Phil., Vol. Ill, pp. 213-244. Geographic Cycle in an Arid Climate, in Jour, of Geol., Vol. XIII, pp. 381-407. The Development of River Meanders, in Geol. Mag., N. S., Decade IV, Vol. X, pp. 145-148. - River Terraces in New England, in Bull. Harvard Mus. of Comp. Zool., Vol. XXXVIII, pp. 281-346. The Physical Geography of Southern New England, in Physiog- raphy of the United States, pp. 269-304. (New York, 1895.) DAVIS and WOOD: The Geographic Development of Northern New Jersey, in Proc. Bost. Soc. Nat. Hist., Vol. XXIV, pp. 365-423. DODGE : The Geographical Development of Alluvial River Terraces, in Proc. Bost. Soc. Nat. Hist., Vol. XXVI, pp. 257-273. DUTTON : Tertiary History of the Grand Canon District; Mono. II, U.S. Geol. Surv. GANNETT: Profiles of Rivers in the United States; Water Supply and Irrigation Paper No. 44, U.S. Geol. Surv. 190 PHYSICAL GEOLOGY GILBERT : Lind Sculpture, in Geology of the Henry Mountains. pp. 99-150 ; U.S. Geog. and Geol. Surv., Rocky Mtn. Region. (Washington, 1877.) Niagara Falls and their History, in Physiography of the United States, pp. 203-236. (New York, 1895.) GOODE : The Piracy of the Yellowstone, in Jour, of Geol., Vol. VII, pp. 261-271. HAYES : The Southern Appalachians, in Physiography of the United States, pp. 305-336. (New York, 1895.) Physiography of the Chattanooga District, in 19th Ann. Rept., U.S. Geol. Surv., Pt. II, pp. 1-58. JEFFERSON : Limiting Width of Meander Belts, in Nat. Geog. Mag., Vol. XIII, pp. 373-384. JOHNSON, L. C. : The Nita Crevasse, in Bull. Geol. Soc. of Am., Vol. II, pp. 20-25. POWELL : Exploration of the Colorado River of the West and its Tribu- taries. (Washington, 1875.) RUSSELL : Rivers of North America. (New York, 1898.) SALISBURY : The Physical Geography of New Jersey; N.J. Geol. Surv., Vol. IV, pp. 65-154. SALISBURY and ATWOOD : The Geography of the Region about Devil's Lake and the Dalles of the Wisconsin; Wis. Geol. and Nat. Hist. Surv., Bull. No. V, Chs. Ill, IV. SHALER : Rivers and Valleys, in Aspects of the Earth, pp. 143-196. (New York, 1889.) WALCOTT : The Natural Bridge of Virginia, in Nat. Geog. Mag., Vol. V, pp. 59-62. WILLIS: The Northern Appalachians, in Physiography of the United States, pp. 169-202. (New York, 1895.) CHAPTER VI GLACIERS CHARACTERISTICS OF GLACIERS Formation of snow fields and ice fields. When the water vapor of the air condenses at temperatures below the freezing point, it is usually as ice crystals, which form snowflakes. Above an irregular surface in the air all points in which have a temperature of 32 Fahrenheit (the isothermal surface of 32), condensing water vapor accordingly usually forms crystals of ice, many of which become snowflakes, while below it the moisture condenses as water, and forms cloud particles or rain- drops. This surface of 32 Fahrenheit is encountered at vary- ing altitudes. It is high near the equator (15,000 to 18,090 feet above sea level), and is at sea level in certain polar regions. In many places, as in northern United States, for example, its position varies notably with the seasons; it is higher in summer and lower in winter. In sufficiently high places in low latitudes and over wide areas in high latitudes, it is at or near the surface during much or all of the year. In such situ- ations more snow falls in the colder months than is melted and evaporated in the warmer ones. The line above which snow is always present is called the snow line. While the position of the snow line is influenced chiefly (1) by temperature, it varies also with (2) the amount of snowfall, being lower when the snowfall is heavy and higher when it is light, and (3) the character of the topography, for some situations favor the gathering of snow and afford protection against the sun, while others do not. In general it does not depart greatly from the summer position of the isothermal surface of 32, 191 (192) GLACIERS 193 Long-lived accumulations of snow constitute snow fields (Figs. 199 and 200). Snow fields become ice fields by the same processes which transform many snow banks into ice banks each winter. The bottom snow is compressed by the weight of that above and becomes more and more compact, the result being much as when snow is packed into an icelike mass in making snow- FIG. 200. Snow fields of Monte Rosa, Switzerland. (R. T. Chamberlin.) balls. Water from rains and from surface melting during the warmer periods sinks into the snow beneath, and when it freezes helps to cement the mass. Still other processes aid in the change, and the originally loose snow passes by degrees into compact ice. Formation of glaciers. When the ice has formed in suf- ficient quantity, it begins to spread from the place of origin. If formed on plains or plateaus, ice fields are thickest at or near their centers, thinning more or less regularly to the mar- gins, where wastage balances snowfall. In such situations the ice accordingly moves slowly under its own weight in all direc- tions from the center. If formed in and about the heads of mountain valleys, snow fields and ice fields acquire a slow movement down valley. When ice fields start to move, they become glaciers. 194 PHYSICAL GEOLOGY A glacier spreading in all di- rections from its center on a plain or plateau is an ice sheet or ice cap (Fig. 212). Glaciers confined to valleys are valley glaciers (Figs. 201, 202, and 203, and Plate X). Com- pound glaciers formed on plains or plateaus at the base of moun- tains by the union of valley glaciers which have spread out in front of the mouths of their mountain valleys, are piedmont (foot of the mountain) glaciers (Fig. 211). VALLEY GLACTERS Distribution and size. There are hundreds of valley glaciers among the mountains of Alaska, western Canada, and northwestern United States. Here high mountains near the coast force the vapor-laden ocean winds to precipitate much moisture in the form of snow. Seward Glacier, the largest valley glacier in Alaska, is over 50 miles long and 5 miles and more wide. Very few glaciers in the United States are more than a mile long. There are nearly two thousand glaciers in the Alps Mountains. The longest of 195 196 PHYSICAL GEOLOGY them measures over 10 miles, but the great majority are less than a mile in length. They vary in width from a few hundred feet in the case of the great majority, to a mile or more. The FIG. 202. Glacier de la Brenva descending from Mont Blanc on the Italian side. (R. T. Chamberlin.) largest ones are several hundred feet thick. Large valley gla- ciers occur also in the Caucasus, Himalaya, and other mountains. Feeding grounds. Deep snow fields occupy the heads of mountain valleys containing glaciers. Fed by snowfall in the valley, by avalanches from the inclosing slopes, and by wind- GLACIERS 197 swept snow from the surrounding crags and peaks, the snow fields constitute feeding grounds for the glaciers which descend from them. The larger part of the snow of such fields is really granular, half -formed ice (neve), mantled and bordered with recently gathered snow. Movement of glaciers. Most glaciers move with extreme slowness. Other things being equal, a glacier moves faster FIG. 203. Glacier des Grandes Jorasses and the Italian face of the Grandes Jorasses. Chain of Mont Blanc. (R. T. Chamberlin.) when it is thick, when the slope of its surface is considerable, when its bed is steep and regular, and when its temperature is relatively high, than it does under the opposite conditions. The glaciers of the Alps move on the average a foot or two a day, while some of the great glaciers of Alaska and Greenland move several times as fast. Certain Greenland glaciers have been credited with the very unusual rate of 50 feet and more per day. From what has already been said, it is evident that glaciers move faster in summer than in winter. The ice of a glacier also moves more rapidly in the center at the surface, than along the bottom and sides (Why?). Since in the gathering 198 PHYSICAL GEOLOGY FIG. 204. End of the Alsek Glacier, Alaska. Com mission . ) (Netland, U.S. Boundary ground of a glacier the surface of the snow and ice is usually concave, the movement is inwards toward the center, as well as down valley. Farther down the valley, the surface is commonly convex, in part because the marginal ice is melted faster by heat reflected from the walls of the valley, and there is accordingly movement toward the sides of the valley, as well as along its axis. FIG. 205. The Zwillinge and Grenz Glaciers, Switzerland. Shows debris on the ice, crevasses, etc. (R. T. Chamberlin.) GLACIERS 100 The exact nature of the movement of glacier ice is a mooted question. The effect, so far as the form of the glacier is con- cerned, is much the same as in the movement of a thick mass of tar or wax. It is doubt- ful, however, if the mo- tion is flowage. Lower limits of gla- ciers . Glaciers descend from their parent snow fields to a level so low and so warm that the wastage of the ice bal- ances its forward move- FIG. 206. Muir Glacier, Alaska. ment. Many large gla- ciers reach far below the snow line ; some of those in Switzerland end near grain fields and orchards. In high latitudes glaciers may reach the sea (Fig. 204). Turbid streams, fed by the melting ice, flow from the lower ends of many valley glaciers (Fig. 202) . Character of the surface of valley glaciers. The surfaces of valley glaciers are in many cases notably irregular (Figs. 205 and 206). Varying in compactness, the surface ice melts unevenly. Changes in the slope of the surface down which the glacier moves cause the ice to crack open (Fig. 207) . Where steep or precipi- tous descents occur in the bed, icefalls correspond- ing to waterfalls in rivers form, and the ice is often shattered by a multitude FIG. 207. Portion of a glacier, showing o f Cracks. Great Cracks crevasses in the ice due to changes in the , N , c -, slope of the bed. (crevasses) may be formed also by the more rapid motion of the center of the glacier, as compared with the sides. One or more crevasses, often large, sometimes form where the neve of the lower part of the parent snow field moves away 200 PHYSICAL GEOLOGY from the thinner snows of the portion above. This fissure, or zone of fissures, where the glacier proper is sometimes con- sidered as beginning, is called the bergschrund (Fig. 208) . The FIG. 208. Bergschrund on east side of Fremont Peak, Wind River Range, Wyoming. (Baker.) upper walls of crevasses formed in these or other ways, being more exposed to the sun and weather than the lower walls, melt faster, so that the openings often become conspicuously V-shaped, and are separated by a complex of crests and sharp ridges. Were it not for melting follow- ing cracking, most crevasses extending crosswise of a glacier would probably be closed by its onward movement. Rock debris weathered from the slopes L^^H above may accumulate in quantity on FIG. 209. - Rock-capped the ice - If such fragments are too ice pillars. The rock thick to be heated through in the course retards the melting of / i ,-, J.T_ T_ 0.1 the ice on which it rests, of a da y> the y Protect the ice beneath, and the melting away The surrounding ice melting mean- while > the y COme to stand On Columns of ice (Fig. 209). Thin deposits of earthy matter such, for example, as wind-deposited dust, have an opposite effect. Dust absorbs heat faster than ice does, and thin deposits, heating through readily in the course of a GLACIERS 201 FIG. 210. The spreading end of a glacier, Alaska. (Brabazon.) day, occasion the relatively rapid melting of the ice below. Depressions known as dust wells result. Miniature dust wells may often be seen in rapidly melting snow banks. Summer FIG. 211. The Malaspina Glacier, and numerous valley glaciers, right by Univ. of Wis.) (Copy- 202 PHYSICAL GEOLOGY melting of the surface ice in the lower portion of a glacier sometimes forms streams which cut ice valleys in the glacier. The above considerations help to explain the rough, broken surfaces of such glaciers as shown in Figure 206. Travel across them is difficult and often dangerous. PIEDMONT GLACIERS Unrestrained by valley walls, glaciers which extend beyond the mouths of their mountain valleys tend to spread (Fig. 210), and may come to occupy a considerable area. As already indicated, when several glaciers descending from neighboring mountain valleys spread out along the base of the mountains, they may unite to form a piedmont glacier. The Malaspina Glacier of Alaska is the type example of this class (Fig. 211). It is about 1500 square miles in ex- tent (larger than Rhode Island), and its stagnant margin is covered deeply with rock waste which locally supports a dense forest. ICE SHEETS Some ice sheets or ice caps are rudely circular, and others are irregular in form. The largest attain great size. South polar ice sheet. Antarctic explorers have made known the existence of a great ice FIG. 212. Map of Greenland ice sheet, s heet surrounding the GLACIERS 203 South Pole. Its area is not known, but it is believed to be more than 3,000,000 square miles (about the size of the United States, exclusive , of Alaska). The ice moves slowly outwards toward the margins of the ice sheet, where great masses are detached as icebergs, and float away. The Greenland ice sheet. Save in a narrow, rugged coastal strip, all Greenland is covered deeply with ice and snow (Fig. 212). The area of the ice is probably some 400,000 to 500,000 square miles (seven to nine times as large as the state of Illinois), and its thickness toward the center more than a mile. Occasional mountain tops (called nunataks) rise as islands through its marginal portions. Close to its edge the ice contains many crevasses and carries more or less rock rubbish on its surface, but over the vast interior the sur- face is smooth and free from rock material. Thinning to- ward the coast, the ice sheet in places gives off great arms, which move along the valleys, FlG 213 _ An iceberg often reaching the ocean. From the ends of these glaciers, some of which rise as cliffs 200 or 300 feet above the sea, great masses are detached, and floated away as icebergs (Fig. 213). Icebergs from Greenland are carried south by ocean currents and winds to the latitude of Newfoundland, and sometimes beyond. Rock material that was frozen in the glaciers is carried away by the icebergs and as they melt it is dropped on the ocean floor. Icebergs, however, are not important agents of trans- portation, and most of what they carry is soon dropped. Small ice caps occur on various Arctic islands. Apart from the geological work which existing ice sheets are doing, and their climatic and other influences, they are inter- esting because th^y make it easier to understand the former existence of great ice sheets in regions now free from ice. 204 PHYSICAL GEOLOGY ANCIENT GLACIERS In much of Canada, in the United States east of the Mis- souri River and north of the Ohio, and in northern Europe, the mantle rock consists of a mixture of bowlders, gravel, sand, and clay, ranging in thickness from a few inches to more than 500 feet. These materials occur separately in some places, and elsewhere are mixed confusedly in all possible proportions. This mantle rock was not produced by the weathering of the underlying rock, for in any given locality it contains material to which the decay of the bedrocks of that locality could not give rise. This fact is further shown by the contact between the FIG. 214. Diagram showing gradual transition from residual soil into the unaltered rock below. (U.S. Geol. Surv.) mantle rock and the underlying rock. Mantle rock formed in place normally grades more or less insensibly into the firm rock below (Why? Fig. 214). In the areas in question, however, the surface material gives place abruptly in most places to the unaltered rock beneath as suggested in Figures 229 and 230. The mantle rock of these areas, therefore, was brought to its present position by one or more of "the agents which transport materials upon the land. It is known as drift, the term having been applied under the impression that it had been drifted by waters to its present position from outside sources. Figure 215 shows a typical exposure of unstratified drift (till). As shown in the illustration, till consists usually of material of many kinds and sizes, and is not in layers. The stones and bowlders are sometimes of kinds which do not occur as bedrock within many miles. Some of them are subangular in form and have flat faces, often highly polished and covered with minute scratches (Fig. 231). The drift GLACIERS 205 is often disposed unevenly, so as to occasion hilly belts and undrained depressions (Fig. 226 and Plate XII). The trans- porting agent, therefore, gathered its load from an area FIG. 215. Section of unstratified drift near Henry, Illinois. (Crane.) FIG. 216. Shows the accumulation of drift beneath an existing glacier. Extremity of the lower Blase Dale Glacier of Disco Island, Greenland. (U.S. Geol.Surv.) B. & B. GEOL. 12 206 PHYSICAL GEOLOGY large enough to yield many different kinds of rock, and was capable of carrying large bowlders as well as fine clay, some- times for great distances. It was capable, furthermore, of giving a part of the stones it carried the characteristics noted above, but was incapable of arranging its irregular deposits FIG. 217. Map showing the areas in and about the borders of North America covered by ice at the maximum stage of placiation. The ur- shad">d parts wore covered by ice; the dotted portions were land aro;.s free from i.-c. (Modified after Willis.) GLACIERS 207 in layers. It is evident that the transporting agent in ques- tion was neither the wind nor running water. The size of much of the material would at once exclude the former, while various considerations as effectually dispose of the latter. The largest bowlders of the till, weighing many tons, are far beyond the transporting power of common streams. Streams tend to round the stones rolled along their channels, FIG. 218. The shaded area shows the part of Europe covered by the con- tinental glacier at the time of its greatest extent. (James Geikie.) and are unable to develop flat faces. Stream-laid beds are in layers. The surfaces of water-deposited beds are without notable irregularities, such as often characterize the till. Figure 216 shows deposits being made beneath a Green- land glacier that possess all the characteristics of those shown in Figure 215. So far as observed, all the deposits being made by existing glaciers show these same characteris- tics. Since existing glaciers are developing exactly the features belonging to the drift of the great American and 208 PHYSICAL GEOLOGY European areas referred to, and since no other agent is known capable of doing so, it has been concluded confidently that these regions were formerly covered by glacier ice. These glaciers were as extensive as the till is widespread, and therefore a,re known to have covered at their maximum development the areas shown in Figures 217 and 218. In a similar way other great areas in various parts of the world are known to have been glaciated at still earlier, but widely separated times (pp. 337, 394). Some of these areas are within the tropics, and now enjoy very warm climates. Glaciers are, then, one of the great geological agents that have modified numerous ancient as well as present land surfaces. Various phases of their work may be studied in most parts of northern United States. THE GEOLOGICAL WORK OF GLACIERS Like winds and rivers, glaciers transport rock waste, wear the surfaces over which they move, and deposit their loads to form characteristic features. TRANSPORTATION AND DEPOSITION As snow gathers to form a snow field it surrounds and covers loose pieces of rock on the surface, and incloses pro- jecting ledges of firm rock. When the snow field becomes an ice field, and begins to move, it carries much of the loose material in its bottom with it, and may also break off and remove pieces of the bedrock, so that the glacier has a load from the beginning. Wherever the water in the soil upon which the glacier advances is frozen, it cements the soil particles into a firm mass. Wherever this ice-cemented soil is frozen to the glacier ice above, it becomes, in effect, a part of the glacier, and is likely to be carried on by its further movement. Most of the material carried by ice sheets, and possibly also much of that transported by many valley gla- ciers, is gathered in these and other ways by the under sur- GLACIERS 209 face of the ice. The material moved in the bottom of a glacier or lodged beneath it constitutes the ground moraine. The ground moraine deposits of the ancient ice sheets were FIG. 219. Diagram to show how debris in the body of a glacier may come to be on top through the lowering of the surface of the ice by melting. frequently pressed by the weight of the overlying ice into very dense, compact beds. These are sometimes called hardpan. Valley glaciers often carry heavy loads of rock debris on their surfaces. This is partly material weathered from the mountain slopes above, partly material worn from elevations in the bed of the glacier and brought to the surface by the melting of the ice above (Fig. 219), and partly also material FIG. 220. Moraine on south side of Hayden Glacier, in west-central Ore- gon. Note the constitution of the moraine. West Sister Peak, Cascade Mountains, in background. (Russell, U.S. Geol. Sum.) 210 PHYSICAL GEOLOGY brought up ! in .-other ways from the bottom. Belts of sur- face debris on the sides of valley glaciers are called lateral moraines (Figs. 220 and 221). If valley glaciers melt away, their surface lateral moraines are deposited on the valley floor beneath, along with material left by the bottom ice which moved from the center to the sides of the glacier (p. 198). In most cases the latter material makes up much FIG. 221. The Corner Glacier with its feeders the Grenz, Schwarze, Brei thorn, and Theodule Glaciers. Shows lateral and medial moraines, and the sources of the morainic debris. (R. T. Chamberlin.) the larger part of the deposits. Lines of surface debris in or near the center are medial moraines (Fig. 221). In many cases medial moraines are the result of the union of two valley glaciers, whose adjacent lateral moraines have joined and occupy a medial position on the main glacier. Debris on the surface of a glacier near its head may be buried by accumulations of snow and carried forward in the body of the ice. Surface material may also work its way through GLACIERS 211 FIG. 222. Sketch of a valley glacier in western Canada, showing terminal moraine. cracks in the ice toward or to the bottom. On the other hand, material may be brought in different ways to the surface of a glacier from a position within or beneath the ice, as noted above. Material carried at the bottom of a glacier may be dropped and picked up again many times before reach- ing a final resting place. Debris may lodge just beyond elevations over which the ice has passed. Moving vigorously over surfaces yielding material readily, the ice may obtain a load which later, under new conditions, it cannot carry. At its end, the moving ice is melting continually, the excess of forward movement over melting being the measure of its advance Material obtained by the glacier back from its end will therefore, if not dropped, find itself sooner or later at the end, where it will be deposited as the inclosing ice melts. Overridden by the advancing glac er, it may be taken up once more, to be dropped again after a longer or shorter journey. Where the end of a valley glacier, or the edge of an ice sheet, remains essentially stationary for a long time, a heavy deposit results at and beneath the margin of the ice. This is called the terminal moraine (Fig. 222 and Plate XII). Obviously, the longer the margin of the ice remains stationary, the larger the terminal moraine becomes. Very massive terminal moraines left by ancient glaciers accord- ingly register very long stands of the margin of the ice. The terminal moraines of valley glaciers are more or less cresceiitic, the convex side pointing down valley (Fig. 222 ). 212 PHYSICAL GEOLOGY As already indicated, glacier deposits are unstratified, and consist commonly of materials of many kinds and sizes. Ice-ground clays usually retain the chemical character of FIG. 223. Drumliri near McFarland, Wis. (Alden, U.S. Geol. Surv.) the parent rocks. Stream-borne silts, in contrast, are gen- erally the product of weathering, and therefore differ chemically from the rocks from which they were derived. Melting ice has sometimes left great bowlders in seemingly insecure positions, forming " perched bowlders," " rocking stones," etc. In some places till has lodged beneath ice sheets to form oval hills, called drumlins (Figs. 223 and 224). Drumlins vary in length from less than 100 feet to more than a mile, and in height from 15 or 20 to 150 or 200 feet. They are common in eastern Massachusetts, in parts of New York and Wisconsin, and in some other localities. In contrast with FIG. 224. Drumlin one mile northeast of Gleasondale, Mass. (Alden, U.S. Geol. Sun.) ice-worn rock hills (p. 219), the shorter and steeper slopes of drumlins generally face the direction from which the ice sheets came. The surfaces of glacier deposits are characteristic (p. 205). PLATE XI. FIG. A. DRIFT TOPOGRAPHY. Contour interval, 20 feet. Scale, about 1 mile per inch. (Dexter, Michigan, Sheet, U. S. Geol. Surv.) FIG. B. TOPOGRAPHY DEVELOPED BY STREAM EROSION. Contour inter- val, 50 feet. Scale, about 2 miles per inch. (Lawrence, Kansas, Sheet, U. S. Geological Survey.) 214 PHYSICAL GEOLOGY The drift is usually disposed irregularly, so that mounds and hills without systematic arrangement are associated with depressions of varying form and size, many of which have no outlets. The streams of recently (geo- logically speaking) gla- FIG. 225. Section of a lake lying in. a ciated regions COm- monly follow aimless and roundabout courses, and in many cases are interrupted by lakes and marshes (Plate XI, Fig. A). All this is in contrast with topographies due to river erosion. Since such topog- raphies have resulted from the cutting of valleys, the ele- vations are distributed systematically with reference to the depressions, all of which have outlets (Plate XI, Fig. B). As we have already seen (p. 207), it is in contrast, too, with topographies due to stream deposition. The lake basins and other surface hollows of drift areas have been formed in several ways. Some are sections of preglacial river valleys in which drift was deposited unevenly. Where the ice deposited more material around than on a given area, the latter came to stand lower than its surroundings (Fig. 225). Still other basins were gouged out of the underlying rocks by the ice. The thousands of lakes in the northern part of the United States are practically all of glacial origin. The features described above as distinctive of drift sur- faces are most pronounced in terminal moraines, which are often characterized by notably hummocky topography (Figs. 226 and 227, and Plate XII). Numerous mounds, hillocks, and short ridges, ranging in diameter from a few feet to a half mile and more, and reaching occasionally a height of 100 to 200 feet, are associated with depressions varying in depth from inches to scores of feet, and in area ranging up to many acres. Many of the depressions contain ponds or lakes. Elevations and depressions are huddled together in confusion. Ground moraine surfaces are usually less 215 PLATE XII. TERMINAL MORAINE AND OUTWASH PLAIN. Contour in- terval, 20 feet. Scale, about 1 mile per inch. (St. Croix Dalles, Wisconsin- Minnesota, Sheet, U. S. Geological Survey.) 216 PHYSICAL GEOLOGY irregular. Hollows are not so deep, swells are not so high, and slopes are gentler. Certain ground moraine drift plains are almost flat. FIG. 226. Terminal moraine topography six miles southwest of Glen- buelah, Sheboygan Co., Wis. (Alden, 'U.S. Geol. Surv.) The deposition of drift may render a surface rougher than before (Fig. 228), or may reduce the relief (Fig. 229). FIG. 227. Terminal moraine topography near Oconomowoc, Wis. The elevations are kames. (Alden, U.S. Geol. Surv.) GLACIERS 217 The latter seems to have been the result over most of the lake and prairie plains in northern United States. FIG. 228. Diagram showing how a nearly level surface may be replaced by a rough one through the uneven deposition of drift. TOPOGRAPHIC FEATURES DEVELOPED BY GLACIER EROSION How glaciers erode. Since it is much softer than rock, pure ice accomplishes little or no wear upon smooth, firm surfaces ; rather is it worn by the harder rock. As already indicated, however, the bottom ice is likely to be charged with rock frag- ments, and thus armed, glaciers become efficient agents of erosion. Their rock tools are pressed with tremendous force upon the surfaces over and against which they move, and each kind does its appropriate work. Clay particles tend to smooth and polish, sand grains and hard pebbles to scratch (striate), and bowlders to gouge and groove the bedrock (Fig. 230). Meanwhile, the tools are themselves worn. The weaker ones may be ground into fine bits, even to rock flour. The stronger ones often are marked typically ; their sides are worn flat, and, like the bed- rock, are polished by clay and striated by sand (Fig. 231). Thick ice moving over much-jointed surfaces sometimes quarries out blocks of rock by a process known as plucking. The bottom ice is pressed by the great weight of that above into the joints, bedding planes, and other openings of the rocks, and as the glacier moves onward, fragments, some- FIG. 229. Diagram showing how glacial drift may be so disposed as to replace a hilly surface with a comparatively level one. 218 PHYSICAL GEOLOGY FIG. 230. Glaciated rock surface. The view shows also the relation of drift to the bedrock beneath. Northern Ohio. (Stauffer.) times of considerable size, are dislodged. The freezing of water in the openings of the rocks beneath the ice helps in the process. General effect of erosion upon relief. Other things equal, ice sheets erode most in regions where many slopes oppose the advance of the ice. In flat regions the frozen mantle rock has sometimes been overridden by thick ice, and little disturbed. In rugged regions ice sheets tend to FIG. 231. Glaciated stones. GLACIERS 219 plane away the angularities of the surface, reducing and smoothing the slopes. Where hilltops are worn, the tend- ency is to reduce the relief. Where glaciers move along the axes of valleys, they tend to widen and deepen them, and so to increase the re- lief. Ice-worn hills and ba- sins. Hills that have been eroded vigorously by ice sheets are usually of characteristic form (Fig. 232). The Side against and FIG. 232. Lamberts Dome. A glaci- , . , , , . , ated hill of granite. Upper Tuolumne up which the ice moved River> (Fairbanks.) (the stoss side) suffered most wear, and was lengthened and smoothed. The side away from and down which the ice moved (the lee side) is commonly the shorter and steeper, and was sometimes left rough and irregular by plucking. Where it crosses valleys and basins, and erodes them, an ice sheet usually wears chiefly the sides opposed to its advance, making them gentler and smoother (Fig. 233). The shapes of glaciated rock hills and basins, then, record the direction of movement of ancient glaciers, the longer and smoother slopes facing the direction whence the ice came. Since minute projections and depressions are similarly the ice > the FIG. 233. - Diagram showing sha P ed change which may be made in tion of any Small Surface of glaci- the cross section of a valley by t d bedrock wiU usua ll y snO W the an ice sheet which moves across J it. Dotted line shows side of direction in which the ice moved. valley before glaciation. ( How much CQuld be toid CQn . cerning the direction of movement by the trend of the striae?) Ice-shaped valleys. Valley glaciers tend by erosion to widen and deepen their valleys and to steepen and smooth 220 PLATE XIII. A PORTION OF THE SIERRA NEVADA MOUNTAINS, SHOW- ING GLACIATED VALLEYS. Contour interval, 100 feet. Scale, about 2 miles per inch. (Mt. Whitney, California, Sheet, U. S. Geological Survey.) GLACIERS 221 their sides. Thus V-shaped valleys are changed to U- shaped troughs (Figs. 234 and 235, Plate XIII). The en- larged heads of glaciated valleys have broad bottoms, often containing ice-worn rock basins, and high, precipitous walls. Such valley heads are called cirques (Figs. 236 and 237, Plate XIII). In winter the neve and ice of the upper glacier freezes to the valley walls. In spring, the ice pulls away from them and dislodges and carries with it many rock fragments. During the summer the walls of the val- ley head may be more or less exposed to the agents of weathering, and ma- terial prepared for later removal by the ice. This process helps to drive the sides and head of the valley back into steep cliffs. Lakes dot the bot- toms of most glaci- FIG. 234. Glacial trough near Green River lakes, Wind River Range, Wyoming. Shows contrast between glaciated topography be- low, and unglaciated topography above. The lake in the foreground is held in by a morainic dam. (Baker.) ated valleys (Fig. 238, Plates X and XIII). Some of them occupy rock basins gouged out by the glacier (Fig. 239), and others fill depres- sions on the up-valley sides of morainic dams (Fig. 240). Tributary valleys normally join their main valleys at even grade. But main valleys are often deepened by glaciers more than their tributaries. Because of this, and because 222 PHYSICAL GEOLOGY FIG. 235. Glacial trough with hanging valleys. River. (Baker.) Upper canon of Green of the widening of the bottoms of the main valleys, the floors of the tributary valleys at their mouths are left stand- ing higher (sometimes 1000 feet or more) than the opposite bottoms of the main val- leys. After the disap- pearance of the ice, the streams of the tributary valleys descend in rapids or falls to the main streams. Such elevated tributary valleys are known as hanging valleys (Figs. 241 and 235). The same condition is of course brought about where a main valley is FIG. 236. View in the Bighorn Moun- tains, Wyo. The cirque in the back- ground contains Cloud Peak Glacier, which has a length of nearly a mile. The cirque walls are in places about 1500 feet in height. (Trowbridge.) glaciated, while its tribu- taries remain free from ice. The topographic fea- GLACIERS 223 tures described above occur in western United States and Can- ada, among the Alps, and elsewhere, in many valleys now ice- free. They have been more or less modified, however, since FIG. 237. Near view of the walls of a cirque. the disappearance of the glaciers, and will ultimately be de- stroyed. The glaciated rock surfaces not covered with drift are being weathered. The steep sides and heads of the val- leys favor landslides, the accumulation of talus, and the formation of alluvial cones. The streams are grading the of- ten irregular beds of the former glaciers, lowering the hanging valleys, and filling or draining the lakes. The relative extent of these changes in different valleys is a rough measure of the 224 PHYSICAL GEOLOGY relative amount of time which has elapsed since the glaciers melted away. Fiords. Where thick glaciers push into the sea through nar- row bays, they may scour the bay bottoms much deeper, and at the same time wear the bay heads back into the mainland. Where ancient glaciers have disappeared from such bays, the sea has en- tered to form long, narrow, steep-walled embayments, called fiords (Figs. 242 and 243). Typical fiords abound along the Norwegian, Alaskan (Fig. 244), and certain other high-lati- tude coasts. In most cases their depth is due partly to sub- mergence of the coast. Many islands fringe these shores, representing for the most part higher land whose lower surroundings were drowned. FIG. 238. Lakes of glacial origin in a moun- tain valley. The nearest lake is in an ice- scoured rock basin ; the others are held in by drift. Note the U-shaped cross section of the valley in the middle distance. Piney Creek Valley, Bighorn Mountains. (Trow- bridge.) FIG. 240. Section of a lake behind a barrier of drift. FIG. 239. Section of a lake lying in an ice- scoured rock basin. Which way did the glacier move which formed this basin ? GLACIERS 225 THE WORK OF WATERS ASSOCIATED WITH GLACIERS Water from the surface melting of summer and from rains sometimes forms streams that flow in valleys which FIG. 241. Hanging valleys, Lyngen Fiord, Norway. The hanging valley in the center contains a glacier. (R. T. Chamberlin.) they have cut in the ice (p. 202). Water also finds its way through cracks and crevasses to the bottom to form sub- Fio. 242. Troldfjord, Lofoten Islands, coast of Norway. (R. T. Cham- berlin.) 226 PHYSICAL GEOLOGY FIG. 243. Fiord at North Cape, Norway. Photograph taken at 12.08 A.M., July 7, 1909. (R. T. Chamberlin.) glacial streams. Subglacial waters are formed, too, by the melting of the bottom ice because of friction between the glacier and its bed, and in other ways. Ice-fed streams, in most cases heavily charged with gravel, sand, and silt, flow from the ends of val- ley glaciers, and at many points from the edges of ice sheets. The streams be- neath glaciers and beyond their ends and edges are com- tlPHi " 181P$ ; monlv {i.u^Tadintf rather than degrad- ing streams. There- fore the deposits made by glacial 152* op i3o- waters are the only FIG. 244. Alaskan fiords., 7 matters in connec- GLACIERS 227 tion with their work which need be discussed. Like other stream-laid beds, such deposits are in layers and consequently unlike the till deposited directly by the ice. FIG. 245. Diagram to illustrate the building of a valley train. Describe and account for what you see along the front of the ice sheet. Valley trains. Streams flowing away from glaciers in valleys of moderate slope are generally overloaded with debris derived from the ice and washed from tributary slopes beyond the ice. They therefore make deposits along their braided channels, building river plains of sand and gravel. Such aggradational plains are valley trains (Fig. 245). The stream deposits more and coarser material near the ice, and less and finer sediment farther from it. The downstream slope of valley trains is accordingly steepest near the ice and increasingly gentle away from it (Fig. 246). Much of the material of valley trains is cross-bedded. Many remnants of valley trains, in the form of terraces, occur along the rivers of northern and northeastern United States. The longer the edge of the ancient ice sheet from FIG. 246. Diagram of a valley train, showing the slope of its surface, its structure, and its relation to the terminal moraine in which it heads. which the aggrading streams issued remained stationary, the greater the valley fillings. Heavy valley train deposits, like massive terminal moraines, therefore indicate protracted stands of the edge of the ice. 228 PHYSICAL GEOLOGY Outwash plains. Where overloaded streams that issued from the ancient ice sheet did not find valleys for their ac- commodation, as was often the case, they spread their material in fanlike deposits in front of the ice. Many FIG. 247. Section of a such deposits made by neighboring lake at the margin of an s t rea ms often joined to form alluvial ice sheet. . ,77- plains, known as outwash plains, which slope gently away from the terminal moraines which they front (Plate XII). Deltas. Marginal lakes were sometimes formed at the edge of the ancient ice sheet where the land sloped down- wards toward the ice, forming a temporary basin (Fig. 247). Where streams issued from the ice at the edges of lakes, they deposited their loads in the form of deltas. Such deltas are common in parts of New England. Kames. The edge of the ancient ice sheet was doubt- less jagged and irregular (Why?). Subglacial streams, flowing in tunnels beneath the ice, were often under great pressure, like the water in a long tube. When such streams issued from beneath the ice in reentrant angles of its edge, the pressure, and therefore their velocity and carrying power, were reduced. This caused them to make deposits, which were shaped by the par- tially inclosing ice walls. In this way irregular mounds and hillocks of rudely stratified and water-worn material were formed in association with the unstratified deposits of the terminal moraine. Such deposits are called kames (Figs. 248 and 227). Eskers. Subglacial streams sometimes deposited sand and gravel along the floors of the ice tunnels through which FIG. 248. Kame east of Kewaskum, Dodge County, Wis. (Alden, U.S. Geol. Surv.) GLACIERS 229 FIG. 249. Bridgewater Esker, Rice County, Minn. (R. T. Chamberlin.) they flowed. On the melting of the ice these deposits re- mained as serpentine ridges, called eskers (Fig. 249). Like the material of kames, that of eskers is usually rounded and poorly stratified (Fig. 250). The bulk of the stratified material of the ancient drift sheets does not form distinct topographic features, but is scattered in irregular belts and layers within and beneath the till, as well as upon it. FIG. 250. Section of an esker near Randolph, Wis., showing its com- position and structure. Many eskers are composed of much coarser ma- terial. (Miller.) B. & B. GEOL. 13 230 PHYSICAL GEOLOGY SUMMARY A chief function of glaciers is to return to lower and warmer levels moisture which otherwise would be imprisoned in- definitely as snow and ice. Geologically, glaciers, like rivers, have as their principal mission the wearing of the land and the moving of the waste toward the sea. In the aggregate, however, they are much less important agents of change than rivers. Streams are, and since the very early history of the earth have always been, at work nearly everywhere upon the land. Even in deserts there are very few large areas without valleys, although such valleys may be occu- pied only by temporary streams. At present, glaciers affect but a small fraction of the land surface, and while, as we have seen, their extent has been much greater than now at various times in the past, this was true, so far as known, for only comparatively short periods. Glaciers are at a disadvantage, too, from the fact that their work is entirely mechanical. On the other hand, their activities are not so conditioned by the hardness and structure of the surfaces upon which they work as are those of streams. Although not so important geologically as they, ice takes its place with air and water as one of the three great grada- tional agents which modify land surfaces. QUESTIONS 1. Why is the snow line much lower on the southern (sunny) side of the Himalaya Mountains than on the northern (shady and cooler) side? 2. Why have the Sierra Nevada and Cascade Mountains more glaciers than the Rocky Mountains ? Why are there more in the northern than in the southern Rockies ? 3. What are the factors upon which the size of a given valley glacier will depend ? 4. What things limit the height which rock-capped ice pillars such as those shown in Figure 209 may attain ? 5. Do all parts of the medial surface line of a valley glacier move at the same rate? Why? GLACIERS 231 6. In what part of a valley glacier should erosion be greatest ? Least ? Why ? 7. (1) What will be the effect of the slow degradation of glacier- bearing mountains upon the snowfall they receive? (2) What influence will this have upon the position of the snow line ? (3) How will the facts involved in the two preceding questions affect the size and length of the glaciers ? (4) When will the mountains cease to have glaciers ? 8. How could one determine in the field the approximate thick- ness of the glaciers which formerly occupied the valleys shown in Figures 234 and 235 ? 9. What conditions would produce valley trains (1) of high, and (2) of low average gradient ? 10. Why are eskers usually roughly parallel with the direction of ice movement ? 11. Compare and contrast typical topographies due to river erosion and to glaciation. 12. Moraine topography and dune topography are sometimes similar. How might the two be distinguished in the field ? REFERENCES ALDEN : Drumlins of Southeastern Wisconsin; Bull. 273, U.S. Geol. Surv. BRIGHAM : The Fiords of Norway, in Bull. Am. Geog. Soc., Vol. XXXVIII, pp. 337-348. CHAMBERLIN : The Rock-Scorings of the Great Ice Invasions, in 7th Ann. Kept., U.S. Geol. Surv., pp. 147-248. DAVIS : Glacial Erosion in France, Switzerland and Norway, in Proc. Bost. Soc. Nat. Hist., Vol. XXIX, pp. 273-322. - Hanging Valleys, in Science, N. S., Vol. XXV, pp. 833-836. GANNETT : Lake Chelan, in Nat. Geog. Mag., Vol. IX, pp. 417-428. GEIKIE, J. : Land-forms modified by Glacial Action, in Earth Sculp- ture, Chs. X, XI. (New York, 1898.) GEIKIE, SIR A. : Scenery of Scotland, Chs. IV, X, XI, XIV, XVII. 3d ed. (London, 1901.) GILBERT : Alaska; Glaciers and Glaciation. Vol. Ill of Harriman Alaska Expedition. (New York, 1904.) NANSEN : The First Crossing of Greenland. (London, 1893.) PEARY: Northward over the ^ Great Ice." 2 vols. (New York, 1898.) RUSSELL : Glaciers of North America. (Boston, 1897.) Glaciers of Mount Rainier, in 18th Ann. Rept., U.S. Geol. Surv., Pt. II, pp. 349-415. 232 PHYSICAL GEOLOGY Malaspina Glacier, in Jour, of Geol., Vol. I, pp. 219-245. Glaciers of the St. Elias Region, in Nat. Geog. Mag., Vol. Ill, pp. 176-188. SALISBURY : Salient Points Concerning the Glacial Geology of North Greenland, in Jour, of Geol., Vol. IV, pp. 769-810. - The Drift, in Jour, of Geol., Vol. II, pp. 708-724, 837-851 ; Vol. Ill, pp. 70-97. - The Glacial Geology of New Jersey ; N.J. Geol. Surv., Vol. V. SALISBURY AND ATWOOD : The Glacial Period, in Bull. No. V, Wis. Geol. and Nat. Hist. Surv., Ch. V. SHALER : Glaciers, in Outlines of the Earth's History, Ch. VI. (New York, 1898.) SHALER AND DAVIS : Illustrations of the Earth's Surface; Glaciers. (Boston, 1881.) TYNDALL : The Glaciers of the Alps. (London, 1860.) CHAPTER VII OCEANS AND LAKES Oceans and ocean basins. The oceans have an area (143,000,000 square miles) nearly three times as great as that of the lands (54,000,000 square miles). They cover the low edges of the continents, so that their area is greater (by some 10,000,000 square miles) than that of the ocean basins. The ocean basins are a little more than twice as extensive as the continental plateaus. The submerged edges of the continental blocks are called the continental shelves (Fig. 251). The shallow seas on the continental shelves may be thought of as remnants of the vast, shallow seas which at various times in the past covered large portions of the continents. Continental 5ea level FIG. 251. Diagram showing a continental shelf, and its relation to the land on one side and to an ocean basin on the other. Soundings have shown that the bottoms of the ocean basins are generally smooth. Mountain chains and plateau- like swells are not altogether wanting, while great volcanic cones are numerous in parts of the Pacific Ocean, many of them rising as mountainous islands thousands of feet above the level of the sea. There are also submarine fault scarps and relatively small areas much lower than the surrounding ocean floor, called deeps. Nevertheless, these features occupy but a small fraction of the ocean bottom, nine tenths or 233 234 PHYSICAL GEOLOGY more of which forms a monotonously flat plain. As already indicated (p. 62), the absence of the familiar hills, valleys, and many other features of the land is due (1) to the fact that the bottoms of the ocean basins are protected from the attack of wind and weather, of streams and of glaciers, the agents which sculpture land surfaces, and (2) to the effects of the deposition of sediment in the ocean. The average depth of the ocean basins is a little less than two and one half miles (about 13,000 feet). This is nearly six times the average elevation (some 2300 feet) of the lands above sea level. Offices of the ocean. (1) Nearly all the moisture which is condensed upon the surface of the land as rain or snow, or in less important forms, comes directly or indirectly from the ocean. Together with the atmosphere (p. 86), the ocean therefore makes possible the work of streams, of ground water, and of glaciers. Without the moisture which is evaporated from the ocean and carried by the winds to be precipitated over the land, neither plant nor animal life could flourish. This constitutes perhaps the greatest service which the ocean renders. (2) The ocean tends to regulate the distribution of tem- perature over the earth's surface. The temperature of the winds is modified by that of the ocean surfaces across which they blow, and the heat or cold gained is carried over the land for greater or lesser distances. Warm ocean currents from low latitudes carry great quantities of heat poleward. Cold currents from high latitudes carry lower temperatures equatorward. Just as oceanic islands have more uniform climates than great land masses, so in past ages widespread invasions of the lands by the sea resulted in periods of uni- form (oceanic) climate, while great extensions of the land areas coincided with periods of variable (continental) climate. (3) In preceding chapters it has been pointed out that the ocean is the ultimate goal of all the waste of the land, which is spread out upon its floor as layers of sediment. OCEANS AND LAKES 235 Throughout the geological ages a chief service of the ocean has been to receive, arrange, and preserve the materials from which new land areas were later formed. While aggra- dation has always been the dominant gradational process in the ocean, degradation has always been of chief importance upon the land. (4) Finally, the sea has always been engaged in eroding portions of its shores. Thus it tends persistently to reduce the area of the land, and to increase its own extent. The movements of sea waters. The geologically im- portant movements of the sea are wind waves, currents, and tidal waves. Earthquake waves and certain other occa- sional and unusual movements are at times important. Wind waves are caused by the pressure of the wind upon the surface of the water. In the open sea, the water is pushed forward very little and slowly, even though the wave form advances with rapidity. Each particle moves through an elliptical path every time that a wave passes, but returns essentially to the point of starting. The movement of the water particles in a wave has been likened frequently to that in a field of tall grass across which the wind is blow- ing. Each blade is bent up and down, back and forth, yet retains its place. Waves are propagated with gradually lessening height far beyond the area of the storm which generated them; here the diminishing waves are called swells. On approaching land, waves drag bottom and the oscilla- tory movement passes into a true onward movement. The unimpeded top of the wave moves faster than the lower part, which is retarded by friction with the bottom, and the front of the wave accordingly becomes increasingly steep, until the crest topples over and the wave breaks with all its weight upon the shallow bottom or upon the shore line (Fig. 252). The water of the broken wave rushes up the beach, and then returns seaward under gravity, forming the undertow. 236 PHYSICAL GEOLOGY The great ocean currents are caused primarily by the winds. Their courses are determined (1) by the direction of the winds, (2) by the arrangement of the land masses, (3) locally, by the configuration of the ocean bottom, and (4) by the earth's rotation, which deflects them toward the right hand in the northern hemisphere, and toward the left hand in the southern hemisphere. The importance of ocean cur- rents in connection with the distribution of temperature has been referred to. Warm and cold currents influence greatly the present distribu- tion of marine life, and the ocean currents of earlier geo- logical periods, some of which flowed across the centers of the continents (then sub- merged), have to be taken into account in explaining the dis- FIG. 252. Breakers on the coast tribution of former life. The (kirb a anks n ) ia ** ***' BUCh n ' mechanical work of ocean cur- rents is in general unimpor- tant. Deep currents in shallow places may scour the ocean bottom, but the bottoms of currents are usually far above that of the sea. Certain ocean currents carry away sediment brought to them by the streams of the neighboring land, but large quantities are never carried far. (Why are ocean cur- rents not so efficient transporting agents as rivers on the land?) The work of shore currents is discussed later (p. 246). The regular rise and fall of the waters of the ocean, twice in about twenty-four hours, constitute the tides. In the open ocean the tides are imperceptible. Along the shores the change of level ranges from 2 or 3 feet to 50 feet and more in narrow bays. For about six hours the water rises and advances upon the shore (flood tide), and then for an equal time falls and recedes (ebb tide). Wide flats are in consequence often alternately exposed to the atmosphere and covered by the sea. In V-shaped bays and OCEANS AND LAKES 237 estuaries, and in narrow passages between islands, tidal cur- rents may be of great strength, and sometimes sweep quan- tities of sediment back and forth and erode the beds and sides of their channels. Tides aid the work of wind waves ., FIGS. 253, 254. High tide and low tide on the coast of Maine at North Haven. The rocks exposed at low tide but under water at high tide are heavily covered with seaweed. Such vegetation often helps to protect rocky coasts against wave erosion. (Bailey Willis.) by lifting and lowering them, and so increasing the width of their zone of attack (Figs. 253 and 254). THE SHORES OF THE OCEAN The shores of the ocean are zones of great activity. Here is the meeting place of land and air and sea. The principal coast-line features and offshore deposits are discussed in 238 PHYSICAL GEOLOGY the following paragraphs. A knowledge of these things aids in determining the geographic changes of the past. The characteristics of shore lines, and the agents which shape them. The shores of the northern continents are characterized by great projections of the land into the sea, and by great extensions of the sea into the land. Large irregularities like Florida, Lower California, the Iberian Peninsula, and Hudson Bay are due to diastrophism. In a late geological period an upbowing of a part of the marginal sea bottom made an island of Florida which, by continued movement, was attached to the mainland as a peninsula. A geologically recent subsidence let the sea in over the area of Hudson Bay. The submergence of a coast land having hills and valleys produces a new shore line which is irregular (Fig. 271). The drowned valley bottoms form bays, while the inter- valley ridges stand forth as headlands. Isolated hills of the old lowlands front the new coast as islands. Chesa- FIG. 255. Diagram of a young coastal plain, peake Bay, Dela- with the old land in the background. ware BaV and many other smaller bays along the eastern coast of the United States are drowned valleys or valley systems. On the other hand, the emergence of a coastal strip tends to produce an even, regular shore line, for the edge of the sea rests against the gently sloping former bottom (Fig. 255). (What should be the general height of new coasts due to (1) submergence, and (2) emergence?) In addition to the features formed by diastrophism, many coastal irregularities are due to the work of gradational agents. Under normal conditions rivers erode but little at their mouths, but may build deltas into the sea (p. 185). Glaciers descending into the sea help to develop fiords OCEANS AND LAKES 230 (p. 224), and may build islands by depositing drift. Loose material is often incorporated in ice formed along high- latitude coasts in winter ; when the ice breaks up in the spring, this material may be carried away to be dropped where the ice melts. Weathering agents reduce sea cliffs and loosen material along shore, preparing it for removal by other agents. But most important in shaping the details of coast lines is the work of wind waves and of the shore currents which they generate. The features they develop are discussed below. EROSION BY THE SEA How the sea wears its shores. Clear waves dashing against cliffs of firm, unjointed rock accomplish little or no wear. The inability of waves to erode under these circum- stances recalls the similar dependence of winds, streams, and glaciers upon their rock tools. But the conditions suggested rarely occur. Usually the rocks of the seashore are traversed by joints. If stratified, they contain bedding planes. There are still other openings, and all form weak places. With the impact of strong waves, water is forced into the openings with great pressure. Furthermore, the air in the openings is compressed by the invading water, and then expands with force as the water withdraws. In these ways pieces of rock are broken and sucked off, and the openings enlarged. Ordi- narily, too, the water offshore is sufficiently shallow for the waves to obtain from the bottom sand, stones, and some- times, when very strong, even large bowlders, which are hurled as battering-rams against the shore. Locally, the sea dissolves the rocks of its shore. Rate of erosion. The rate at which a given coast is eroded is determined by several factors. (1) Other things being equal, strong waves obviously erode faster than weak ones. The velocity of the winds which generated them, the depth of the water they have traversed, and the distance they have 240 PHYSICAL GEOLOGY come before reaching .the coast, all influence the strength of the waves. (How does each factor affect the result?) The force of waves has been measured in connection with cer- tain engineering enterprises. On the coast of Scotland and among the outer Hebrides, storm waves sometimes exert a pressure of nearly three tons per square foot. (2) The rate FIG. 256. Wave erosion near Santa Cruz, Cal. The parallel channels in the foreground are the result of rapid wear along joint lines. (U.S. Geol. Sun.') of wear is influenced by the character and structure of the rocks at the shore. Soft rocks wear faster than hard ones, soluble rocks faster than insoluble ones, rocks with many joints (Fig. 256) and openings faster than rocks with few. (Other things equal, which structure would occasion most rapid wear, (a) horizontal beds, (6) beds dipping abruptly toward the sea, (c) beds dipping away from the sea? Why?) (3) Finally, the rate of wear is influenced by the number and character of the tools of the waves. The shallower the water immediately offshore, the greater the number of tools that are likely to be accessible to the waves. But, on the other hand, if the water be very shallow for any considerable distance from the shore, the velocity of the waves will be so reduced by friction with the bottom that, on arriving at the shore line, they will be unable to erode effectively. It may be noted OCEANS AND LAKES 241 that usually deep water fronts high coasts, and relatively shallow water, low coasts. On the eastern coast of England, where the rocks are rela- tively weak, entire parishes have been washed away within a few centuries ; in some places the shore line has retreated as much as 15 feet in a single year. The south shore of Nan- tucket Island, Massachusetts, has lost in places as much as 6 feet in a year, and as early as 1835 the opinion was expressed that within a few centuries the entire island would be devoured by the sea. Sea cliffs and terraces. The chief topographic effects of wave erosion are illustrated by Figure 257. The original slope near the water level is indicated by the dotted line. Erod- ing waves have notched this slope, forming a sea cliff. The develop- ment and recession of a sea cliff involve also the formation and widening at its base of an under- water platform, called the wave-cut FlG> 257 _ Diagram of sea terrace. Its surface represents the cliff, wave-cut terrace, and lower limit of effective wave action. wave ' built terrace - It slopes gently seaward because, as its width increases, the strength of the waves at its inner edge decreases (Why?), and they are accordingly able to cut a less and less distance below sea level. (Should you expect the slope of wave-cut terraces to vary? If so, why?) At first the material worn from the cliffs is swept to the edge of the wave-cut terrace, and de- posited in deeper water. Here it accumulates to form the wave-built terrace, which extends the wave-cut terrace seaward. Later, more or less of the waste of the cliffs remains here and there upon the terrace at their base (Why?) to form a beach. Still later, when the beach is developed more continuously, much of the waste is washed along it by waves and shore cur- rents (p. 246). Wave-formed terraces may become land by lowering of the sea, or by uplift of the coast line (Figs. 258 and 259). 242 PHYSICAL GEOLOGY FIG. 258. Wave-cut terraces on the California coast. (U.S. Geol. Sun.) How many terraces are shown ? What is their relative age ? Outline the history of the coast as recorded by the terraces. What changes are now in progress ? The height of sea cliffs depends upon the elevation above sea level of the land at the coast. Their steepness varies with (1) the strength and structure of the rocks, and (2) the rapidity of wave cutting and of weathering upon the cliffs above. Loose material usually cannot stand in steep cliffs. Firm rocks may form vertical and even overhanging cliffs FIG. 259. Raised beaches, near Elie, Fife. (Laurie.) (Figs. 260 and 261). (What rock structures favor, and what ones oppose, the formation of steep cliffs?) Rapid cutting by the waves tends to keep the cliffs steep, while the weathering of the rocks of the upper cliffs and the removal of the loosened material tend to lessen their declivity. (What inference may be made from the fact that even sea cliffs containing OCEANS AND LAKES 243 rocks capable of standing in vertical faces, commonly slope sharply back toward the land?) The rapid weathering of sea cliffs is favored by the absence of protecting talus (Why absent ?) and often of vegetation, and by the frequently wet condition of the rocks due to the spray. The active issu- ance of ground water as seepage and springs near the level of the sea often helps to undermine sea cliffs. FIG. 260. Sea cliffs on the northern coast of France. Sea caves, stacks, natural bridges. The enlargement by the waves of a joint or other opening in the face of a sea cliff may result in a sea cave (Fig. 262), provided the overlying rock is strong enough to form a roof. Occasionally a sea cave is worn back and up to the surface of the ground some distance back from the cliff. Again, a fissure or joint may form an opening between the inner end of a sea cave and the surface of the ground. Storm waves sometimes drive spray and water up through such openings, which are then called blowholes. Taking advantage of joint systems, waves sometimes quarry out the rocks about a section of a cliff, leaving it as an 244 PHYSICAL GEOLOGY island in front of the retreating shore. From their form, such islands are frequently called stacks or chimney islands (Fig. 263). Waves may cut through a rocky headland in such manner as to form a natural bridge (Fig. 264). If the roof covering a FIG. 261. Sea cliffs in northwestern France. sea cave near its mouth remains after the roof above the inner end of the cave has collapsed, a natural bridge also results. While interesting because of their picturesqueness, these special features of cliff shores have little geological importance. The goal of sea erosion. Just as rivers seek to wear the land to sea level, so the waves of the sea, acting as a horizontal saw, seek to cut the land to a level slightly below the surface OCEANS AND LAKES 245 of the sea. Extensive peneplains have been developed re- peatedly in the past, but, so far as known, wave-cut submarine FIG. 262. Sea caves on the southern coast of California. (Fairbanks, U.S. Geol. Surv.) When the upper cave was cut it stood in the same relation to sea level that the lower one now does. Since it was formed th land has therefore been elevated with reference to the level of the ocean. plains of great extent have not been formed. This is because, as already indicated, waves drag bottom across the sub- marine flat which they cut, and so become weaker as the flat becomes wider. The gradual subsidence of a coast and marginal sea bottom aids in the ex- tension of a wave-cut plain by gradual^ in- creasing the depth of the water upon it, and so maintaining the vigor of the waves at the shore. Gradual emergence, on the other hand, opposes ' FIG. 263. Stacks on the west coast the formation of an exten- of France. B, "* l-s 02 0) If (320) THE ARCHEOZOIC ERA 321 The presence of limestone may mean that shell-bearing animals were already in existence, but the fact that some limestones are even to-day formed by the direct precipitation of lime carbonate from water leaves the question in doubt. Beds of graphite, which are thought to be simply metamor- phosed coal seams, indicate that plants lived in Archaean time. No fossils have been found in the Archaean rocks and so we know nothing of the real character of the life which may have existed then. All we can be safe in imagining about the plants and animals of the era is that they were on the average much simpler and lower in the scale of life than those which exist to-day. Length of the Archaeozoic era. It is difficult to imagine the vast length of time embraced in the Archaeozoic era. Since the bottom of the Archaean rocks has never been reached it is clear that we know nothing of the earlier part of that era. Yet even the knowable part gives us a fragmentary story of many successive volcanic disturbances and several distinct periods of folding and metamorphism. From the clearer record of later times we know that such changes take place slowly and are separated by periods of quiet, often to be measured in millions or tens of millions of years. Considera- tions such as these have led to the conjecture that the Ar- chaeozoic era may have been longer than all the later periods together. QUESTIONS 1. In some places the Archaean system is found to contain both gneiss (once granite) and schistose basalt. Which of the two would you consider the older (1) if the gneiss contained rough frag- ments of the basalt within itself and if the main mass of basalt were cut by branching layers of gneiss, or (2) if the gneiss were crossed by layers of basalt continuous with the main mass of the latter ? 2. Why should we not expect to find fossils in schist even though the original mud from which it was derived was filled with shells ? 3. An old name for the Archaean system is "Basement Complex." Why is this a good descriptive phrase ? CHAPTER XII THE PROTEROZOIC ERA What it represents. The oldest rocks which contain numerous fossils are those of the Paleozoic group. Between these fossil-bearing sedimentary rocks of the Paleozoic and the intricate complex which records Archeozoic time, there is in many places a thick group of systems, partly sedimentary and partly igneous in origin, which represents a vast lapse of time between these two eras. This time is the Proterozoic 1 era (often called also the Algonkian period). PROTEROZOIC ROCKS OF THE LAKE SUPERIOR REGION The Proterozoic rocks are nowhere better known than in the vicinity of Lake Superior. In certain parts of this region the entire group is divided into four systems which are separated from each other by unconformities. In other localities only three or two divisions are distinguished. Where four systems are known, they are called lower and upper Huronian, Animi- kean, and Keweenawan. Basal unconformity. The basal formation may in- clude a conglomerate which contains rounded pebbles of at a locality in Michigan. (After Schist and gneiss derived V w^r- A * A from the Archaean rocks be- Wmch is the younger of the two formations, and what is the evidence ? neath (Fig. 320) . No better 1 From two Greek words meaning "earlier life." 322 THE PROTEROZOIC ERA 323 FIG. 321. Stereogram showing down-folded rem- nants of Proterozoic rocks surrounded by the Archaeozoic complex. proof could be desired that the Archaean rocks had been folded, metamorphosed, laid bare as land, and profoundly eroded before the Proterozoic rocks were deposited upon them. Huronian system. The Huronian rocks are quartzite, limestone, and slate, with the addition of beds of iron ore and jasper. Where metamorphosed the predominating rocks are schists. The beds are usually much folded and they are exposed at the surface as long down-folded bands within the out- crops of the Ar- chaean (Fig. 321). In at least one dis- trict a well-marked unconformity di- vides the Huronian strata into two systems, the lower of which is evidently much older than the upper. Igneous intrusions of different ages cut through them here and there, and lava flows are sometimes found interbedded with the sediments themselves. Around the batholiths of granite and the other large intrusions, the rocks may be altered to schists ; and it then becomes difficult to discriminate them from those other schists which belong to the Archaeozoic group. Animikean system. Unconformably above the Huronian rests the Animikean, 1 another system of sedimentary rocks and lava flows, which is in general much like the Huronian (Fig. 322). On the average, however, the rocks are less folded and less metamorphosed, in some places not at all. The quartzites and slates are traversed by a few dikes and larger intrusions of later age. Iron ore has been men- 1 The geologists of the U.S. Geological Survey class the Animikean as Upper Huronian. 324 HISTORICAL GEOLOGY O u > 0> if 3.9 ^ ^ W&ji a) tn mm S^ ffif After Va 3ntary ro i n //' / Jv **^**~^' //7/fjLr^ - IHaC^r^^^f ijfik^-^*- ^^&d jil swgz-Z^: /JlggE G g IS^F Is C^x x x o XXX e g t X X" X S | NX X x X x x 1 \ '^^^^ o ^ .s'S 03 ~ V^V^S^i^C^Ii o 'G K^Q^W^v^ 8 * ^^-C^C; a v ^s^-^- *s ^c^^^ "s " ^^^g O be l| "o S */ \ SI ^ ' v 3 ^3 v /\"/ v iC~ x ~ 2 "** \/*"^ f f \\' o 3 ^~/-^- ? X JLN^ 1 S tioned as a constituent of the Huronian. In the Animikean the largest and richest deposits of that indispensable ore that are yet known have been found (Fig. 323). They occur in the form of thick beds in the sedimentary rocks. Some of the Animikean formations originally contained a large amount of iron minerals, together with quartz and other im- purities ; this was further enriched in certain spots where the underground waters dissolved out everything except the iron minerals and, in some cases, even rilled the pores thus left with still more oxide of iron. The mines of the Lake Superior region supply more than 80 per cent of the ore from which is derived the iron used in the great industries of this country. This is equal to more than one third of the world's output. More ore is now taken from a single mine in the Mesabi district of Minnesota each year than was mined in the entire United States before the Civil War. Keweenawan system. Still a third great system lies upon the eroded edges of the Animikean strata and occasionally laps over upon even older formations. In this we have the record of one of the greatest episodes of local volcanic activity known in geologic time. The eruptions seem not to have come from definite craters, but the fluid lava simply welled up through cracks in the surface and spread over wide areas. A series of these flows accumulated one above the other to a depth estimated at more than 6 miles. The great number of the flows may be ap- preciated when we consider that most of THE PROTEROZOIC ERA 325 them were less than one hundred feet in thickness. Later in this period the eruptions apparently came at wider intervals, and meanwhile coarse sandstones were deposited in the same region. Finally the lava ceased to flow out and so, toward the close of the period, only sedimentary rocks were made. These lavas and sandstones form the Keweenawan system. Since they were laid down, they have been moderately tilted but not much altered. It appears that the lava originally contained minute quantities of cop- FIG. 323. Distribution of ore deposits in the Proterozoic rocks of eastern United States ; iron districts are shown by the black patches, and copper deposits by the crosses. per. Part of this copper, furnished in solution to the active underground waters, was deposited in certain porous layers in the sandstones and gravels, as well as in the cindery portions of the lava flows themselves. From these enriched bands vast quantities of pure copper have been mined during the last few decades. PROTEROZOIC ROCKS IN OTHER REGIONS Rocks of Proterozoic age are found in many parts of this and other continents, but the formations cannot be matched 326 HISTORICAL GEOLOGY closely with those of the Lake Superior region. This is true chiefly because the necessary fossils are lacking. In the Grand Canon of Arizona Proterozoic strata are again well exposed, but they are unlike the Lake Superior formations in details. The lower walls of the canon reveal the complex schists of the Archaean. Upon these rests un- conformably a tilted pile of. sedimentary strata (Fig. 324). FIG. 324. Ideal cross section of the Grand Canon of the Colorado River in Arizona. In spite of their great age they are neither folded nor notably metamorphosed. These in turn were largely removed during a still later period of erosion, so that the Cambrian sandstone was deposited horizontally, not only upon the beveled edges of the Proterozoic formations, but out over the Archaean also: Proterozoic rocks are well known in Scotland, Sweden, and China (Fig. 325), and have been studied in considerable FIG. 325. A section through the ancient rocks at a point in Northern China, showing the Archaeozoic rocks (A), overlain by a thick series of folded beds of Proterozoic age (B), and upon both resting Proterozoic lime- stone and shale (C), much less folded. The Cambrian rocks (Z>) rest un- conformably on the others. detail. In each case there appear to be two or more systems between the Archaean and the Cambrian, separated from each by a pronounced unconformity. Where there are two systems the older is usually intensely folded and metamor- phosed, although still plainly made up of sedimentary rocks ; THE PROTEROZOIC ERA 327 while the younger consists of slates, quartzites, and lime- stones which are neither closely folded nor much altered. GENERAL CHARACTERISTICS OF THE PROTEROZOIC GROUP Sedimentary rocks but with some igneous. Having learned something about the Proterozoic systems in widely separated regions, we may proceed to consider the things which are characteristic of the group as a whole and of the long periods of time during which it was being formed. In each case the rocks which make up the group were derived chiefly from ordinary sediments. They were once gravel, sand, clay, and ooze spread out upon the sea floor or upon the low-lying lands. They have since been cemented into solid rocks; they have been folded, mildly in some places and intensely in others; and some of them have been metamor- phosed into slates, schists, and gneisses. Many kinds of lava have been forced up through them at different times. These either spread out on the surface as flows, or solidified beneath in the form of dikes, sills, batholiths, and other intrusions, which interrupt the stratified rocks and complicate the study of the structure. As would be expected, the older Proterozoic formations are often much more deformed than the younger, because they have passed through more epochs of folding. The ArchaBan system, we learned, includes some beds of sedimentary rock, but the vast body of that ancient mass is of either igneous or doubtful origin. In the Proterozoic group, on the other hand, the proportions are reversed, and the sedimentary strata predominate overwhelmingly. Unconformity general but not universal. We have seen that in each district where the rocks have been fully studied the Proterozoic group is separated from the Archeozoic by a great unconformity. This clearly shows that the regions had been lands cut down by weathering and erosion until the very roots of the Archaean mountains were laid bare and planed off, and all this before the Proterozoic sediments began to 328 HISTORICAL GEOLOGY be deposited. This unconformity evidently tells of a very long lapse of time between the deposition of the Archaean and Proterozoic rocks, a time otherwise unrecorded in the rocks which we know. Because of this, and because of the wide distribution of the unconformity, it is generally regarded as one of the greatest interruptions in the geologic record. But no unconformity, however widespread, can exist all over the globe. The very same facts which indicate that the lands were deeply eroded prove that the material worn off was as continually being deposited elsewhere; and in those areas where deposition was in progress no unconformity resulted. It has been suggested that the sediments which were deposited then, as now, in the deep ocean basins have never been raised into land, and hence are still unknown to us. Unconformities within the Proterozoic group. Other notable unconformities serve to divide the Proterozoic into two or more systems. In Minnesota the Animikean sand- stones and shales rest at a moderate inclination upon closely folded slates and quartzites of the Huronian. Furthermore, some of the dikes in the Huronian rocks do not pass up into the Animikean system. Such an unconformity is conspicuous when it can be seen in the side of a quarry or a ravine. No one of these interruptions in the strata has, however, been traced across any continent, much less over several conti- nents; and the divisions themselves, therefore, can be used only in the region where they are known to apply. Protero- zoic rocks are generally separated from the Cambrian system, which overlies them, by another great unconformity, a de- scription of which will be found in the next Chapter. Duration of the Proterozoic era. Just as the remote ancient periods of human history are long in comparison with the subsequent centuries, so the Proterozoic era was im- mensely long as compared with later periods. In the course of a century only a few feet of average sediments are deposited, and of limestones perhaps not even one foot. Yet the Huro- nian sediments of Michigan are alone said to be more than THE PROTEROZOIC ERA 329 13,000 feet thick, while the Keweenawan lava flows and sand- stones may have a thickness of 35,000 feet. If to the time required for the making of these rocks we add the long lapses of time represented by the various unconformities, it becomes evident that the Proterozoic era was one of the longest. By comparing the thicknesses of younger systems it has been esti- mated that it may have been as long as all the subsequent periods combined. LIFE IN THE PROTEROZOIC ERA Evidence from the sediments. In the Archseozoic era living things are believed to have been present, but the evi- dence of their existence is somewhat indirect, for no fossils have been found. Again, in the Proterozoic systems of rocks, we find limestones, this time in thick layers, which may have been made partly of the shells of minute animals, just as more recent limestones have been. It is well known that coal beds have been derived from compressed masses of the vegetation which accumulates in swamps. Coaly layers and beds of graphite among the Pro- terozoic rocks probably had the same origin. Still other facts make it almost certain that both plants and animals were abundant throughout the era. Fossils very rare. Among the younger strata of the Pro- terozoic group a few poorly preserved fossils have been dis- covered. They are the remains of animals, and among them are forms which seem to belong to the brachiopods (p. 298) and the crustaceans (p. 300). The crustacean group is one of the most advanced of all the invertebrates, and it is therefore somewhat surprising that it should have appeared so early. Few though they are, these fossils justify us in believing that the living world had been in existence for untold ages before the strata which contain them were deposited, and that the slow changes of evolution had already produced some types not altogether unlike those of modern times. 330 HISTORICAL GEOLOGY QUESTIONS 1. Pebbles of Archaean schist are of ten found in the basal layers of the Proterozoic rocks. Under what conditions are schists pro- duced? And what does this tell about the depth to which the erosion of the land had penetrated at this time ? 2. What events are recorded by the unconformities in Figures 326,327, and 328? FIG. 326. An irregular contact between hori- zontal beds. FIG. 327. Horizontal sandstone resting up- on folded beds. u- 328. Horizontal sandstone resting up- on granite, schist, and slate. 3. With which type of volcanic eruption are cinders and ashes usually associated, the fissure or the crater type ? 4. Why should the gravel beds and cindery layers of the Ke- weenawan contain richer copper ores than the dense lava flows ? 5. Why should the older Proterozoic formations be more folded and metamorphosed on the average than the younger? 6. Beds of conglomerate thought to be of glacial origin have recently been found in theHuronian rocks of Canada. With which theory of the origin of the earth is this more harmonious ? CHAPTER XIII THE CAMBRIAN PERIOD The Cambrian rocks. Most of the rocks which consti- tute the Cambrian system in the United States were origi- nally sands, clays, and oozes, deposited in nearly horizontal layers upon the bottom of the seas of the Cambrian time. That portion of the deposits from which the sea has since been withdrawn and which has been exposed to view by the removal of such younger strata as were deposited on them, was laid down chiefly in the shallow waters near shores. For this reason the clastic sediments predominate in the Cambrian system as we know it. Embedded in these sediments we find the shells of some of the animals which lived in the same seas. The fossils in the lower layers differ somewhat from those found in the upper beds of the system, and by the gradual changes in the fossils from level to level, several stages, or horizons, have been recognized within the Cambrian system. A threefold division of the system is usually made, giving us Lower, Middle, and Upper Cambrian series, corre- sponding to similar epochs of time. Basal unconformity. The lowest layers of the Cambrian sediments generally rest upon an uneven eroded surface of the older rocks. In some places the underlying strata are ot Proterozoic age ; in others of Archaean age. Some of the older rocks were folded or even metamorphosed before the Cam- brian strata were laid down. As evidence of this, it is common to find, in the lowest Cambrian beds, pebbles which are water- worn fragments of the older rocks. The unconformity thus indicated has been observed in many parts of the continent, and, as very few exceptions have been discovered, it is evi- dent that before the Cambrian period began, most of North America had been for a time dry land and subjected to ero- 33l" 332 HISTORICAL GEOLOGY sion. Where the eroded surface of the older rocks has not been deformed by later movements of the crust, it is nearly level ; and from this fact it is thought that the denudation of the continent, before the land was submerged by the Cam- brian sea, must have continued for a very long time, suffi- ciently long to allow the streams to reduce large areas to the condition of peneplains (p. 147). On account of the great duration of this interval of erosion, and because of the very general presence of the unconformity in all continents where the Cambrian has been studied, the interruption is regarded as one of the greatest in the geologic record. Gradual submergence of the continent. Further light is cast upon the geography of the times by the discovery that, in North America, the layers which contain the oldest Cam- brian fossils exist only near the eastern and western borders of the continent. Farther inland it is the Middle Cambrian that rests on the eroded pre-Cambrian surface; and in the interior, from New York to Michigan, the strata above the unconformity contain the Upper Cambrian fossils. From this we infer that the sea encroached so slowly upon the gently inclined land surface that nearly the whole of the long Cambrian period was required to accomplish the submergence. Not all of the continent seems to have disappeared beneath the sea even at this time. A large area of ancient rocks in eastern Canada, another occupying what is now the Atlantic seaboard, and also some parts of the West seem to have remained as land masses. These continued to be eroded and hence to supply sediments to the seas of the time. The name " Appalachia " is used to designate the large island which then lay just east of the present Appalachian Moun- tains, from New England to the Gulf states. Its influence on the rocks formed in later periods will be mentioned in succeeding Chapters. Where seas have encroached upon the land, it is often im- possible to decide whether the ocean surface actually rose or whether the lands sank. In the Cambrian, it is significant THE CAMBRIAN PERIOD 333 that the sea advanced gradually and almost simultaneously over central Europe and eastern Asia, as well as North Amer- ica. Since continents can hardly be supposed to subside evenly over so large a portion of the globe, the facts in this case suggest a general rise of the ocean waters. The very sediments which were being carried into the sea all through the Cambrian period would inevitably displace a considerable amount of water, and raise the sea level correspondingly. Cambrian strata differ according to locality. The Cam- brian rocks are by no means alike in all localities, for the con- ditions of sedimentation varied from place to place. Where the sea advanced over a low shelving surface its waves and currents reworked the soils and alluvial deposits already pre- pared by weathering and wash, and sifted from them an abundance of sand which was spread widely along the shores. This may be the explanation of the very widespread Middle and Upper Cambrian sandstone which represents the system wherever it is exposed in the interior of the United States. In the West, and in the Appalachian Mountains, the deposits of Upper Cambrian age are principally limestones and shales, indicating that in those districts conditions for clastic sedi- mentation along shore had passed. During much of the period the water may have been too deep to receive the coarser sediments, but it is more probable that the lands were low and remote, and were for that reason unable to furnish much debris. Owing to the differences in the conditions of sedi- mentation and in the time it continued, the total thickness of the Cambrian strata is in some places great and in others small. Over the flat interior region the sea was apparently shallow and came in late in the period, so that the sandy for- mation (Potsdam sandstone) then produced is rarely more than one thousand feet thick. In some places, on the other hand, as in the Appalachian Mountains and in Nevada, deposition of varying sediments seems to have continued nearly or quite throughout the period, and to have resulted in a succession of strata several thousand feet in depth. B. & B. GEOL. 19 334 HISTORICAL GEOLOGY Shifting of volcanic activity. In striking contrast to the Keweenawan system, the Cambrian rocks of North America contain scarcely a trace of volcanic materials. As later periods are studied it will be seen that volcanic activity is prevalent in one region for a time and then dies out, only to break forth again in some other district. So in the Cambrian period, Wales and Scotland, to-day entirely without volcanic activity, were the scenes of many eruptions. Later changes in the Cambrian rocks. The sediments of which we have sketched the origin have since been changed in various ways. Almost all have been converted into firm rocks : the ooze into limestones, the muds into shales, and the sands into sandstones or even quartzites. Along both the Atlantic and Pacific coasts they have been in part metamor- phosed into slates, schists, and gneisses by exceptional com- pression and at the same time their fossils were obliterated. Wherever the sea is, there sediments are being deposited ; and to these must be added the debris laid down in lakes and other low places. The rocks of any period therefore originally formed a layer somewhat more extensive than the seas of their time. Most of that blanket of rock which we call the Cambrian system is still beneath the sea or, if raised above it, remains concealed by the formations afterwards laid upon it. Around the borders of the old Cambrian lands the system now out- crops in an irregular band adjacent to the older rocks, and, in certain mountain regions both east and west, the Cambrian has been exposed by the deep erosion of raised or folded tracts. FIG. 329. Block diagram of a dome fold like that of the Black Hills of South Dakota, showing the relation of the Cambrian (solid black) and later sedimentary rocks to the highly folded rocks of pre-Cambrian age. THE CAMBRIAN PERIOD 335 Cambrian life highly developed. The existence of life in the earlier pre-Cambrian periods of the earth's history is known only from the indirect evidence of organic sediments and the like, or from the testimony of a few imperfect fossil shells. In the Cambrian rocks, for the first time, we find such shells abundant and varied in form. It would not be unnatural to expect that these early animals and plants would prove to be very primitive in their structure and low in the scale of evolution, but such is not the case. Of the eight or nine primary divisions of the animal kingdom, all but the highest, the vertebrates, have Cambrian representatives. It is probably not too much to say that more than one half of the development of the animal kingdom was accomplished before the Cambrian. We thus get a hint of the long ages which preceded the time of which the geologic record gives us an intelligible story. In spite of the great development of life before the Cambrian, enormous progress was made in the later periods, and, as compared with the animals which suc- ceeded them, the Cambrian types show many primitive characteristics. Plants existed. Concerning the plants of the Cambrian time, little is known ; but since plants provide the ultimate food supply of most animals, it is evident that they must have been then in existence. We may perhaps attribute the lack of fossils to the fact that the Cambrian rocks thus far studied are of marine origin, and most marine plants are too soft and succulent to be readily preserved as fossils. Only when in younger strata we come to the deposits made in marshes and rivers by the plants which possess woody tissues, do we find vegetable remains well preserved. The more prominent animals of the Cambrian. Two groups of animals, the brachiopods and the trilobites, have left fossil remains in such abundance that they are regarded as the most important of all that numerous assemblage of species which is called the Cambrian fauna. The early brachiopods had pairs of small oval or rounded shells which 336 HISTORICAL GEOLOGY FIG. 330. a and b. A Cam- brian brachiopod. Interior and exterior views of the shell. are commonly ornamented only by concentric lines of growth (Fig. 330). Internally they exhibit the simplest type of brachiopod structure, the spiral feeding arms not supported by hard skeletons, and hence not preserved, and the two shells held together by muscles only, rather than by a solid hinge. The trilobites had attained somewhat greater variety of form even before the Cambrian period began, and were seemingly more advanced in their cycle of evolution. Some very simple types (Figs. 331 and 332) were present, species which were eyeless and had only two body segments between the broad head and tail. Others were of large FIG. 331 size (even exceeding two feet in length, in exceptional instances) and were ornamented with spines and raised lines (Figs. 333 and 334). Most of them possessed prominent compound eyes not unlike those of insects, and they were provided with a generous number of jointed legs of a type adapted to swimming. These crustaceans, by vir- tue of their advantage in size and their greater intelligence and activity, doubtless held the dominant place in the animal world of their day. Many other groups, such as the corals, mollusks (Figs. 335 and 336), worms, and graptolites, have left representatives among the fossils of the Cambrian strata, but they scarcely attained prominence until later periods. As yet we have no knowledge of the existence in the Cambrian of air-breathing animals, such as insects, nor of even the simplest vertebrates. One of the earliest and simplest trilobitea (Agnostus), char- acteristic of the Cambrian rocks. FIG. 332. A larger Cambrian trilobite (Conocoryphe). Compare this with Silurian va- rieties. In which are the eyes visible ? THE CAMBRIAN PERIOD 337 A large trilobite(Olenellus) characteristic of the lower Cam- brian rocks. Climate of the Cambrian period. In the days when the Laplacian or gaseous theory of the earth's origin was generally accepted as true, it was thought that, in a period so remote as the Cambrian, the at- mosphere must have been distinctly warmer, more moist, and more heavily charged with carbon dioxide than now. There was no direct evidence, however, that such condi- tions really existed, and in more recent years some facts have been discovered which effectually show that they did not. FIG. 333. Glacial deposits of early Cambrian age exist in Norway, China, and probably elsewhere. In China the glaciers were not far from sea level in about the latitude of New Orleans. From this it is reasonable to infer that the general climate of the earth in the Cambrian pe- riod was not radically different from that which prevails at present. Close of the period. The Cambrian sys- tem is somewhat ar- bitrarily set off from the Ordovician FIG. 335. Supposed because of a difference in the fossils HtheTln Sh a 1S bH Iy of which the rocks contain. It is probable Cambrian shale. that, when the history of the two periods is better known, a more rational means of separation will be found. Estimates of the length of the Cam- FIG. 336. A cap- brian period. There is no satisfactory from Pe the Sa c S ambrian means of determining the number of system (Stenotheca). years in any of the geologic periods. FIG. 334. A large trilobite (Dikelo- cephalus) charac- teristic of the late Cambrian rocks. 338 HISTORICAL GEOLOGY Nevertheless calculations, based chiefly on the thickness of sediments deposited, give a rough approximation to the truth, sufficient to show that geologic history is exceedingly long. It has been estimated that from 2,000,000 to 3,000,000 years would be necessary for the deposition of the sand, mud, and ooze which formed the thick Cambrian strata. Similar es- timates made for later periods indicate that the majority of them were of some such duration. Their combined length must then have been many millions of years, a lapse of time almost too vast for comprehension. QUESTIONS 1. How can the extent of the sea at a particular time in geo- logic history be ascertained ? 2. Why should limestone be deposited close to the shore of a low, densely forested land, but not near a rugged or less verdant coast ? 3. Why should the Cambrian system be thicker on the average where it is made up of sandstone and conglomerate than where it consists largely of limestone ? 4. Over which kind of a surface would a rising sea spread most rapidly, a peneplain or a mountainous plateau ? Why ? 5. At several points in the interior of the United States the basal layers of the Cambrian sandstone contain great angular bowlders of quartzite, granite, and other rocks. What do these indicate about the Cambrian shore line at those particular places ? FIG. 337. FIG. 338. 6. Compare the diagrams, Figures 337 and 338. In which do you find evidence of the existence of an island in the Cambrian period ? Can you suggest how the other has come to resemble it in general structure ? CHAPTER XIV THE ORDOVICIAN PERIOD Expansion of the sea in North America. By the end of the Cambrian period the sea had overspread the greater part of North America. Neglecting certain retreats and read- vances of this sea, the salient fact is that the general submer- gence seems to have been greatest during the Ordovician period (Fig. 339), gradually giving place to the reverse tend- ency toward the close. On the east side of the continental sea lay the island of Appalachia, an extensive land stretching from New England to the Gulf states entirely east of the present Appalachian ranges. Westward from this island an open sea spread over the interior of the continent, probably joining the Pacific. Some interrupting islands, whose out- lines are imperfectly known, are thought to have existed in the western part of the country. On the north lay other lands now represented by the ancient rocks of eastern Canada and adjacent parts of the United States. That much of this sea was shallow is indicated by the remains of corals of the reef-making type and other animals which to-day are unable to live in deep water. Such a shallow body of salt water lapping up over the continent is termed an epicontinental sea. Many single species of Ordovician fossils are found alike in Europe and in the United States, a fact which seems to mean that it was possible for the animals of the shallow waters to mi- grate freely from one continent to the other. As some of these animals find it almost as difficult to cross the deep parts of the ocean as to pass a barrier of dry land, we may suppose that the shallow sea which spread over parts of Canada was directly connected with the similar sea of northern Europe. 339 340 HISTORICAL GEOLOGY FIG. 339. Approximate distribution of land and sea in North America in the middle of the Ordovician period. (Modified after Willis.) Sedimentation under varying conditions. In different parts of this interior sea the conditions were not alike, and hence the sediments are not the same in different localities. Over the great central and western interior region limy ooze, composed partly of the shells of animals, was the most impor- tant sediment, and there we now find thick beds of limestone. THE ORDOVICIAN PERIOD 341 This implies clear water, for, although shell-bearing ani- mals are often abundant in turbid waters, their remains are there mixed with so much mud or sand, that shale or sand- stone is the resulting rock. Thus along the western flank of Appalachia there is less limestone in the Ordovician system because the land supplied greater quantities of sand and clay. Where lands are high they are more rapidly eroded, and when the mountains are near the sea a correspondingly rapid accumulation of coarse sediments is likely to take place off shore. When, however, broad low plains clad with vegeta- tion border the seas, it may happen that little material is worn from the surface thus protected, and likewise little sediment may be washed into the sea in that vicinity. Such considera- tions as these serve to explain the fact that the period is represented by over 4000 feet of strata in eastern Ten- nessee, but by only a few hundreds of feet in Missouri. Similarly, there may be differences in the rate at which cal- careous sediments accumulate, for in warm, shallow waters shell-bearing animals are likely to be far more numerous than in cold waters and far from shore. Subsequent changes in the sediments. The Ordovician sediments were laid down in nearly horizontal beds, and were almost entirely buried by sediments deposited at a later time. Since then they have been consolidated into hard sandstone, limestone, and shale. In some places they have been folded or bulged up in such a way that they have been uncovered by the erosion of the land. Thus the outcrops of Ordovician rocks are now found adjacent to those of Cambrian age. In the Appalachian Mountains these outcrops lie in parallel bands, while on the other hand they form rings about certain upraised masses of older rocks in the northern and western states, as in the Adirondacks, in Missouri, and in the Rocky Mountains. On the Pacific coast, as well as in New England, the Ordovician rocks have been severely metamorphosed, so that it is now a matter of extreme difficulty to distinguish them at all 342 HISTORICAL GEOLOGY Lead and zinc deposits. In parts of the Mississippi valley ores of lead and zinc are now found abundantly in the Ordovician limestone. Apparently minute particles of lead and zinc minerals were deposited sparsely through the sedi- ments while they were accumulating, and, at a later time, these scattered particles were dissolved out by the waters which saturate the rocks, and were redeposited along joints and bedding planes in the lime- stone (Fig. 340). Thus concentrated in veins, the minerals may be profitably mined. Wide distribution of the sea life. The broad, shal- l w seas f the Ordovician period afforded a congenial home for many species of marine organisms, and, al- though it is certain that the majority of the forms which existed then have left no traces in the rocks, yet enough have been preserved to show us the variety and advancement of the ani- mals of the time. The wide expansion of the seas, and the free communication which seems to have prevailed be- tween them, permitted the individual species to migrate readily from one part of the globe to another. This was par- ticularly true of animals which floated in the water, such as graptolites and young corals (p. 296). Hence some of the Ordovician fossils of the United States are much like those of Europe and even Asia and Australia. Such a widespread assemblage of animals is called a cosmopolitan fauna. In any one place the animals of Ordovician time were in part FIG. 340. Vertical section of a zinc- and-lead ore deposit in southwestern Wisconsin. (After Chamberlin.) THE ORDOVICIAN PERIOD 343 istic Ordovician brach- iopod (Orthis). descended from those which lived there in the Cambrian period, and in part from others which had come in from elsewhere. Progress of the brachiopods and trilo- bites. Among the members of this fauna the brachiopods and trilobites still held a prominent position. The little Fig. 341. A character- oval varieties of the former were at this time associated with larger types, many of which were ornamented with radiat- ing ridges (Fig. 341). The species that had hinged shells were more numerous and even the spire-bearing group was represented (Fig. 342). During the Ordovician period the trilobites had FIG. 342. A common . . . Ordovician brachiopod risen rapidly to their culmination, and with hooked beak were even more nume rous than in the (Rhynchotrema). _. . . _ Cambrian. As we shall see, it was not until the next period, however, that they ex- hibited to the fullest their propensity for adopt- ing queer forms and orna- ments. Some of the Or- dovician trilobites went to the extreme of sim- plicity (Fig. 343) in their adornment; a few are quite smooth, and are all but devoid of even the pair of furrows (Fig. 344) which impart to most members of the group their trilobate aspect. FIG. 343. A remarkably smooth trilo- bite (Bumas- tus) from the Ordovician rocks. telus) of the Ordovi- addition to the brachiopods and trilo- cian period. Compare j^es, other groups rose to prominence TUG GyGS Wltll TJIlOSG OI Cambrian types. in the Ordovician. Some were repre- 344 HISTORICAL GEOLOGY sented in a subordinate role in the Cambrian fauna, while others seem to have made their appearance after the close of the period. Of these none is more important than the graptolites (Figs. 346 and 347), those colonies of little polyps strung on stems. Being freely float- ing animals they were easily transported by ocean currents, and hence single species had an almost world-wide range. Their relatives, the corals, here became important for the first time. It is to be FIG. 346. A colony noted that in the early stages of their evolution the corals were represented chiefly by the solitary hornlike forms (Fig. 348), FIG. 345. Head whereas the habit of living in compact colo- pL a cL e noid S ^f nies ( Fi S- 349 ) became re prevalent in the Ordovician later periods, until to-day the compound period. corals far outnumber the solitary varieties. The mollusks (p. 298), of which only the pteropods and cap-shaped gastropods had been noteworthy in the Cambrian, of graptolites. Each little tooth on the blades held an indi- vidual polyp. The central portion may have served partly as a float. FIG. 347. A branching colony of graptolites impressed upon a piece of shale. THE ORDOVICIAN PERIOD 345 subsequently expanded into considerable diversity. tropods developed many variations of the spiral (Fig. 350) and flat-coiled shells (Fig. 351), lacking in fact only the orna- mental spines and tu- bercles of our modern species. The two- - shelled mollusks, or pe- FIG. 348. A small horn coral (Strepte- lecypods, seem to have lasma) from the Or- mac j e slower progress I do vician limestones. ' yet there are many of them in certain Ordovician rocks (Fig. 352). Like the brachiopods, they first ap- peared with simple unornamented shells, gathering complexity of structure and decora- tion as they advanced. The highest, and in some respects the most re- markable, group of mol- lusks, the cephalopods, rrmlrAQ ifs fir+ smnpnr makes its nrst appear- ance in numbers in the Ordovician strata. The earliest types had FIG. 350.-AnOrdo- straight tapering shells (Fig. 353), open at the d divided partitions The gas- FIG. 349. Broken fragment of one of the earliest com- pound corals, show- ing several coales- cent tubes each built by an individ- ual coral animal. stout flat-coiled gastro- pod (Bellerophon) common in the Or- dovician period. vician gastropod (Hormotoma) with j tall, spiral shell. by sagging into a series of chambers. vance is shown in curved (Fig. 354) or even >CyP d A seeming ad- FIG. 352. A small, plain pelec: (Ctenodonta). tightly coiled shells (Fig. 355), which ap- peared at this time. The remarkable folding of the dividing partitions did not, however, set in until the Devonian. 346 HISTORICAL GEOLOGY There is evidence that the great vertebrate branch had become distinct as early as the Ordovician period, for scales, which appear to be those of fishes, have been found in rocks of that age in the Rocky Mountains. Still another long period must be passed, however, before fishes come into prominence. Land plants and animals. Considering the fact that the continents were so largely sub- merged and that the known Ordovician strata in which our only record of the life is preserved are of FIG. 353. One marme origin, it is not sur- of the earliest . . ,,,{,., j . , and simplest prising that the land animals cephaiopods anc [ pl an ts of this period are (Orthoceras). , , , ,, scarcely better known than are those of the Cambrian. An insect's wing from the rocks of Sweden proves that the land-inhabiting arthropods had already come into being, and it adds confirmation to Our previous FIG. 354. Broken suspicion that land vegeta- alopod (Cyrto- tirm PYiVpH in ceras)> Com P are the sutures with those early those in Figures times; for the 391 and 418. winged insects are almost wholly dependent upon plants for their sustenance. Crustal disturbances at the close of the period. The long, quiet reign of the epiconti- nental seas, which had begun in the Cambrian and continued FIG. 355. A coiled Ordovician through the Ordovician, was cephalopod related to forms still living. partially interrupted by events THE ORDOVICIAN PERIOD 347 which are used to mark the close of the latter period. It is believed that, during ages of tranquillity of the earth's sur- face, the forces which at times produce warping and moun- tain folding accumulate power until finally the resisting strength of the rocks is overcome, and the outer layers are wrinkled and broken. This wrinkling is usually confined to a small belt or district, but within that area the folding and crushing may be intense. In the present instance the first premonition of a change is afforded by the fact that the clear seas of the Middle Ordovician in eastern United States later became turbid with mud, so that the last strata of the system are shales overlying the limestones. Evidently changes in the activities of rivers or currents, or both, were in progress, although it is not easy to prove just what the changes were. In eastern New York the early Silurian strata are found lying unconformably upon highly folded rocks which are known to be of Ordovician age. From this it is known that the recently deposited Ordovician and older strata, in that region and somewhat farther south- ward, were intensely deformed ; and also that the same region became land and was subject to long-continued erosion. The wide extent of the unconformity shows that much of the eastern interior of the United States emerged at the same time. During the compression of the rocks in the East, shales became schists, and fossil-bearing limestone was altered to marble in which nearly all trace of fossils has disappeared. The local nature of this disturbance becomes evident when it is found that in the adjacent regions of New York and New Jersey the Ordovician rocks were only slightly disturbed at this time, while in some portions of the Mississippi Basin they did not even emerge from the sea. The obvious result of the folding must have been a belt of mountains, perhaps of notable height. Although these have since been totally cut away by the erosive agencies, their site is occupied by the newer Taconic Mountains of to-day, and so this disturbance which 348 HISTORICAL GEOLOGY closed the Ordovician period is frequently spoken of as the " Taconic revolution." Similar events in Europe. In Europe the deposition of sediments in Ordovician time was in many ways like that in the United States, and at the close it suffered a similar inter- ruption. The rocks of Wales and Scotland were highly folded into a series of mountains which were gradually worn down during the Silurian period. The fact that the crust was simultaneously wrinkled on both borders of the Atlantic Ocean suggests that a slight subsidence of the great oceanic area may have been directly responsible for the disturbance. Yet it cannot be said that this is proved. QUESTIONS 1. Sun cracks have been found on the bedding planes of the Lower Ordovician limestone in the Mississippi Valley. From this, what do you infer as to the depth of the water in which this lime- stone was deposited ? How does this compare with limestones in general ? 2. Judging from what you know of the Archaean and Algonkian systems, what was the character of the rocks from which the Ordo- vician sediments were derived ? Why does the Ordovician system consist of limestone, shale, and sandstone, rather than pieces of these older rocks cemented together ? 3. What is the chief process of change at work on the surface of a land which is too low to be eroded by streams ? 4. Can you suggest why nothing is known about the sediments which were deposited in the Ordovician period off the eastern shore of Appalachia ? 5. What phase of metamorphism would be most likely to obliter- ate all traces of fossils in the Ordovician rocks of the Taconic Moun- tains ? 6. Can you see any reason for thinking that vertebrates were in existence long before the fishes whose plates have been found in the Ordovician rocks ? CHAPTER XV THE SILURIAN PERIOD Transition from Ordovician to Silurian. In eastern United States and western Europe the Ordovician period seems to be distinctly set off from the Silurian by the so-called Taconic revolution. Elsewhere, however, the transition from the one to the other was quiet and not marked by notable disturb- ances. Some portions of this continent emerged from the sea and became low plains, from the surfaces of which little debris could be eroded. In Oklahoma, on the other hand, and in the western states generally, the surface appears to have remained submerged beneath the sea. These things are clearly shown by the succession of the sedimentary rocks. Thus, as mentioned on a preceding page, the Silurian strata lie in marked unconformity upon the folded Ordovician rocks in the New England region. In Tennessee, Minnesota, and some other states, the two systems are parallel in bedding, but are separated by an irregular weathered surface which is in reality an unconformity. In Utah and Montana the Silurian system is only a part of a thick succession of lime- stones which contain Ordovician fossils below and Devonian fossils above. Clastic sediments along the eastern land. The oldest sediments referred to the Silurian period are unlike in different parts of the country. Along the western flank of the newly made eastern highlands quantities of gravel and sand brought down by swift rivers were spread out in thick banks which thinned toward the west. The gravel, now consolidated into hard conglomerate, is known as the Oneida formation. Where it has since been tilted up on edge it forms mountain ridges, because the softer rocks on each side of it have been more B. & B. GEOL. 20 349 350 HISTORICAL GEOLOGY rapidly removed by erosion. The sand and finer sediments sifted from the gravel were carried farther westward, forming the Medina sandstone. As the high lands were worn down, the rivers became less active, and less gravel was strewn along the front of the mountains. As FIG. 356. Diagram showing the relation of the the zone of gravel Silurian limestone in the Mississippi Valley to the accumulation be- conglomerate, sandstone, and shale in New York. came narrower, the zone of sand deposition encroached upon it, and it thus happened that the Medina sands extended continually farther and farther eastward until they came to lie partly upon the Oneida beds (Fig. 356). The Clinton iron formation. In the more remote parts of the interior this rejuvenation of the New England region seems to have exerted no influence. In Illinois, for example, the first Silurian beds were of shale and limestone, and the deposition continued without change in the character of the sediments until the latter part of the period. Between the sandy coastal plain and this clear, open sea there was an irregular belt over which sediments rich in compounds of iron were deposited on a large scale. This phase of the Silurian rocks has been named the Clinton formation. The iron ore is usually of the red variety or hematite; in some places, where massive beds several feet in thickness are found, pro- ductive iron mines are located. The microscope shows that some of this ore has the structure of limestone, that is, the rock is composed of bits of shells, corals, etc., but the material is largely iron oxide instead of lime carbonate. Students of the subject are not yet agreed as to the exact conditions under which these unusual deposits were made, but there seems good reason to believe that the sediments were laid down in shallow water not far from land. In the process of smelting iron ore it is mixed with limestone and coke, and when the mixture is heated in the furnace, the iron is released from the ore and flows out into the molds. At Bir- THE SILURIAN PERIOD 351 mingham, Alabama, Clinton iron ore, coal, and limestone are found together. This fortunate combination has made that region one of the great centers of the iron and steel industry, and a place of much importance in the industrial upbuilding of the southern states. The interior sea again enlarged. As the period progressed, the sea seems to have encroached slowly upon the land, much as it did during the Cambrian. One broad arm extended northward across Canada and perhaps into the polar regions. As the lands were worn lower and the shores ad- vanced eastward in the United States, the zones of deposition migrated accordingly, so that not only did the Medina sandstone come to over- lap the Oneida conglom- erate, but the limestone of the West, with its peculiar iron-bearing shoreward phase, over- spread the Medina as far as central New York. From the fact that its massive layers form the FlG - 357 - Diagram of a limestone cliff in rC i i Montana, showing levels (x) at which fos- Cllff Over which the sils of different ages were found. Niagara River plunges in its famous cataract, the limestone is known as the Niagara formation. It is, of course, much thicker in the Mississippi Valley, where it seems to have accumulated through most of the period, than in the New York region, where it began to be deposited considerably later. The Silurian furnishes an illustration of the well-known fact that a single rock formation in one part of the country may be equivalent in time to several distinct and unlike formations in another place. How may the absence of Silurian fossils be explained ? 352 HISTORICAL GEOLOGY In western North America the Silurian system, where found, consists of limestone. It is in fact merely a part of a thick limestone series which contains faunas characteristic of the Ordovician and Devonian periods as well as of the Si- lurian. The implication is plain that for long periods of time the open sea held uninterrupted sway. In the region in- cluding Colorado and part of Wyoming, however, Silurian rocks are unknown and it is possible that here was a land mass in Silurian time. Animals of the Niagaran sea. Our knowledge of the living things of the Silurian is largely confined to the rich and varied society of animals which in- habited the clear though shallow seas of the time. The Oneida conglomerate has yielded no fossils, and the Medina very few, perhaps because the turbulent FIG. 358. One of the , i i i i i_ v j- > M commonest triiobites and sand-choked streams which distrib- (Caiymene) of the Ni- uted them were not attractive to aquatic animals. The Niagara fauna, then, may be considered by itself. Of the groups mentioned in discussing the Ordovician period all but two made notable progress in the Silurian, the excep- tions being the graptolites and the triiobites. The decline of the graptolites from their position of importance in the pre- ceding period was rapid. They are not numerous even in the Niagara rocks, and the Devonian period witnessed their complete extinction. Among the triiobites, however, the descent from supremacy was more gradual. In the Silurian they were still abundant, and never were they more diversi- fied in form than at this time. Like the decadent nations revealed to us in human history, they indulged in extrava- gant and futile eccentricities, ill befitting their approaching overthrow. Odd and highly ornate forms appeared in pro- fusion (Figs. 359, 360, and 361), and in most instances the THE SILURIAN PERIOD 353 spines, tubercles, and horns which they produced seem to have had little or no real value in their life activities. We FIG. 359. An unu- sually spiny trilo- btte (Acidaspis) from the Silurian of Bo- hemia. FIG. 360. Atrilobite (Lichas) of the Si- lurian period. Com- pare with Cambrian trilobites. FIG. 361. A highly specialized Silurian trilobite of peculiar form (Deiphon). shall see in studying the later periods that similar eccentric- ities mark the fall of other groups, such as the ammonites and the reptiles. Among the rising groups only a few require special mention. The corals show an increase in the number of composite types, such as the " honey- comb coral" (Fig. 362) and the " chain coral " (Fig. 363), as against the horn corals. Although the echinoderms had been represented even as early as the Cambrian and had attained some importance in the Ordovician, they did not reach commanding prominence until the Silurian. The clear, shallow seas in which the Niagara ooze was produced furnished congenial life conditions not only for corals but for communities of the crinoids (Fig. 364), FIG. 362. A piece of honeycomb coral (Favosites). 354 HISTORICAL GEOLOGY graceful animals attached to the sea floor by flexible stalks and provided with feathery arms or tentacles around the mouth (p. 297). The mollusks (Figs. 365 and 367), and their companions the brachi- opods (Figs. 366, 368, and 369), developed steadily along the lines already defined in earlier times, and be- came constantly more numerous. Clearly preserved fishes appear FIG. 363. The chain f , ,, *\. coral (Halysites), here for tne first time > common in Silurian but as yet they are rare, and the considera- tion of them may best be deferred until the Devonian is discussed. They were extreme^ primitive types, unlike any that are now living. Life on the lands. The plants, which we FIG. 365.-A stout ma ^ wel1 believe Silurian gastropod clothed the Silurian (Strophostylus). j^ are almogt un _ known, doubtless for the same reason that has been suggested to explain the similar ab- sence of information about the flora 1 of earlier periods : the FIG. 366,- Alarge and knOWn r cks afe chief ^ strongly beaked of marine origin. brachiopod (Conchi- Equally scant ig the J FIG. 364. A nearly perfect crinoid, as found in the Niagara limestone of Indiana. The roots served merely for attach- ment. dmm) of the Silurian period. record of air-breathing arthropods, but 1 The flora of a country or of a period is the entire assemblage of trees, shrubs, herbs, and other plants living in that place or time. THE SILURIAN PERIOD 355 FIG. 367. A coiled Silurian ceph- alopod (Phragmoceras). the fossils already discovered show that, even before the Silu- rian period, this group had become divided into its constituent classes, such as insects, scor- pions, and others. Relations with Europe. A fauna very similar to that just described lived in a sea which occupied the site of England and the Baltic region during the same time. It is thought that the route of intermigra- tion between the two conti- nents lay along a shallow-water tract which extended up through Canada and Alaska and perhaps even the polar regions. So easy was the communication along this path that peculiar Swedish corals and trilobites found their way over to Iowa, and crinoids characteristic of the United States became residents also of England. Silurian deserts. The quiet continu- ance of these broad epicontinental seas (Rhynchotreta) char- was interrupted in both continents by changes of far-reaching importance. The deposition of limestone in eastern United States gradually ceased, and, in some areas, if not in all, this was occasioned by the emergence of the sea bottom into a low-lying land. In the West at this time much of the region from Montana south westward remained under water. FlG 369 _ A gii ur ian In the East the Niagara limestone is brachiopod (Ortho- frequently found lying unconformably be- *f J^ %* neath the later deposits. In the districts long hinge line. Fl pointed " ri < an I ro t cks? f 356 HISTORICAL GEOLOGY adjacent to lakes Erie and Ontario, sediments continued to be deposited. While part of the beds were laid down under water, this was evidently not the water of the open sea. The rocks (Salina beds) consist of shales and sandstones of reddish and gray colors interbedded with seams of gypsum and rock salt. The salty beds are covered by a peculiar limestone, parts of which are valuable for the manufacture of hydraulic cement, and in this limestone are found, not the Niagara fossils, but peculiar arthropods (Fig. 370) and fishes of types which are almost unknown in strictly marine formations. The Silurian salt beds of New York have long furnished a large part of the salt used in this country. Wells have been bored through the overlying strata into the salt beds, and the salty water is pumped to the surface. There the water is evaporated and the salt remains. At the present time beds of salt and gypsum are produced in excessively salt lakes, such as Great Salt Lake and the FIG 370 A large ar- Dead Sea. These saline lakes are con- thropod related to those fined to desert regions where evaporation WatlL ton! is ra P id ' U is ^&^t also that the sediments deposited in some desert basins are of a red or brownish color. From these considerations it appears that, in the late Silurian, northeastern United States had a distinctly arid climate. Most deserts are now situated in the interiors of continents, either where they are sheltered from moist winds by barrier mountain ranges, or where drying winds, like the trade winds, blow constantly. The emergence of the continent which seems to have occurred in the late Silurian largely increased the area of land, and, if highlands of sufficient elevation were so situated as to exclude the moist winds from the Gulf of Mexico and the Atlantic, which now bring rain to the Ontario region, the conditions for local THE SILURIAN PERIOD 357 deserts would have been present. The upper limestone, or Water-lime formation, is thought to have been deposited partly in fresh or brackish lakes, which were perhaps made possible by an increase in the rainfall of that region. Closing incursion of the sea. In the vicinity of lakes Erie and Ontario, where the Salina beds are best known, the Water-lime formation grades upward into limestone with coral reefs and marine shells. A second incursion of the epiconti- nental sea is thus recorded. Between these Monroe strata, as they are called, and the overlying Devonian rocks there is no sharp dividing line, but merely a gradual change in the kinds of fossils. QUESTIONS 1. Why are the divisions of the Silurian system as recognized in New York not suitable for Illinois ? 2. The pebbles of the Oneida formation consist largely of pure quartz. Can you suggest how pure quartz gravel could be derived from a complex mass of igneous and metamorphic rocks such as those which were exposed in the ancient continent of Appalachia? 3. By what process may loose gravel be transformed into a hard rock capable of forming mountain ridges ? 4. Why should fossils be rare in the Oneida formation, even if shell-bearing animals were abundant at the time and place it was deposited ? 5. At Cobalt Lake, in northern Ontario, the Niagara lime- stone lies directly upon the surface of Huronian and Archaean igneous rocks. What different hypotheses may account for this relation ? 6. Small patches of Niagara limestone are found northeast of the edge of the continuous formation in Canada and the United States. What is the significance of these outliers (Fig. 371), as they FlG " 371. Diagram of outliers. are called, with reference to the former distribution of the Silurian system ? CHAPTER XVI THE DEVONIAN PERIOD Relations to the Silurian. In North America the Silurian and Devonian systems are not sharply separated from each other, either by a striking unconformity or by noteworthy changes in the character of the sediments. For this reason there has been some dispute as to where the division should be made. The fact serves to illustrate the general principle that geologic time itself is unbroken and that the divisions which we recognize must necessarily be somewhat arbitrary and local in their application. At the close of the Silurian period the great central part of North America seems to have been land. In many parts of the country, for example, northern Illinois, Alabama, and Colorado, no sediments of earlier Devonian age exist, and it is thought that much of this area was land at that time. In some other places, as in Iowa, an unconformity has been found at the base of the Devonian system. The detection of this interruption is usually difficult, inasmuch as the beds below are parallel with those above; upon careful examina- tion, however, the irregularity of the contact, the slightly weathered surface of the uppermost Silurian beds, and the abrupt change in the fossils serve to prove the existence of the break. Such an unconformity clearly indicates two things, namely, that the older rocks were not deformed, as were those of New England at the close of the Ordovician period, and that when the sea withdrew it left a land surface of very slight relief. Had the land been high above the sea, the rivers would have cut deeply into it and would either have developed a very hilly surface, or, if the erosion cycle had gone on to old age, the Silurian strata would have been 358 THE DEVONIAN PERIOD 359 largely carried off and the Devonian sea would have en- croached upon a plain underlain by still older rocks. North America at the beginning of the Devonian period. Although this low-lying land seems to have stretched from Michigan and Virginia westward over much of the present Mississippi Basin, the sea had by no means entirely retreated from the continent. In what is now the lower Great Lakes region and again in Utah, Nevada, and Montana, the deposi- tion of limestone and shale went on from the Silurian far into the Devonian. Considering the isolation of these localities, it is not surprising that the fossils in the one place bear little rela- tion to those of the other. No more do the animals which inhabit the seas off California and New England to-day. DEVONIAN IN THE WEST As the Devonian period progressed, the events in one region were not necessarily the same as those in another. In Utah, for example, lime ooze and mud were deposited uninterrupt- edly throughout the period, with the result that limestone about 1000 feet thick now represents the Devonian in that region. So free from disturbing influences was this part of western United States that the animals of the western sea underwent only very slow changes. The fossils in the youngest beds of the system do not seem to differ widely from those in the oldest. The conditions elsewhere were in contrast to this. DEVONIAN IN THE EAST Helderberg limestone. In eastern United States the period was marked by the gradual reexpansion of the epicon- tinental sea, attended by important changes in the relations of the land and water bodies. At first all the eastern lands seem to have been low, for if land masses had been eroded rapidly, the derived sediments would surely have formed clastic rocks in the adjacent seas. As it was, only limestone 360 HISTORICAL GEOLOGY was laid down and that chiefly in a restricted sea extending from the St. Lawrence region to Virginia. This is called the Helderberg limestone because it is well exhibited in the Hel- derberg Mountains of eastern New York. At the same time, apparently, a bay extended up from the south into Tennessee and Indian Territory. Oriskany sandstone. As the clear sea with its limey bottom spread slowly westward into the Mississippi Valley and perhaps south to Alabama, some radical change in the middle Atlantic states allowed coarse, sandy sediments to be spread out over the Helderberg formation. The result- ing Oriskany sandstone is several hun- dreds of feet thick, and the sand of which it is made represents the decomposition of a vast amount of solid rock. (How might this be accounted for (1) by cli- matic change, (2) by diastrophism ?) Onondaga limestone. Gradually the deposition of sands became restricted, and the sea which occupied the Appa- lachian depression was again clear. In FIG. 372. A bit of ., , ,. , ,, ~ organ-pipe coral (Sy- ^ a second limestone, called the Onon- ringopora) from the daga, was deposited over the Oriskany sandstone. The warmth and shallow- ness of the Onondaga sea are shown by the abundance of corals (Fig. 372) and other animals which frequent coral reefs. By this time, also, the northwestern part of the con- tinent, from Alaska to Alberta, was covered by the waters of the northern ocean. They were apparently not cold waters in those days, for reasons not yet well understood. Hamilton shales. Muddy sediments succeeded the Onondaga deposits in the East, and even in Illinois the lime- stone is less pure. The Hamilton shales are usually dark and bituminous, implying an abundance of minute plants as well as the animals whose shells abound in the same beds. The THE DEVONIAN PERIOD 361 slow decay of these organisms is believed to have produced most of the petroleum and gas which are now obtained from the Devonian rocks in Ontario, Ohio, and Pennsylvania. From the fact that the Hamilton formation thickens as it is followed eastward it seems probable that the mud was largely derived from lands along the present Atlantic slope. The basins coalesce. Toward the close of the Devonian the epicontinental sea attained still greater extent. By the spreading of the northwest Canadian sea southward and east- ward, the western and eastern basins of the United States seem to have been joined (Fig. 373). In the West and North- west muds and oozes continued to be deposited, and from this we may infer that in those remote times the western part of America had none of its present rugged mountain ranges, but that it was a flat or undulating lowland. As the upper Devonian strata are traced eastward to Ohio and beyond, they become increasingly thicker and more sandy. We have already learned that a change from ooze to mud and thence to sand is to be expected as one approaches the shore line. The Chemung formation, as these sandy shales are called, grades finally into thick sandstones which contain few fossils except leaves of plants and bones of fishes. It is from these non-marine strata that the Catskill Mountains have been carved. The imperfect assortment of the sediments suggests that they were strewn by rivers rather than by waves, and that the Catskill beds represent the alluvial apron built out into the shallow sea on the west by streams which descended from the highlands of the Appalachian continent. MIGRATIONS AND CHANGES OF THE SEA LIFE As the continuance of the clear sea over a comparatively isolated province such as the Nevada region allowed the animals which lived there to develop quietly along their own lines of advance, so, on the other hand, the shifting relations of land and sea and changing character of the sediments in 362 HISTORICAL GEOLOGY eastern United States afforded conditions for the rapid and conflicting evolution of species. At the outset of the period FIG. 373. Supposed geography of North America in late Devonian time. The dotted pattern represents sediments on land. The limits of the land mass north and south of the United States are wholly unknown. the animals of the sea were confined to the edges of the con- tinent and such bays as lapped over its surface. Being iso- THE DEVONIAN PERIOD 363 lated, they developed independently, and after the lapse of sufficient time became notably different in the several embay- ments. As the sea later spread over the land these distinct faunas invaded the interior region from different directions, one from the northeast, another from the south, another from the northwest, and so on. As the widening seas mingled, the faunas were one by one brought into conflict, much as the invasion of North America by the French and English brought them into opposition in Canada and the Mississippi Valley in the eighteenth century. Just as we now have a mixed French-English people in Quebec, so the mingling of the Hamilton fauna of the East with the McKenzie fauna of the Northwest produced a mixed race in which the influence of the Northwest immigrants was strongest, and left its stamp on the result. The commingling of two marine faunas results in something more than a mere mixture of the two. The struggle between two faunas usually crowds into extinction certain weaker members of each assemblage, and it often results in the rapid rise of entirely new forms not found in either of the original faunas. In late Devonian times the result of this succession of immi- grations and intermixtures was a fairly cosmopolitan fauna inhabiting the seas from Alabama to Alaska and having close relations with the animals of distant China and Russia. Changed conditions of life. From what has already been said of the Devonian faunas and their migrations it will be readily inferred that the fossils are abundant and locally well preserved. The same groups which were impor- tant in the Silurian are represented also in the next period, although with different relative standings. In the Silurian the animals of the clear seas were almost the only forms extensively preserved. Our best-known Devonian rocks are, however, chiefly shales and sandstones, and so the fossils in them tell us of the animals which frequented the mud banks and the sandy shores rather than the clear, open sea. Con- ditions which are congenial for one group of animals may be 364 HISTORICAL GEOLOGY FIG. 374. Two common Devonian pelecypods. adverse or even fatal to another. Thus many of the mollusks prefer somewhat turbid water and a muddy bottom, while the corals are exterminated by any large admixture of sediment in the water in which they live. With this principle in mind we shall be prepared to find the crinoids, corals, and other animals which were abundant in the Niagara sea relatively uncommon in the Devonian formations except the limestones. Mollusks and brachiopods numerous. Their places were taken by hosts of two-shelled mollusks (Fig. 374) and brachiopods (Figs. 375 and 376), with other groups in subordinate positions. The brachiopods in particular were prob- ably near their zenith in the Devonian. Most of the important types had made their appearance in full force, and it re- mained for later periods only to carry out the lines of progress already defined. Decline of the trilobi es. The trilobites had by this time dwindled to a few forms (Fig. 377) which, however, clung to their Silurian propensity for useless excres- cences and ornaments. Although the two cases may not be similar, there is a resemblance to certain decadent families among our own race who cling to the traditions and outward appear- FIG. 376. A large brachio- an ces of former rank, long after they pod common in the Devo- have been shorn of power and wealth. nian rocks (Spirifer). _ . t ,. Cephalopods take a new line of ad- vance. The chambered mollusks, or cephalopods, now enter upon a new career which eventually leads them to the extreme Fig. 375. One of the commonest De- vonian brachiopods (Atrypa). THE DEVONIAN PERIOD 365 FIG. 377. Alargetrilo- bite (Dalmanites) of the Devonian period. of complexity and diversification, as regards their internal structure. The early Paleozoic types had shells which were divided into chambers by a series of flat or saucer-shaped partitions. In some of the Devonian species these partitions became slightly folded at their edges, and the suture lines on the outside of the shell show corresponding lobes or angles. These simpler varie- ties are called goniatites (Fig. 378) . It will be interesting to compare them with the complex forms of later times. Profusion of fishes. Of all the new developments among the Devonian animals, none is more important than the apparently sudden rise of the fishes. From meager beginnings in the previous periods they spread out into many different types, and be- came so abundant that the Devonian is sometimes called the Age of Fishes. Being among the earliest to make their ap- pearance, it is but natural that the Devonian repre- sentatives of the class should have been primitive in their structure. Lowest in the FIG. 378. A coiled cephalopod (Goniatites) in which the sutures are slightly folded. B. & B. GEOL. 21 scale are the O.s- 3(56 HISTORICAL GEOLOGY tracoderms (literally "shell skin "), which were not fishes at all, in the strict sense (Fig. 380). It is not cer- tain that they possessed jaws, but if they did, there is some reason to think that the jaws worked horizontally as in beetles. Strong resemblances to some of the early arthropods are seen in their bony head shields and in the closely spaced eyes. In fact, their claim to a place among the vertebrates rests chiefly on the possession of a tail fin which seems to imply that they had a rudimentary spinal column. Since none are now in existence it is hard to determine their real character. The true fishes, which are furnished with jaws of the customary type and one or two pairs of fins along their flanks, are represented in the Devonian fauna by many strange and some very large species. Some, on the other hand, were not FIG. 379. A Devo- nian coral, showing $he cup with radiat- ing partitions. FIG. 380. An ostracoderm. so unlike those of to-day that the untrained eye would readily note the difference. Others had the head cased in heavy plates of bone, with only the rear part of the body left in a flexible condition. In modern fishes the limb bones do not extend out into the fins, but end in blunt plates to which the fin rays are attached (Fig. 383). The Devonian fishes, FIG. 381. Tail of a primitive fish with fringe above and below. THE DEVONIAN PERIOD 367 on the other hand, had fully vertebrated fins (Figs. 381 and 382). Again, there are some very peculiar things about the teeth of these ancient members of the finny tribe. Unlike the FlG . 382 . _ Asymmetrical tail of sharp, spikelike teeth of modern fishes (Fig. 385), many of them were rounded or corrugated the sturgeon, in which the body axis follows the upper blade of the fin. plates (Fig. 384) adapted for grinding food rather than for seizing live prey. Altogether the De- vonian fishes were massive and clumsy. As in the arthropods, their skeletons were FlG . 384 .-A single chiefly On the Outside corrugated tooth of in the form of bony Fro. 383.-Fan- 3 hIped armOT > f r the limb tail fin characteristic bones and Spinal COl- of the higher types of umn were often Uttl fishes. more than cartilage. As time went on, the advantage of speed p IG 335. Pointed over armor seems to have led to the tooth of an extinct strengthening of the internal skeleton with bone, and to the development of a more flexible body. a Devonian shark- like fish. FIG. 386. A modern lung fish from Australia, not unlike certain Devo- nian fishes. LIFE ON LAND For the first time we have among the Devonian fossils a fair representation of the animals and plants of the lands. 368 HISTORICAL GEOLOGY The rivers and other land waters supported a variety of fishes and mollusks. Vegetation was luxuriant in favorable places and included trees as well as the lowlier growths. The trees of this period were not, however, like those of the present day. Most of them were relatives of the ferns. Insects and their allies have been found in some numbers. Considering the small chance of preserving such delicate creatures in the rocks, it would not be reasonable to expect many fossils. QUESTIONS 1. Why is the present distribution of the Devonian system less than it was originally ? 2. Can you suggest why the outcrops of the Devonian system are usually narrow bands ? 3. Oil is found in Devonian strata in certain parts of eastern United States. It is usually concentrated beneath anticlines. A well piercing the fold usually encounters first natural gas, deeper oil, and still farther down water. Can you suggest why there should be this arrangement ? 4. Why should the wells obtain more oil from sandstone than ___^_^ from shale ? ^J^J^j^2lS^' <=>.Q: 5. An instance is known of the g^ IT occurrence of black mud containing JL- I teeth and plates of Middle Devonian I fishes in cracks exposed in a quarry _f 3 in the Niagara Limestone (Fig. 387). ' Can you suggest an explanation? FIG. 387. ' Cracks in the Ni- Q Tjiider wnat conditions will two agara limestone filled with faunag ^ and becQme legg and black mud and fossils. , ... . ,1 less like each other .' 7. Why should the fossils of the Chemung formation be less like those of the Catskill beds, which are of the same age, than like those of the Hamilton formation, which is distinctly older ? 8. Can you suggest why crinoids and corals are rarely found in the Oriskany formation ? 9. Why is it not so easy to use the small divisions of geologic time in widely separated countries, as it is to use the larger divi- sions, such as eras ? 10. When a local and a cosmopolitan fauna are permitted to mingle, because of some geographic change, which of the two usually exerts the stronger influence on the new fauna thus formed, and why ? CHAPTER XVII THE MISSISSIPPIAN PERIOD The Carboniferous divided. The Mississippian, Pennsyl- vanian, and Permian periods were formerly combined under the name of Carboniferous. Evidence is accumulating, however, which indicates that the three divisions are really quite as distinct from each other as are such periods as the Devonian and the Silurian ; and so it is thought best to make three separate periods out of the old Carboniferous. Each is named for a region in which the rocks are well exposed and well known. Transition from the Devonian. The transition from the Devonian into the Mississippian period was not marked by abrupt changes in most parts of the North American conti- nent. The chief event which characterizes the Mississippian is the further expansion, over the greater part of the United States and the Northwest, of the epicontinental sea which, even in the late Devonian, was fairly extensive. This expan- sion of the sea was followed later in the period by a corre- sponding retreat. For the eastern interior, it was the last period of purely marine conditions. Clastic sediments in the East. Over what is now the Mississippi Basin, as far east as Ohio, and as far west as Nevada at least, was the open sea. In that portion of this vast region which the Devonian ocean had also covered the strata of the two systems are generally conformable. In much of the West, however, the Mississippian extends be- yond the Devonian and rests directly upon more ancient rocks, in some places even on the Archaean. About the northeastern border of this sea, notably in Pennsylvania and Ohio, coarse sands and muds were accumulating rapidly. 369 370 HISTORICAL GEOLOGY The rocks, as we now find them, are thick clastic formations, usually called the Pocono sandstone below, and the Mauch Chunk shale above. Ripple marks and sun cracks in the shales indicate that they were deposited in shallow water; and a close study of them has recently made it fairly certain that they represent a great flat delta plain over which rivers in a semiarid climate spread silts and sands in times of flood. Occasional coal seams tell of the existence of marshes upon the surface of this delta plain. Limestone in the central and western states. As we trace them farther west and south, the land-derived sediments become finer, and limestone increases in prominence. From Indiana westward massive limestones form the bulk of the Mississippian system. The same formation reappears in the Black Hills, parts of the Rocky Mountains, and the Arizona plateaus, and is believed to underlie nearly all of the Great Plains. This extensive limestone series implies a clear open sea remote from rugged lands. That its genial waters abounded in animals of the sea is proved by the crinoids, corals, and other fossils with which the strata are locally crowded. This is true especially in the Mississippi and Ohio Basins, or, in other words, near the border line between the muddy and the limy bottoms. The deep sea explorations of the " Challenger Expedition " some years ago brought out the fact that animals are always extraordinarily abundant near the mud line or the outer edge of the muddy area ; there the conditions of life seem to be more favorable than else- where. Sedimentation outside of the interior sea. A body of water covering the southern peninsula of Michigan was at this time more or less isolated from the great interior sea. The strata which accumulated there are associated with salt and gypsum, suggesting that the local climate was not moist, and that the basin was cut off from direct connection with the sea. Again, in Nova Scotia sediments were laid down in basins THE MISSISSIPPIAN PERIOD 371 probably not filled by the sea. Thick sandstone and conglom- erate are there succeeded by shales with gypsum. Decreasing seas at the close. The uppermost strata of the Mississippian in the middle states are shaly and even sandy, like the beds which immediately followed the Devonian. Above these sandy beds there is usually a distinct unconform- ity, which separates the Mississippian from the overlying Pennsylvanian system. The lower shaly beds we have inter- preted as the mud banks along the borders of an expanding sea, in which the deposition of mud was gradually being re- placed by that of limy ooze. Toward the end of the period the sea was evidently being restricted in eastern United States. As the shore line migrated west and south, the mud and sand which are usually deposited near shores were spread out over the limestone that had been deposited in the clearer sea earlier in the period. Finally by withdrawal of the sea the eastern part of the country became land. The erosion of this low land was attended by slight warping of the surface, and even a few faults and gentle folds were produced. The result of the disturbance and the erosion together is the unconform- ity between the Mississippian and Pennsylvanian systems. In the far West, changes of land and sea at the close of the Mississippian period were less pronounced. No distinct line of separation between the two systems has been recognized in the Arizona-Nevada region ; but in the Rocky Mountains a widespread unconformity indicates the emergence of the sea bottom. The Paleozoic Alps. During the four preceding periods sediments were being deposited rather generally over western Europe, much as in eastern United States. It is noteworthy that Britain and Germany were also volcanic districts through much of this time. After the close of the Mississippian period, these deposits were locally folded up (Fig. 388) into two great mountain chains, one extending from Ireland into Germany and the other from southern France into Bohemia. These folds now 372 HISTORICAL GEOLOGY FIG. 388. Trends of the folds produced during the late Paleozoic moun- tain-building epoch in western Europe. are so old that they have been worn down to mere stubs, such as the low mountains and hills of the Black Forest and Cornwall. But in their prime they may well have been lofty, snow-capped ranges, and for that reason they are styled the Paleozoic Alps. LIFE OF THE MISSISSIPPIAN SEA On account of the wide extent of the clear Mississippian sea, it is but natural that of all the life of the period we should know the marine animals best. Very few animals and plants of the dry land have escaped destruction, nor are those of rivers and swamps well represented among the fossils of the time. Abundance of the crinoids. In the limestones of earlier periods we have noted the abundance of either corals or cri- THE MISSISSIPPIAN PERIOD 373 FIG. 389. One of the last represen- tatives of the trilo- bites (Phillipsia) found in Mississip- pian rocks. iioids or both. They were preeminently animals of the clear seas ; but the conditions of depth, temperature, and food supply which are essential for one are not quite those which are required for the other. In some parts of the Mississippian sea of the United States, corals seem not to have been favored, although they were common elsewhere. Where the corals were few, crinoids were locally so abundant that some strata are composed mainly of their stems and scattered plates. At no time in their history were the crinoids more diversified or more highly ornamented. Like the trilobites of the Silurian, some of them assumed eccentric and seemingly useless changes of form, with spines, ridges, and knobs upon the plates. Similarly, the crinoids were at this time on the verge of a rapid decadence ; by the close of the Mississippian period the lG i\ 39 ii'T c o m " majority of them had become extinct, plete blastoid. One J J of the stemmed leaving a decreas- echinoderms, espe- mg line Q f descend- cially common in . the Mississippian ants which are but limestones. poorly represented in our modern seas. The cause of their decline is yet a mystery. Development among the fishes. In the eastern part of the interior sea, fishes were numerous, and, as we may well believe, the most formidable predatory animals of the time. Sig- nificant changes had taken place FlG - 391 -A Mississippian gomatite with moderately among them since the Devonian. The folded sutures. 374 HISTORICAL GEOLOGY queer ostracoderms had disappeared, and the heavily protected sluggish types of the true fishes were replaced by more active varieties which relied upon swiftness rather than upon armor. The place of prominence was occupied by the sharks and their relatives, but the Missis- sippian forms of sharks (Fig. 392) were by no means so for- midable as their modern rep- resentatives. In those days many of them were provided only with flat, corrugated teeth suitable for grinding mollusks and other small animals. As weapons of defense against predaceous fishes, such teeth were evidently not effective ; and perhaps to offset this lack, further protection was added to some varieties in the form of sharp spines on the outside of the body. Advent of the amphibians. The vertebrates now show a distinct advance in the advent of a class, some members of FIG. 392. The Port Jackson shark. One of the nearest living relatives of some of the Paleozoic sharks. Like them its mouth is paved with grinding teeth. FIG. 393. A modern salamander or tailed amphibian. (Jordan and Kellogg.) which, at least, were equipped with legs and toes, and were able to live on land and breathe air. As the vertebrates are now predominately land animals, this was a notable step toward the realization of the future destiny of the group. The fossil remains of amphibians are very rare in the Mis- sissippian rocks, and little is known about them. They were long, salamanderlike animals (Figs. 393 and 394), which doubtless spent most of their time in the water. The rela- THE MISSISS1PP1AN PERIOD 375 tionships between these primitive amphibians and the fishes are so close as to leave small doubt that they were actually derived from one of the groups of fishes in the Devonian. Indeed, even among the highest amphibians the young are hardly more than fishes, breathing water through gills and swimming by means of fins. FIG. 3J4. Larval form of a salamander, showing the finlike fringe on the tail and the branching external gills just behind the head. QUESTIONS 1. Many limestones such as those of the Mississippian system contain nodules of flint and chert, a very dense form of quartz. During the weathering of the limestone what should become of these nodules? 2. What different types of animals could make five-toed foot- prints ? Which of these groups is the lowest in the scale of evolution ? 3. An important part of the salt now used in the United States is obtained from Mississippian strata. From your knowledge of the formations of this age, where should you expect to find the center of this salt industry ? 4. What structure should you expect to find in the Paleozoic rocks of Belgium ? Why ? 5. At a locality in Illinois the Pennsylvanian and earlier Paleo- zoic systems have the relation shown in Figure 395. What events are indicated ? 6. On what grounds is it jus- FIG. 395. Section of Paleozoic beds in Illinois. tifiable to separate the Mississippian as a distinct period? CHAPTER XVIII THE PENNSYLVANIAN PERIOD THE system which contains the most important deposits of coal in both the United States and Europe is called in America the Pennsylvanian. Because of the great value of the coal beds, this division of the old Carboniferous has received more attention than the earlier and later portions. Land interval at the beginning. At the close of the Mis- sissippian, a large part of the United States emerged from the sea, and the fact is recorded by an extensive unconformity. Sediments continued to accumulate in certain low or sub- merged regions, for example in Arizona and Utah, and there we find transitional formations; but in the eastern interior especially, land conditions prevailed. The long-continued weathering and erosion of the land removed part of the Mis- sissippian rocks, and locally uncovered still older formations. As the limestones crumbled and decayed, a residual layer of clay, with bits of flint which had formed part of the original rock, was left upon the surface. These insoluble grains and nodules, worked over by the currents of the rivers, and the sea of the ensuing period, contributed to the formation of the basal part of the Pennsylvanian system. Marine conditions in the West. In the Southwest, where changes of geography had been slight, the interior sea had been maintained. Early in the Pennsylvanian it extended itself over a much larger part of the West. In its clear waters limestone was deposited in Nevada, while shales and sand- stones are found in Arizona and Montana. Corals, crinoids, and other marine invertebrates (Figs. 396 and 397) flourished in these waters, as in the preceding age, although the number of fossils which have been found is far 376 THE PENNSYLVANIAN PERIOD 377 FIG. 396. A brachiopod (Spirifer) from the Penn- sylvanian limestone of Col- orado. less. We are not to suppose, however, that the West was entirely submerged at this time. Reddish sandstones in the Black Hills of South Dakota and coarse red conglomerates in parts of Colorado were probably deposited upon land by streams. They contain no marine shells. These red strata are linked with the more widespread red beds of the Permian period and with the peculiar conditions of its climate, a topic discussed in later pages. Transformation eastward. In the Rocky Mountains the Paleozoic rocks have been exposed by the upturning of the beds (Fig. 450). Traced eastward, they dip beneath Mesozoic strata which underlie the Great Plains, reappearing hundreds of miles away in eastern Nebraska, Kan- sas, and Oklahoma. Where they re- appear the Pennsylvanian system is notably changed. Marine limestones are subordinate, and are interbedded with shales, sandstones, and beds of coal. Still farther east the coal be- comes more abundant and the marine Pennsylvanian strata correspondingly less conspicu- ous. Evidently the eastern part of the country was not the site of a clear, open sea. Coal measures of the East. The Pennsylvanian rocks of eastern United States contain so many beds of coal that they are often called the Coal Measures. Only a small part (about 2 per cent) of the total thickness of the system actu- ally consists of coal. The section (Fig. 398) shows FIG. 397. A large spiny brachiopod (Productus) of the period. FIG. 398. Cross section of Coal Measures, heavy black lines represent coal seams. Tho 378 HISTORICAL GEOLOGY 60 \ \ that the coal seams, which average but a few feet in thickness, are interbedded with thicker laj^ers of sandstone, shale, and other rocks. The details of this section hold good for a single locality only. Elsewhere we may find fewer or more coal seams, and the thicknesses of the individual beds vary from place to place. But the general re- lations are typical of the whole region. Origin of coal. There is ample proof that coal is composed of vegetable matter much altered from its original condition. Stumps of trees are sometimes found standing in the coal seams as they grew; delicate leaves are matted upon the shales which accom- pany the coal; and it is often possible to identify, even with the naked eye, the cellular structure of plant tissues in pieces of the pure coal itself. Vegetable substance is composed chiefly of carbon, hydro- gen, and oxygen in very complex compounds. When wood decays, chemical changes take place and new substances are produced. If this decay goes on in the open air, the carbon, hydrogen, and oxygen (most of this from the air) unite in such a way as to form water and the gas carbon dioxide. As these are volatile, the entire substance of the plant soon disappears. Wood rtsh Peat .ignite Soft Coal Hard Coal Graphite 40 20 FIG. 399. Curves showing the changes which take place in the alteration of wood through coal to graphite. Why so little change in the ash? What proportion of the changes may be passed through while the marsh is still unburied ? THE PENNSYLVANIAN PERIOD 379 If, however, the tissues decompose under water, where the air is excluded, the changes are quite different (Fig. 399). There is not enough oxygen present to form much carbon dioxide and water. The principal products are a carbon-hydrogen gas, known as marsh gas, and other compounds which contain less carbon. While the bulk of the hydrogen and oxygen are thus removed, the carbon is only moderately reduced, and thus it comes to form a proportionately larger part of the solid mass which is left. The result of this process is coal. (See curves in Fig. 399.) For the formation of coal, then, two things are needed: abundant vegetation, and decay under water. In forests we have the first condition, but not the second. In the sea, decay takes place under water, but the vegetation is not usually deposited in great quantity. In swamps and marshes, however, both conditions are favorable. That coal has actually come from marsh deposits is plainly indicated by many facts. The Seams of COal Fi G . 400. Petrified stump and roots are basin-shaped, being of a tre( ; uncovered in a coal mine in ,, . , .,* Scotland. thickest near the middle, and thinning out into mere black soils at the edges ; this is just the shape of existing marshes. Again, we find the old stumps and rootlets embedded in the clay beneath the coal (Fig. 400), showing that the vegetation grew where the coal now lies ; and remains of aquatic animals in the midst of the coal tell of the presence of water while it was being deposited. Coal, then, is nothing but the half-decomposed vegetable matter of swamps, long buried by later sediments, compressed by their weight, and converted into a hard rock. Varieties of coal. The varieties of coal mark stages in the process by which the volatile components are gradually 380 HISTORICAL GEOLOGY lost. Peat is merely a compressed but spongy mass of car- bonized plants, such as we may now find beneath swamps. Soft or bituminous coal has lost far more of the gases and liquids and is a firm rock. Anthracite or hard coal is nearly all (91 to 95 per cent) carbon a hard rock bearing little trace of its origin. The loss of the volatile parts of the coal is a very slow pro- cess. Thus we find that the marsh deposits of more recent periods are but partly converted into coal, while, at the other extreme, the most ancient beds are reduced to impure carbon alone, in the form of graphite. It is not, however, altogether a matter of age. In some places, as in Colorado, igneous intrusions have baked the soft coal 1 into anthracite, or even coke. Wherever the coal-bearing strata have been strongly folded, the coal is found to be much harder than in strata of the same age where they have not been folded. Thus the Coal Measures of eastern Pennsylvania contain anthracite because the strata were crumpled, while in the western part of the state they are flat and afford only soft (or bituminous) coal, the age of the rocks being the same in both localities. Coal resources of the United States. The wonderful development of manufacturing in the United States is due in no small degree to the presence of great coal beds in the popu- lous eastern states. No other country, except perhaps China, is so well provided with this essential resource (Fig. 401). Marshy plain in the East. Returning to our picture of the United States in Pennsylvanian times, we may think of the eastern part of the country as generally low and monoto- nously flat. Vast swamps probably bordered the sea which lay to the west, as they now fringe the coast of New Jersey, the Carolinas, and Florida. On the east they were flanked by the land mass of Appalachia. Inland, along the sluggish rivers, fresh-water marshes, like those of the Mississippi and the Yukon, probabty covered large areas. Gradual but halting submergence of the region seems to have been in progress. 1 These particular coal deposits are of much later age. THE PENNSYLVANIAN PERIOD 381 Now and again the swamps were inundated, allowing sand, mud, and even lime ooze to be spread over them. If the movement soon ceased, the sea bottom was gradually built up by the sediments until the water again became so shallow as to favor the growth of marsh plants. That the sea was not COAU FIELDS OF EASTERN UNITED STATES SCALE OF MILES FIG. 401. The coal fields of eastern United States. Those east of the Great Plains are largely of Pennsylvanian age, but in part Triassic and younger. (U.S. Geol. Sure.) always encroaching on the land is shown by the presence of unconformities in the Coal Measures. While some of these are due merely to the shifting of stream channels traversing the marshes, others imply temporary land conditions during which the rocks were eroded slightly. In short, the land was very near sea level, but was sometimes above it and B. & B. GEOL. 22 382 HISTORICAL GEOLOGY sometimes below. During the Pennsylvanian period many hundreds of feet of strata with many distinct coal beds accu- mulated. Coal Measures in Europe. The marshy plains were duplicated on a smaller scate in western Europe ; and, from the coal seams there formed, England, Germany, and adjacent FIG. 402. Little wheatlike shells of protozoans (Fusulina) in a Pennsyl- vanian limestone. countries now derive most of their supply of coal. Russia, the Mediterranean region, and southern Asia, however, were occupied by clear, open seas in which thick beds of limestone were deposited. In some places these strata are so full of the little wheatlike shells of the protozoan Fusulina (Fig. 402) that they are generally known to geologists as the " Fusulina limestone." LIFE OF THE COAL SWAMPS Plants well recorded. Plants are known to have been plentiful in the Devonian, and there is reason to believe that they clothed the land surfaces even in much earlier periods; but by the accident of having large coal beds preserved, we have in the Pennsylvanian rocks for the first time a satisfac- tory record of the plants of the land. Dominance of the fernlike forms. As in our modern swamps, so in those of the Pennsylvanian period, plants of all sizes lived together in the wet places. Little floating algae, hardly visible to the eye, low sedgelike forms, and even large trees were present. But there was this difference : the plants belonged more largely to the lower branches of the vegetable kingdom. Neither the cypress nor the mangrove, nor even the THE PENNSYLVANIAN PERIOD 383 FIG. 403. Leaflet of a seed fern from the Coal Measures of Pennsylvania. tamarack and rushes, were in existence then. The prevalent plants were the seed ferns (Fig. 403) with some true ferns and other pteridophytes (see p. 293). They were free to occupy all the stations in life now held by the higher seed plants. Some were low herbs, like our modern ferns, while many had developed woody trunks with bark, and these rivaled our present day trees in stature. The nu- merous stumps and fallen logs which have been found embedded in the coal show that extensive forests of these trees (Fig. 404) were common in both the United States and Europe, as well as in the tropics. The graceful fronds which crowned the palmlike trees may often be found matted between layers of shale, where they have been preserved as in a botanist's press. We can gain a fair idea of the aspect of the Carboniferous forests by comparing the tree ferns which still inhabit New Zealand and Australia. With the seed ferns were mingled dense thickets of reeds, resembling our familiar horsetail grass (Equisetum). Many reached the size and perhaps the strength of the tall bamboo of Asia, although their modern descendants are of lowly stature. Probably the largest trees of the period were the so-called " scale trees " (Lepidodendron and others). Unlike the pre- ceding forms, the trunks branched as in our familiar elms, and instead of broad, feathery fronds their^eaves were short and stiff, and were attached closely to the trunk and branches. The nearest living relative of the Lepidodendron is $i,e trailing club moss (which is not a true moss at all), one of the frailest little herbs of our modern forests. Higher plants appear. Thus far all the coal plants which have been mentioned have been members of the Pteridophyte 384 HISTORICAL GEOLOGY group or of that transitional class which we have called the seed ferns (p. 293). There is still no evidence of the existence of the plants with incased seeds and prominent flowers, but FIG. 404. Ideal view of the trees in a Carboniferous swamp. The large cone-bearing tree in the center is the scale tree (Lepidodendron). On the right are gigantic horsetail reeds and on the left Cordaites, one of the ear- liest Gymnosperms. (After Horsfall.) THE PENNSYLVANIAN PERIOD 385 FIG. 405. A small brachi- opod (Chonetes) common in the later Paleozoic rocks. the gymnospcrms were represented in the Pennsylvanian forests by Cordaites, a tree which com- bined many of the characteristics of the conifers and the palmlike cycads. They had long sword-shaped leaves and appear to belong to a distinctly higher level of development than any of the fernlike plants. Land animals diversified. In older systems of rocks the remains of land animals have been found only rarely. In the Pennsylvanian, however, with its well- preserved plants, the finding of many air- breathing animals should be expected. Among the arthro- pods, a variety of insects (Fig. 408), scorpions, centipedes, and spiders testifies to the wide diver- sification through which the group had passed in the periods before. As yet, however, the bees, butterflies, and other highly specialized insects had not ap- peared, a fact which gains added in- FIG. 406. A small brachiopod (Pug- nax) with sharply folded shell. FIG. 407. A Pennsyl- vanian scallop shell ( Aviculopecten) . terest from the reflection that the flowers on which these animals now depend were like- wise yet to come. Amphibians take the lead. In the Mississippian period amphibians are known to have been present. Far more abun- dant remains of them are found in the coal-bearing rocks which followed. Nearly all appear to have been like the salamanders in form, but many FIG. 408. A large winged insect from the Coal Measures of France. 386 HISTORICAL GEOLOOY of them had more substantial bony frames and were of larger size (Fig. 409). Certain degenerate types had lost the use of the limbs and doubtless adopted the habits of snakes. FIG. 409. A large amphibian of crocodile-like form and habits, as it prob- ably appeared in life. i ' '(..-, ' First appearance of the reptiles. 'Recently the bones of true reptiles have been discovered in the Coal Measures of Illinois and Pennsylvania. Not until the next period, how- ever, does the class come into prominence, and so the discus- sion of them is deferred until tliat Chapter is reached. Climate of the Pennsylvania!!. The abundance of vege- tation in the coal swamps has been thought to indicate that North America and Europe were covered with tropical jungles in which''l!he' growth' of plants was luxuriantly rapid. This would imply a climate warmer ithan that of the present, and perhaps m6ister. By others, 'however, it is- pointed out that the largest accu- mulations 6 : f 'peat are now being formed in cool regions, such as Canada ' and -northern Europe. 'Singularly enough, the microscopic -structure of the leaves of the coal trees is much more like that o"f our northern conifers and other hardy plants than like' the- clel&aie' and ; iJiih-skihned leaves prevalent in THE PENNSYLVANIAN PERIOD 387 tropical jungles. Thick bark is another feature shared by the coal trees and those of our cooler countries to-day. It seems not improbable, therefore, that the climate under which many of the coal marshes nourished was more like that of Canada than of Florida or the Amazon. Great length of the period. While it is not possible to calculate exactly the duration of geologic periods, some rough estimates made for the Pennsylvanian are of interest. For the growth of the vegetation which made the coal seams in a single locality 1,000,000 to 2,500,000 years would seem to be required, and, for the sediments in which they are inter- bedded, at least as much more. It is therefore possible that 5,000,000 years were included in this single period. QUESTIONS 1. In Pennsylvania there are thick beds of conglomerate and sandstone which contain no fossil shells, but an abundance of plant leaves in certain layers. Can you suggest the origin of these rocks ? 2. The sandstone which lies at the base of the Coal ^^^^^^: : : : . Sandstone :.- Measures in many parts of this country and Europe is sometimes called the mill- stone grit. Can you suggest why this name was given it ? FIG. 410. A sandstone "cut out" in a coal seam. 3. The accompanying cross section (Fig. 410) shows a narrow, winding bed of sandstone lying in a coal seam. Such things are known to the miners as "cut outs." How may such a feature have been produced ? 4. Tell all the events and changes which you find recorded by the accompanying sec- tion taken from the Coal Measures of Ohio (Fig. 411). 5. Coal seams are often broken by faults. 411. Section Jf in flowing a certain coal seam in a mine, you should encounter such a fault, how could you tell whether to hunt for the lost continua- tion of the bed at a higher or lower level ? 6. The Pennsylvanian strata are the latest widespread Paleozoic FIG. from Ohio Measures. Coal 388 HISTORICAL GEOLOGY deposits in eastern United States. Why should their outcrops, as shown in the map (Fig. 401), be so different in shape from those of the Cambrian system? 7. In Rhode Island the coal is very hard and graphitic. From this fact what do you suspect with reference to the structure of the Coal Measures in that district? 8. From which variety of coal could illuminating gas be made to the best advantage, and why ? 9. Of the mineral products which you have studied thus far, which most resembles coal in method of occurrence, iron, copper, or zinc ? 10. Iron ore is not infrequently deposited in bogs at the present time. That being true, in what part of the United States might such ore be expected in formations of Pennsylvanian age ? CHAPTER XIX THE PERMIAN PERIOD A transition period. The Permian marks the transition from the Paleozoic era to the Mesozoic. In eastern United States the Permian rocks are a mere continuation of the Penn- sylvanian system, while in the Rocky Mountains it is often impossible to separate Permian strata from those of the Trias- sic period. Only locally are the systems sharply marked off from each other. Emergence of the eastern region. --Throughout the Per- mian, the interior sea was slowly being withdrawn. The deposition of the Coal Measures continued on into the early Permian in the Ohio Valley. Some of the sediments were laid down in rivers or fresh-water lakes, and contain abundant leaves of plants. Later in the period the region seems to have been drained, leaving a broad lowland which was only feebly eroded. The sea lingers in the Southwest. In the southern part of the tract which we now call the Great Plains the sea lingered somewhat later. Shales, sandstones, and limestones quietly accumulated in Texas, and perhaps in Kansas. The red beds. The later Permian rocks of northern Texas, however, tell of very different conditions; they are reddish shales, with layers of gypsum and rock salt. Evi- dently the sea had by that time receded still farther, leaving a desert region in western United States, with saline lakes in the depressions. In Colorado and some other places the formation of these red beds began during the Pennsylvanian and continued on into the Triassic period, indicating that the arid climate was of long duration. 389 390 HISTORICAL GEOLOGY Expansion of Permian lands. The gradual withdrawal of the epicontinental sea eventually left almost all of the continental platform dry land. On studying other countries we find evidence that there, likewise, the land was more extensive than at any other period in the Paleozoic era. The Permian was everywhere a time of expanded continents. To explain such a general withdrawal of the seas the sugges- tion has been made that the deep ocean basins sank slightly, thus leaving the continents relatively higher than before. So widespread and "radical a change marks this as one of the critical periods of geologic history. The Appalachian trough. -- Throughout the Paleozoic era, the thickest layers of sediments had been laid down in the interior sea just west of the old Appalachian land, from New England to Alabama, and even across to Oklahoma. This curved belt had been a subsiding trough, sinking perhaps because of the weight of sediments which were constantly loaded upon it. Much of the time the sinking just kept pace with the deposition of sediment, so that thousands of feet of strata were formed in relatively shallow sea water, as we now learn from the presence of such things as .coral reefs. At other times, the subsidence was more rapid, and deeper water prevailed, or on the other hand was of a halting nature and allowed the coastal rivers to build out the seashore with allu- vial deposits. Finally, in the Permian, the sinking and the sedimentation ceased, and the process was reversed into a slow emergence. The sediments deposited in this trough had then reached a thickness much greater than that of the corresponding strata in the Mississippi Basin. Crumpling of the east flank of the continent. Near the close of the Permian, whether as a result of a sinking in the Atlantic basin, or from some other cause, the 'east side of North America was subjected to powerful horizontal com- pression. The Appalachian trough was a weak zone 'in the crust, just as the bend in a crooked stick determines the point at which it will break when pressure is applied at THE PERMIAN PERIOD 391 the ends. From Newfoundland to Alabama and even into Oklahoma the rocks were thus crumpled. The rocks of the old Appalachian land had been folded more than once before, and so the Permian deformation added little to the com- FIG. 412. An ideal representation of the west coast of Appalachia, during the Paleozoic era. The ancient rocks on the east are being eroded, and the sediments laid down in the sea gradually become finer westward* (Modified after Willis.) plexity of their structure. The Paleozoic rocks in the great trough, however, were now folded for the first time (Figs. 412 and 413). In Pennsylvania, the strata were bent into a series of open anticlines and synclines (Fig. 46). Farther FIG. 413. The same, after folding of the Paleozoic sediments. The unshaded portion shows the folds restored as they might have been if they had not been affected by erosion. (Modified after Willis.) south, where deformation was greater, the folds were com- pressed and overturned westward (Fig. 47). Here and there the stiff limestone and quartzite formations were broken and thrust over the adjacent rocks (Fig. 414). At the same FIG. 414. Closely folded strata in the southern part of the Appalachian mountains. (U.S. Geol. Surv.) time the soft shales were crumpled and crushed beneath the stronger beds. So numerous are these overthrusts in some districts that the original folds cannot now be reconstructed. 392 HISTORICAL GEOLOGY Folding a slow process. It is probable that the folding was accomplished very slowly, as are the larger earth move- ments of the present time. The anticlines probably rose so gradually that the decay of the rocks and the work of streams partially kept pace with the growth, so that the young moun- tains were at all times ragged and gashed with valleys. This is true of growing mountains to-day, such as the Coast Range of California and the St. Elias chain in Alaska, and it is safe to judge the Permian by the present. Even the great thrust faults, along which massive limestones have been pushed several miles over younger beds, were doubtless made by a succession of slippings, each advancing not more than a few feet, and each slip separated from the next one perhaps by months or years of time. While recognizing, however, that the folding and the uplift were very slow, we may well imagine the first Appalachian and Ouachita (Oklahoma) Mountains as a series of lofty, rugged ranges comparable to the Alps, or to the modern Pacific ranges in North America. Effects of expanding the continents. --The widespread emergence of the continents into dry land must have produced important changes, not only in the geography of the period, but in the conditions of life for plants and animals, and even in the condition of the atmosphere and in the climate. It will be best to inquire into these matters separately. Adversities inflicted on the sea life. During the Paleo- zoic era the shallow epicontinental seas had harbored abun- dant marine animals and plants. With the exclusion of these broad sheets of water from the continents the home of such forms of life was much reduced in size. It would be entirely natural to expect that as a result the competition for a living would become much keener, and that many of the forms less able to adapt themselves to the new conditions would be exterminated. This may be the explanation of the well- known fact that very few of the distinctly Paleozoic fossils pass on into the Triassic system. Few of the large groups entirely disappeared, although some were much diminished, THE PERMIAN PERIOD 393 while others, such as the corals, were largely reorganized on new plans. The change is seemingly one of the most abrupt and profound in all the geologic record. Yet in northern India and California, where the Permian seas lingered on into the Triassic, the change in the fossils is gradual and no sharp dividing line can be drawn. More ample opportunities for the land life. The very changes which restricted the habitation of the corals, mollusks, and their kin gave wider room to the denizens of the land. A great abundance and variety of plants and insects were already present. The salamanderlike types of amphibians were even more numerous and better constructed than in the Pennsylvanian. In fact, they were never afterward as promi- nent as at this time. Amphibians nowadays are small crea- tures, and most of them have soft bodies; but some of the Permian types were large and were more comparable to rep- tiles like the crocodiles of to-day with their bony-plated heads, powerful muscles, and formidable array of teeth. Reptiles gain the ascendancy. It was left for the true reptiles, however, to gain supremacy among the land animals in the Permian period, in spite of their amphibian rivals. When any group first appears in geologic history, it is apt to be 'represented by closely related kinds unlike those which live to-day. These are known as generalized types, because they combine vaguely in one animal the characteristics of several later kinds. Thus in the Permian there was one kind of rep- tile which resembled in some respects the crocodiles, the lizards, and other types now extinct, and yet cannot be classed with any one of these groups more than with the others. Later, these generalized types branched out into the distinct forms now recognized. These will be described in connection with a later period. Prevalence of arid climates. The abundance of salt and gypsum beds in the Permian strata of many countries has already been mentioned as indicative of desert conditions. Deserts are, of course, only possible on land, and to-day they 394 HISTORICAL GEOLOGY are especially well developed in the interior regions of large continents, for example, in central Asia. Wide extension of lands, as in the Permian, therefore favors the making of deserts in appropriate places. Glacial conditions in the tropics. While considering the Permian climate we must not fail to note what is easily the most remarkable fact now known about the period. Asso- ciated with rocks of Permian age layers of glacial till have been found lying upon scratched and grooved surfaces of older rocks. These glacial beds have not been discovered in the polar regions, as we might confidently expect, but in India, South Africa, South America, and Australia ; that is to say, near and even within the tropics. Nor are we to suppose that glaciers were confined to lofty mountains. The limestones and shales with which some of the layers of till are associated show that they were deposited near or even below sea level. The existence of glaciers over so wide an expanse of the earth's surface and even within the tropics themselves points to most unusual climatic conditions. We may well believe that they indicate a colder climate than now over much of the globe ; but mere cold does not account for the strange dis- tribution of the glaciers, and a satisfactory explanation of all the facts is still lacking. SUMMARY OF THE PALEOZOIC ERA Geographic conditions. From the Cambrian to the Permian, the more persistent lands were in eastern Canada and southeastern United States. The central and western parts of the continent were repeatedly submerged by a rela- tively shallow sea. As stated on a previous page, the copious supply of detritus from the Appalachian land built up the thickest Paleozoic formations along the eastern border of the interior sea, while in the middle of the continent sedimenta- tion went on more slowly. In the far West, the Colorado region and parts of the Pacific THE PERMIAN PERIOD 395 slope were occasionally out of water, although there is little evidence that they were as mountainous as now. Where the Great Basin now is, the sea was especially long-lived, and thousands of feet of varying marine sediments were there laid down. On the whole, the seas overlapped the continent more in the early than in the later part of the era. Throughout the Paleozoic periods volcanic activity was confined almost entirely to the Pacific side of North America, much as in recent times. In Europe, however, most of the volcanoes were near the Atlantic. They were particularly numerous at times in the British Isles. (How does this compare with modern conditions?) Climatic conditions. The climates of the Paleozoic periods have left only a scanty record. That there were deserts in several countries at various times is proven by such saline deposits as those of New York and Michigan; and that moist conditions prevailed at other times is indicated by the coal beds. It was formerly supposed that the earth was hotter in Paleozoic times than now, but Cambrian and Permian glacial deposits in regions which are now semi- tropical seem to preclude this view. Of the distribution of the climatic zones we know almost nothing. We can infer their existence only because as long as the earth is round, and the surface part receives its heat from the sun, such zones must be present. Development of plants and animals. In reviewing the progress of the living things we see many noteworthy changes in the course of the Paleozoic era. The early periods were dominated by the invertebrated animals. One by one, new groups made their appearance ; and of these, some, like the trilobites, passed their prime and slowly dwindled to extinc- tion, while others merely retired to a subordinate position in the scale of life. In the later periods, fishes and amphibians successively came to the front,' proved themselves more powerful than the 396 HISTORICAL GEOLOGY invertebrates which preceded them, and in turn yielded par- tially to the reptiles, which began their rise near the close of the era. Of the evolution of the plants we know much less ; but it is clear that the ferns and some of the gymnosperms were the prevalent types in the later Paleozoic periods. The higher groups were still to be evolved. QUESTIONS 1. Is it necessary to assume that salt lakes are detached parts of the ocean ? Can fresh lakes ever become salt, and if so, how ? 2. Can you suggest a reason why desert sandstones like some of those in the Permian system are usually cross bedded ? 3. In Germany a single bed of salt in the Permian system is said to be more than four thousand feet thick. What does this indicate ? 4. What is the significance of the Permian system in India with reference to Laplace's theory of the origin of the earth ? 5. Although in Australia the center of Permian gla- ciation is not known, and neither surface moraines nor drumlins have been identi- fied, the direction of glacial FIG. 415. -Glacial markings on a rock movemen t ^s been deter- surface. mined. Can you suggest how from a single striated surface of rock (such as represented in Fig. 415) one may learn whether the ice was moving toward the left or right ? 6. It is often said that coal has been formed in tropical jungles. What is the significance of the coal beds which lie between sheets of glacial till in Australia ? CHAPTER XX THE TRIASSIC PERIOD Transition from Paleozoic to Mesozoic. The Permian period fades into the Triassic so gradually that a dividing line cannot always be drawn between them. In eastern United States, it is true, great geographic changes had taken place, and the crumpling of the Appalachian sediments had produced ranges of mountains where there had been a coastal plain before ; but the rest of the continent remained much as in the Permian period. Deserts continue in the West. In the Triassic, as in the Permian, red sediments were deposited where the Great Plains and Rocky Mountains now stand. Beds of gypsum, the relics of salt lakes, point to the prevalence of desert condi- tions, much as in Nevada and Utah to-day. Needless to say, the remains of living things are very rare in the red beds. It is probable that much of the West was an inhospitable place for both plants and animals, except such hardy forms as were fitted to live in an arid land. 1 Marine strata on the Pacific slope. Marine rocks of Triassic age are found only on the western border of the continent, and in but few places even there. From this we may infer that the seas which had been drawn off from the continental platform during the Permian were very slow in creeping back upon it. The Pacific is the only ocean known to have encroached upon the North American land during the period. Its shore line lay somewhat farther east than now, especially in British Columbia and Alaska. In the United States it reached Idaho and Nevada. Off this coast thick 1 It may be noted, in passing, that desert conditions were prevalent also in western Europe at this time, as they had been in the Permian period. B. & B. GEOL. 23 397 398 HISTORICAL GEOLOGY banks of mud and ooze accumulated. In consequence of compression at a later time, the sediments are now folded slates and schists. Erosion of the new Appalachian Mountains. No surely marine beds of Triassic age are exposed in the eastern part of the continent. The land probably extended out even beyond the present Atlantic shore. The growth of the young Appalachian Mountains seems not to have ceased entirely in the Triassic period. As the Paleozoic rocks were folded, the eroded surface of old Appa- lachia was likewise warped, and, in the broad downwarps, streams continually spread the abundant sand and silt which they were removing from the mountains. The deposits on the low slopes and bottoms of these basins eventually reached a thickness of thousands of feet ; and, as is common among rapidly accumulating sediments, the particles were not well assorted. Alternations of sandstone and shale of varying colors are characteristic of the Newark formation, as these beds are called. Red is the predominating color. Locally, as in southern Virginia, the Triassic rocks include a few beds of coal, which bespeak the growth of marshes in the low grounds. The fact that these marshes existed prob- ably means that the climate of the Atlantic seaboard was less arid than that of the West. Volcanic eruptions in the Atlantic slope. About the time the Newark sediments were being laid down, eruptions of basaltic lava occurred in the same district. Some of the basalt sheets rise obliquely through the strata, thus proving that they were squeezed into the rocks as intrusions. Others have cindery surfaces and are overlain by sandstones which are not altered at the contact with the lava. Here, evidently, the flows were poured out upon the surface and afterwards buried beneath sediments. Being harder than the sedimentary strata, the lava sheets have been left as ridges in the subsequent wasting of the surface. The palisades of the Hudson and such heights as THE TRIASSIC PERIOD 399 Mt. Holyoke in Massachusetts are merely the outcropping edges of hard Triassic lava sheets, formed during this epoch of volcanic activity, the last in the history of the eastern portion of North America. FIG. 416. A Triassic brachiopod (Terebrat- ula). The majority of Mesozoic brachio- pods have this general form. LIFE OF THE TRIASSIC New aspect of the marine invertebrates. Although the lower animals of the Triassic seas resemble the Paleozoic types in many ways, the differences are nevertheless very distinct. Not only had some of the Paleozoic groups, such as the trilobites, wholly disappeared, but others, as the brachiopods (Fig. 416), had been relegated to an inferior station. The mollusks became the most abun- dant and conspicuous of the shelled ani- mals, and among them the group of coiled cephalopods had a remarkable development during the Triassic and later Mesozoic periods. From the simple types with straight sutures, they had advanced later in the Paleozoic to the posses- sion of lobed or wavy sutures. In the Mesozoic era the folding of the partitions became most intricate (Figs. 418 and 419), producing equally com- ^--^_L - * plicated suture patterns. FIG. 418. An ammonite with Reptiles overreach the amphib- part of the outer shell re- ians The brief supremacy of moved to show the complexly . .. . , , folded sutures. the clumsy amphibians had now FIG. 417. A small pele- cypod from the Triassic limestone of Europe. 400 HISTORICAL GEOLOGY passed. Large alligatorlike forms of powerful build were still common in the Triassic marshes, but after this period the class was represented only by smaller soft-bodied types, such as frogs and salaman- ders. The reptiles were the class in power. The bones of many reptiles have been found in the Triassic rocks, and the existence of others is made known to us by the host of footprints (Fig. 420) which they left upon the muddy flats along the slack rivers and bays, as in the valley of the Connecticut River. The mud has since hardened into stone, but without effacing the footprints. Some of these tracks show the marks of three toes, and were at first very naturally mistaken for those of birds. FIG. 419. One of the simpler ammo- nites (Ceratites) showing the moder- ately folded sutures. FIG. 420. Tracks of three-toed reptiles found in the Triassic sandstone of the Connecticut Valley. (After Hitchcock.) THE TRIASSIC PERIOD 401 The reptiles adopt many roles of life. There seems to be no doubt that the ancestors of the early reptiles were the amphib- ians. We may think of them, then, as originally inhabitants of marshes and inland bodies of water. Reptiles adapted for such a partially aquatic life were rather common in both the Permian and Triassic periods, as their fossil remains attest. The terrestrial type becomes prominent. Some of the reptiles came to spend more and more time on land, and even- tually became fitted for living wholly under such conditions. Some walked on all fours, as do our modern cattle and many other mammals, but the more agile varieties apparently were leapers, using their powerful hind legs and stout tails after the manner of the kangaroo. The majority of these swifter forms seem to have preyed upon other animals, and for this purpose their teeth were sharp and strong. They played the role of our modern beasts of prey, such as the tiger and the wolf, although in a manner no doubt peculiar to themselves. Reptiles find a place in the sea. The tempting source of food which the fishes and mollusks of the sea presented was early appropriated by other branches of the reptilian stem. Two main types were represented in the Triassic period. Of these the Plesiosaurs were large, flattened saurians with long, slender necks and short heads. Their legs eventually became mere paddles like those of the modern sea turtles. The sus- picion that they fed partly upon mollusks, for which they probably delved in the shallows with their long necks, is strengthened by the finding within their bodies of smooth pebbles, thought to be gizzard-stones used to pulverize the food swallowed. Of all the reptiles none were better fitted for living ex- clusively in the open sea than the Ichthyosaurs (fish-reptiles). They had acquired the form of fishes themselves (Fig. 421), with the long powerful tail fin, the short neck, and long jaws set with sharp teeth. These animals seem to have been almost exclusively fish eaters. Most aquatic reptiles, for example the turtles, lay their eggs in sand along the shore; 402 HISTORICAL GEOLOGY but the ichthyosaur, having lost the power of resorting to the shore to lay its eggs, was in the habit of bringing forth its young alive, as is proved by the interesting find of five little ichthyosaur skeletons undamaged within the skeleton of one of these reptiles. FIG. 421. A family of fish reptiles (Ichthyosaurus). (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copyright by Amer. Mus. of Nat. Hist.) Mammals make a feeble beginning. Were it not for the overwhelming predominance to which the mammals after- wards attained, their first appearance would be scarcely worth mentioning. In the Triassic rocks a few little bones have been found which seem to be those of primitive mammals. THE TRIASSIC PERIOD 403 They were as small and insignificant as the moles of to-day. The most distinguishing thing about them is that their teeth were differentiated into incisors, canines, and molars, as in mammals, while almost all reptiles have merely pointed teeth much alike throughout the jaw. The derivation of mammals from some of the Permian land reptiles is now the most favored view. FIG. 422. A living Mexican cycad. (Photograph by C. J. Chamberlain.) Conifers and cycads make the forests. A change in the plants which had been in progress during the Permian was clearly defined in the early Mesozoic periods. The great ferns and allied trees of the Carboniferous forests were sup- planted by a higher group (gymnosperms) containing the conifers and the palmlike trees called cycads (Fig. 422). 404 HISTORICAL GEOLOGY Ferns continued to be common, but there were more of the small varieties, like those now growing in our forests, than of the tree ferns. The woodlands of the Triassic times doubtless had a somber aspect not unlike that of our pine forests to-day. Nor is it surprising that the purely vegetarian land animals were then so little developed, when we consider that the tough, fibrous leaves of palms and the resinous needles of pines and similar plants are among the least palatable foods for our modern cattle and wild animals. The introduction of these higher animals seems to have awaited the evolution of the flowering plants, particularly the grasses. QUESTIONS 1. In the Humboldt Mountains of Nevada the marine Triassic rocks rest on metamorphosed pre-Cambrian beds. What different explanations may be offered ? Fio. 423. Conglom- erate with lava peb- bles resting upon a sheet of lava. FIG. 424. Sheet of lava with secondary minerals in the ad- jacent shale. FIG. 425. A sheet of lava with in- cluded pieces of the adjacent rock. FIG. 426. Sheets of lava inter- bedded with layers of tuff. 2. In the diagrams (Figs. 423- 426), which are the intrusive and which are the extrusive lava sheets, and how are the facts known ? 3. Small bodies of copper ore have been found in the Newark rocks. What do you suspect as to their origin ? 4. Some of the sandstones in the Newark series contain abundant grains of feldspar and flakes of mica. How do these rocks differ from ordinary sandstone ? Can you suggest the conditions under which the two different varieties are made ? 5. What kinds of fossils would you expect to find in the Newark series, and what about their abundance? Why? CHAPTER XXI 1 -^-^^--=--^-~ FIG. 427. Triassic sediments in Connect- icut with a surface flow and intrusions of lava, as they may be supposed to have existed before faulting. THE JURASSIC PERIOD Land in eastern North America. Rocks which were made during the Jurassic period are not extensively exposed in the United States, and for the most part they are less well known than those of other periods. In the eastern half of the country no rocks which are surely 1 of this age have been discovered. From this circumstance it seems probable that the eastern part of the continent was largely out of water, and that the erosion of the Appalachian region was still in progress. After the deposition of the Newark sediments during the later part of the Tri- assic period, the eastern border of the continent was slightly warped. As a result, the Triassic rocks are now tilted, to the east in New England, and to the west in New Jersey and southward. During the same disturbance the rocks were broken by many normal faults which, in some instances, repeat the interbedded sheets of lava (Figs. 427 and 428). Temporary inundation of the Northwest. Little is known of the events of Jurassic times 'in the great interior of the 1 It has been suggested that some doubtful fresh-water deposits along the Potomac River in Maryland are of Jurassic age, but it is equally prob- able that they are Comanchean (Lower Cretaceous). 405 FIG. 428. The same, complicated by several normal faults and eroded to a peneplain. 406 HISTORICAL GEOLOGY FIG. 429. Probable appearance and structure of a com- mon Jurassic mol- lusk (Belemnites) allied to the mod- ern cuttlefish. The shell is the shaded portion below and is the only part usually preserved. (After Pictet.) FIG. 431. A Juras- sic oyster shell (Ostrea). United States. In -the West only barren sands and clays, such as accumulate on land, and probably under conditions of dry climate, seem to have been deposited in the earlier part of the period. Later a broad tract extending from Alaska south to Wyoming and Utah subsided enough to let in an arm of the sea. That this sea was shallow is indicated by the character of the sediments, which are shales and sand- stones, with occasional beds of limestone. The fossils in the limestones are not only the remains of animals which lived in the ocean, thus proving that this was the water of the sea and not of a large lake, but they were most near- ly related to the animals which lived at the same time on the COast of FlG 430 . _ A large and oddly Alaska, and ornamented pelecypod (Trigo- even in Siberia.' nia) of the Jurassic period ' Their affinities with the Californian types are much less close. From this we may infer that the sea came in from the far north rather than from the west (Fig. 432). This inundation of the Northwest during the late Jurassic was of short duration only. At the close of the period, changes of level, 1 At this time Siberia and Russia were largely sub- merged by an expansion of the Arctic Ocean, and broad bays spread southward from this into central and western Europe joining the ancestral Mediter- ranean Sea, which was much larger then than now. THE JURASSIC PERIOD 407 FIG. 432. Geography of North America as it is supposed to have been in late Jurassic time. Dotted pattern represents sediments on land. the reverse of those which had brought in the sea, again excluded it. In neither case were the strata disturbed, and where there is any unconformity above the marine beds it is barely detectable. Marine deposits along the Pacific slope. On the Pacific coast there was a very different state of affairs. Throughout 408 HISTORICAL GEOLOGY the period sediments had been accumulating along the margin of the ocean, which at that time spread eastward as far as Nevada. Probably deposition had been going on along this coast through several of the preceding periods also. By the close of the Jurassic period the result was a very thick body of marine sediments, chiefly shales and sandstones, which had been laid down in shallow water, and, as the general fineness of the material testifies, near a coast which was not rugged. Crumpling on the Pacific border. At the end of the period this long cycle of deposition was interrupted by one of those intense crumplings of the earth's crust, which at inter- vals throughout geologic history have disturbed first one locality and then another. By lateral compression, the driv- ing force of which seems to have originated in the deep basin of the adjacent Pacific Ocean, the shales and sandstones were closely folded so that the beds now stand on edge. At about the same time, great quantities of igneous rock, especially granite, welled up into the folded mass, and solidified in the form of stocks and huge batholiths. On the borders of these intrusions, the sedimentary beds were changed into schists and other metamorphic rocks. Even at a distance from the igneous masses, the intense pressure exerted was sufficient to convert the deeply buried shales into hard slates, and, in some cases, to metamorphose the rocks even more severely. The result of this series of disturbances was doubtless a wrin- kling and cracking of the surface of the land parallel to the Pacific through California and probably as far north as Alaska. Mountain ranges were raised on the site of what had previously been a shallow sea. Even as they were ele- vated, these mountains were being gradually dissected by running water and the other agencies of degradation, just as the rising Sierras to-day are being sculptured. In their youth, we may well suppose that these early ancestors of the present Sierra and other Pacific ranges were lofty and rugged moun- tains. They have since, however, been worn down and have THE JURASSIC PERIOD 409 totally disappeared, the mountains which now occupy the same territory having been made at a much more recent date by the reelevation of the same strip. LIFE OF THE JURASSIC Culmination of the reptiles. In the Jurassic period the reptiles, which had been rising into prominence since the Per- mian, reached the climax of their career. Type after type had made its appearance, until by this time the animals of the reptilian class had assumed most of the roles and taken FIG. 433. A Jurassic dinosaur (Stegosaurus) , as it may have appeared in life. (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copyright by Amer. Mus. of Nat. Hist.) possession of most of the habitats which were open to them. The mammals and the birds were then in a very primitive condition, and occupied an insignificant place. The stations which they have since acquired were then held by the reptiles. The few rather small and unpretentious reptiles which now remain, for example, the snakes, lizards, and turtles, give but a faint conception of the saurian class in its prime. In the Jurassic, there were also large and ponderous reptiles (Fig. 433), 410 HISTORICAL GEOLOGY which more or less resembled the great modern pachyderms such as the elephant and the rhinoceros. They fed upon vegetation, and in spite of their bulk and the formidable array of bony plates, scales, and spines with which many of them were protected, they were probably neither ferocious nor dangerous. There were also smaller and more active reptiles (Fig. 434), which, like the tigers, lions, and other flesh-eating mammals of the present, preyed upon the more sluggish varieties that fed on vegetation. FIG. 434. Carnivorous dinosaurs (Allosaurus) of the Jurassic period. (Restored by C. R. Knight, under the direction of Professor Edward D. Cope.) Besides the land reptiles, there were batlike forms which had developed the power of flight almost as fully as did some of the birds in later times. These pterosaurs (Fig. 435), or flying dragons, as they are sometimes called, had hollow bones and other characteristics which are now peculiar to the birds. One of them had a spread of wings of more than twenty feet, nearly twice that of the largest living bird but the majority were much smaller. THE JURASSIC PERIOD 411 FIG. 4oo. The largest of the pterosaurs. A Cretaceous species. (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copy- right by Amer. Mus. of Nat. Hist.) In the shallow waters along the seacoasts and in the marshes of the rivers and inland bodies of water, other reptiles which had adopted an aquatic mode of life were abundant. Some, like the turtles and crocodiles of to-day, divided their time between the water and the shores, and were provided with legs fairly well adapted for either situation ; while others, as described in the last Chapter, had become adapted for swimming only. Their feet had been changed into flippers not unlike those of a whale, and in the extreme examples of this adaptation, only the front pair of paddles remained. The rear pair, being apparently less useful, had gradually dis- appeared, as in the modern whale. In such types the loss of the rear legs was always compensated for by the develop- ment of a long and powerful tail, flattened so as to serve as an efficient propeller. The mammals still in the background. The birds and mammals have been casually mentioned as occupying a 412 HISTORICAL GEOLOGY subordinate place on the stage of life in the Jurassic period. The mammals had, indeed, made their appearance as early as the Triassic, but they were still very primitive and quite unlike any forms which exist to-day. Not one, the remains of which have been discovered, was much larger than a rat, and there are reasons for believing that many of them belonged to the lowly group of egg-laying mammals, which is now extinct save for the duckbill and spiny anteater of Australia. FIG. 436. The earliest known bird ( Archseopteryx) . (Modified after Hutchinson.) The earliest of the birds. Our first evidence of the exist- ence of the birds comes from the Jurassic rocks of Germany. In the wonderful lithographic limestone of Bavaria several specimens, including even the feathers, have been found. They represent a bird (Fig. 436) which was so unlike the birds of to-day that, aside from the feathers and the warm blood which those feathers imply, it might with considerable justice be looked upon as a reptile rather than a bird. In its THE JURASSIC PERIOD 413 jaws were conical teeth like those of a lizard, and its long tail had bones out to the very tip. The fingers of the front limbs (or wings) were still free and distinct, whereas in all modern birds they have entirely grown together into a single bone, to which the feathers are attached. Its characteristics, then, are essentially intermediate between those of reptiles and of birds, and seem to indicate, with more than usual certainty, that the birds are the direct descendants of some one of the earlier reptiles. QUESTIONS 1. The faulted lava beds of late Tertiary age in Oregon and Nevada now stand out as high mountain ranges, while those in New Jersey make very low mountains, or hills. Can you explain ? 2. What does the thin shale and limestone formation of Jurassic age in northwestern United States tell us about the Rocky Moun- tains in that period ? 3. There are many lava flows interbedded with the Jurassic shales in California, and the shales contain marine fossils. From this what do you infer as to the conditions at that time and place ? 4. Under what conditions does molten lava form granite ? 5. Why are granites the commonest rocks in batholiths ? 6. How does it happen that many batholiths of granite are ex- posed at the surface to-day ? 7. In what kind of a deposit should the most delicate fossil be most perfectly preserved ? B. & B. GEOL. 24 CHAPTER XXII THE COMANCHEAN PERIOD 1 Conditions at the beginning of the period. The opening of the Comanchean period found the continent of North America very largely out of water, the long gulf from the northwest having been excluded at the close of the preceding period. On the west coast the series of rugged mountains which had been produced by the Sierran disturbance were being eroded, and the material supplied by their decay was spread along the shores of the Pacific Ocean. The present Rocky Mountains and most of the numerous ranges of the Great Basin region were not then in existence. From the Pacific mountains to the Atlantic Ocean there were probably no prominent highlands, except some low mountains in the Carolina region and perhaps others in New England. By this time the folded ranges of the Appalachian system had been worn down to a lowland or peneplain, over which sluggish rivers meandered on their way to the Atlantic Ocean and the Gulf of Mexico, and above which a few scattered hills rose as monadnocks. The beginning of the coastal plain. Up to this time the coast line of eastern and southern United States appears to have been considerably farther out toward the edge of the continental shelf than now, for almost no Paleozoic or early Mesozoic rocks of marine origin have been discovered there. In the Comanchean, for the first time, sediments were laid down over a considerable part of the old Appalachian land, now represented by the Piedmont Plateau. In the broad, level flats and marshes back from the coast, deposits of sand and clay with occasional carbonaceous layers were formed. 1 Often called the Lower Cretaceous period. 414 THE COMANCHEAN PERIOD 415 These are known locally as the Potomac series. Similar strata are found in the eastern Gulf states, and there, too, they are not of marine origin. In Texas and Mexico the sea spread far inland during the early part of the Comanchean period. That the submergence came on gradually and disappeared equally slowly may be readily inferred from the character of the rocks of that age. Thus the lowest beds of Comanchean age in Texas do not contain marine fossils. They are mixed sands and clays, with traces of marsh vegetation, facts which indicate that they were laid down upon a low- lying land, perhaps in the lagoons and flats along meandering rivers. Upon these earlier strata shales and limestones are found, and the marine shells which they contain prove that they were deposited in the sea, which was then advancing northward over the land. Above the limestones, however, are more sandy beds, which indicate that the shore line was on its retreat southward, and the sea was becoming shallower, until finally Texas and neighboring regions were left dry again. Reduction of the Pacific mountains. On the Pacific slope a vast thickness of gravel, sand, and mud accumulated during the Comanchean period. No other evidence is needed to show that the mountains which had been upraised at the close of the Jurassic were being rapidly eroded and the products of their decay carried into the adjacent sea. Hundreds and doubtless thousands of feet of rock were carried away by these slow but incessant processes, resulting in the uncovering of even the deep-seated batholiths of granite, which had been intruded at the time of the folding. Emergence of the continent. Above the Comanchean strata there is almost everywhere a distinct unconformity, which tells of long-continued erosion after the sediments were deposited. In the Atlantic and Gulf coastal formations the unconformity is merely an irregular surface dividing the strata above from those below. In that region there was no notable deformation between the two epochs of deposition. On the Pacific coast, however, the unconformity is more promi- 416 HISTORICAL GEOLOGY nent, and in Mexico it is still more conspicuous, for there fault scarps which were made at the close of the Comanchean were base-leveled before the Cretaceous sediments were de- posited. In Europe, also, an unconformity has been observed. From the wide distribution of these conditions, it seems probable that the sea level was drawn down at the close of the Comanchean period, and that the continents were again largely out of water. The plants become modernized. Up to this time the vegetable world had been represented entirely by such fossil plants as the ferns, mosses, and eye ads, plants which belong to distinctly lower groups than those with which we are now most familiar. In size and abundance they probably com- pared well with our modern trees, shrubs, and grasses, but they were different in structure and aspect. In the Coman- chean period, for the first time, the modern flowering plants made their appearance, and in this case with a suddenness which is yet to be explained. By the end of the period they had become the most abundant of all plants in America, and later in Europe, and they have continued to hold their su- premacy ever since. It is a significant fact that not long after the advent of the flowering plants came the great rise of the mammals, and also the first appearance of the higher insects, such as the butterflies, bees, and wasps. These animals are, in fact, largely dependent upon the higher types of plant life for their existence, and their rapid development may be due in no small measure to the entrance of the Angiosperms (p. 294). Lesser changes among the animals. By a somewhat slower development, the fishes likewise had reached almost their modern position by the close of the Comanchean period. The peculiar and in many respects ill-constructed fishes of the earlier periods, from the Silurian on, were gradually relegated to the background, while the modern Teleosts became the most abundant types, fishes like our modern bass, salmon, and many others, which have well-developed bony skeletons. THE COMANCHEAN PERIOD 417 The reptiles had by this time passed their zenith, but the discussion of their decline and the final exit of most of them from the stage of life is reserved for the next Chapter. QUESTIONS 1. Can you suggest from Figure 437 how it may be inferred that the Appalachian Mountains were worn down to a lowland by the Comanchean period ? FIG. 437. Generalized section of the Atlantic slope of the United States, showing the relation of the base of the Potomac sediments to the tilted Triassic beds and the folded Paleozoic strata. 2. Under what conditions might a period of emergence and land conditions not be marked in the section by an unconformity ? Where in the United States at the present time are these conditions realized ? 3. Can you suggest why the Comanchean system contains chalk in Texas but marble in central Mexico ? 4. The Comanchean rocks of California are said to be 25,000 feet thick, while in New Jersey they are only 700 feet thick. What reasons may be suggested for this great difference ? 5. Why are bees now dependent on the angiosperms ? CHAPTER XXIII THE CRETACEOUS PERIOD 1 Renewed inundation of the continent. Throughout the first three periods of the Mesozoic era the seas had over- spread only small portions of the United States. The areas submerged were (1) the Pacific coast in the Triassic, with the addition of the northwestern mountain region in the Jurassic, and (2) the southwest part of the Great Plains in the Comanchean. The Cretaceous period, on the other hand, was charac- terized by a widespread inundation of this and most other continents. For North America, at least, it was the last time that any large part of the continent was covered by the sea. The coastal plain submerged. It will be recalled that along the Atlantic coast the Comanchean strata are sands and clays, which were not deposited hi the sea, but more probably along rivers and marshes. A series of marine beds of Cretaceous age lies upon them, but the two are separated by a slight unconformity. The unconformity represents an interval during which the plain was land, and was subject to erosion. The Cretaceous beds are, like the Comanchean, almost unconsolidated, but they differ in some respects from the latter. Besides clay and ordinary sand, one of the constit- uents of the Cretaceous, especially in New Jersey, is green- sand, which seems to be a chemical precipitate formed in relatively shallow water. 1 As used here, the name Cretaceous refers to the "Upper Cretaceous" of many writers. 413 THE CRETACEOUS PERIOD 419 Farther south in Alabama and Mississippi, the Cretaceous system includes several hundred feet of chalk or soft lime- stone. In the belt where this chalky formation outcrops, the soil is so good that the cotton plantations there are the richest in the South. In the same region the slave popula- tion was densest before the Civil War and the proportion of negroes is now largest. An interior sea divides North America. Very early in the Cretaceous period the sea which lay south of the United States began a slow advance northward over the nearly FIG. 438. Stereogram of part of the Great Plains early in the Cretaceous period, showing the probable relations of the zones of different kinds of sediments. level surface of the land. Along the shores of the sea there were probably marshes and lagoons such as now fringe the low coast of Texas. Still farther inland, there were broad plains over which the sluggish rivers were spreading fine sediments (Fig. 438). As the sea advanced northward, the shore conditions must have migrated slowly in front of it, the streams constantly depositing sediment farther and farther north as the advance continued. These deposits of the alluvial plains appear to be represented in the oldest Cretaceous formation of the Great Plains region, the Dakota sandstone. In this sand- stone many leaves of land plants have been found (Fig. 439). As the leaves are unworn, and are well preserved, one can hardly suppose that they were washed out to sea and 420 HISTORICAL GEOLOGY there deposited; it is more probable that they fell di- rectly into the sandy shal- lows of rivers and bayous, and were in turn covered by the sands. Being a porous layer between beds of dense shale, the Dakota sandstone serves as an important reser- voir for underground water, and from it many artesian wells in the northern part of the Great Plains derive their flow. Beyond the shore line, in the open sea, very different deposits were being laid down, of course, chiefly clays in a broad belt near the shore, and limy ooze farther out in the clear water (Fig. 440). The shells and bones of marine animals became embedded in these deposits, and are now found as fossils in the Cretaceous rocks. These two zones, in which clay and ooze were deposited respectively, like- wise migrated northward as the sea advanced in that direction, until finally they had overspread the Great Plains and Rocky Mountain FIG. 439. - Angiosperm leaves from the j f Oklahoma and Dakota sandstone. A, Poplar ; B, Wil- low ; c, Sassafras. Iowa on the east to Arizona THE CRETACEOUS PERIOD 421 and Utah on the west, and as far north as the Arctic Ocean. At this time, then, North America was divided into two land masses (Fig. 441) : one on the east, which, being very low, furnished little but fine sediments to the FIG. 440. The same region as in Figure 438 near the middle of the Cretaceous period, showing the retreat of the shore and the changed positions of the zones of sediments. interior sea ; and one on the west, which was more rugged, although by no means so mountainous as the same region is to-day. By the advance of the first zone of marine deposition spoken of above, a thick layer of clay was gradually built up over the Dakota sandstone, and this in turn was followed by the zone of calcareous ooze, which produced the chalk now found in Kansas, Nebraska, and the Dakotas. The chalk points to the existence of a clear, open sea beyond the reach of mud-laden currents. In this chalk are found not only marine shells, but the bones of large swimming reptiles, and even of birds and flying reptiles. In the latter we seem to have evidence that the winged animals of Cretaceous times were accustomed to fly far out over the sea, as gulls and albatrosses do now. The interior sea retreats. The duration of the interior sea was evidently long, but before the close of the Cretaceous period changes began which eventually caused its disap- pearance. The sediments which were being constantly swept into it around its borders helped in some degree to 422 HISTORICAL GEOLOGY fill it up, but the chief cause of its withdrawal is probably to be found in gentle changes of level, this time the re- FIG. 441. Probable geography of North America near the middle of the Cretaceous period. The dotted pattern represents terrestrial deposits. verse of those which had caused it to overspread the land. As the sea receded, the zones of sedimentation (ooze, mud, sand, and river deposits) began a slow retreat. This migra- THE CRETACEOUS PERIOD 423 tion is recorded in the series of clays and sands which lie upon the chalk, and by more sands containing coal seams, which in turn are spread upon the clays. The presence of the coal seams records the passing of the shore line. The retreat of the sea seems to have been somewhat halting, however, and interrupted by occasional small ad- vances, for marine strata are found interbedded with the coal-bearing sandstones. That the retreat was exceedingly slow, and that the land FIG. 442. A pelecypod (Inoceramus) of the Cre- taceous shales character- istic of the western plains. stood for a long time not far from sea level, is suggested by the thick- ness of the sediments which accumu- lated and by the number of succes- sive coal beds in the upper part of the system. Each distinct series of beds thus made has a name of its own, the uppermost or coal-bearing series being called the Laramie for- mation. There may be as much coal in the Laramie as in the Pennsylva- FIG. 443. A Cretaceous . . ^ gastropod with curiously man system in eastern United States, formed shell and beaded ornamentation. Qn average it is of poorer quality, and but little of it is anthracite. Local deposits on the Pacific coast. In California and northward at various points as far as Alaska, Cre- taceous sandstones and shales have been recognized. They are usually separated from the Comanchean strata /. , i i FIG. 444. A Cretaceous by an unconformity, because much oi gea urchin or echinoidt the Coastal region Was land for a with the spines removed. 424 HISTORICAL GEOLOGY considerable interval at the close of the Comanchean period. The Cretaceous sediments were evidently derived from the mountains which bordered the Pacific shore. These moun- tains were probably higher in the preceding period, but had been much reduced before Cretaceous times. The thick- ness of the Cretaceous sediments is correspondingly less than that of the Comanchean system. In explanation it may be suggested that the mountains had been eroded down to a subdued hilly tract before the Cretaceous period. A period of quiet throughout the world. We have already seen how the eastern and western portions of the continent had been reduced to lowlands, either before the Cretaceous or by the end of that period. The only notable elevations which seem to have remained in North America were certain hills and mountains along the Pacific coast and others in the Carolinas. The same condition may be traced in Europe and in Asia, where peneplains of enormous extent seem to have developed by this time. While the lands were thus low and monotonous, comparatively little sedi- ment was being worn from them, and even that was fine mud and silt. In the flood plains of the broad river val- leys, clays and silts were spread in very wide but thin layers interspersed with marsh deposits ; while along seashores little except mud accumulated, 'and the deposition of pure lime ooze was permitted comparatively near the coast. Thus in England and France, and in many other parts of the eastern continents, the Cretaceous rocks consist largely of chalk or limestone. Greater uniformity of climate than we now have seems to have been another characteristic of this quiet period of low lands and shallow seas, for plants much like those of the Gulf states lived also in Greenland, where snow and ice now prevail. Conditions favor the sea animals. The conditions of life in such a period must have been somewhat more stable THE CRETACEOUS PERIOD 425 than at periods like the present, in which comparatively rapid changes have been taking place. Uniform climates permitted the migration of both animals and plants over wide stretches of the continents and of the seas. The broadly expanded shallow seas afforded a congenial field for the increase of marine life. Along with shells, corals, and other remains of sea animals, we find in the Cretaceous rocks the bones of many marine reptiles, not only turtles like FIG. 445. A Cretaceous mosasaur. (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copyright by Amer. Mus. of Nat. Hist.) those which now inhabit the oceans, but long, serpentlike forms in which the legs were reduced to short paddles, whilp the long, flattened tail served as a strong propeller (Fig. 445). The sharp teeth of these mosasaurs is ample evidence of their ferocious nature. Eccentric forms of the older reptiles. The large land- inhabiting reptiles, or dinosaurs, which had reached the zenith of their career in the late Jurassic, were still abundant, but had entered upon a period of eccentric diversification 426 HISTORICAL GEOLOGY A " such as is characteristic of the decline of many other animal groups. Like their predecessors, they were ponderous and clumsy in the extreme, and the small size of the cavities in their skulls shows how insignificant was the capacity of their brains and how little intelligence most of them possessed. Externally they took on many pecu- liar and apparently useless styles of ornamentation, such as the great bony plates and spines shown in Figure 433. The coiled shells called ammonites ornamented seem to ^ ave ^ een m tne same sta S e with blunt spines. The of their career, and likewise exhibit complexity of the sutures TYlflTlv nPfM] i; flr is concealed by the shell. *** P 6< forms and or- naments (Fig. 446). Some had spines, others knobs or ridges, while some showed a tendency to uncoil and re- vert to the straight Orthoceras type, although still keeping the highly crumpled suture lines (Fig. 447). The birds in transition. The birds in the Cretaceous period were far more like our modern birds than was the strange Archoeopteryx of the Juras- sic. In fact, the one characteristic which linked them closely with that ancestral form was the possession of teeth resembling those of reptiles and set in sockets or grooves in the jaws. That the birds had developed along Fio 447 . _ Fragment of several widely divergent lines is shown a large partly uncoiled by the fact that some which have ammonite, showing re- markable complexity of been found in the Cretaceous rocks the suture lines. THE CRETACEOUS PERIOD 427 were strong flyers, like the gulls, while others were wingless (Fig. 449) and spent their time exclusively in the water, where they had become as expert divers and fishers as the modern penguins of Antarctica. Crustal disturbances close the period. Even before the close of the Cretaceous period various occurrences gave a hint of the revolutionary changes which finally brought the period to an end. In Mex- ico, British Columbia, and elsewhere volcanic eruptions took place on a con- siderable scale during the later part of Cretaceous time. At about the same time some portions of Colorado, Wyoming, and doubtless other regions were warped upward. That the resulting highlands suffered rapid erosion is shown by the coarser and more abundant sediments which were deposited around their borders. These disturbances mark the beginning of a widespread FIG. 448. A Creta- ceous ammonite (Turrilites) with loose spiral form. FIG. 449. A toothed diving bird (Hesperornis) of the interior Cretaceous sea. (Painting by Gleeson. By the courtesy of McClure, Phillips and Company.) 428 HISTORICAL GEOLOGY epoch of crustal deformation, which resulted at the close of the period in the formation of many important mountain ranges, especially in the western hemisphere. Besides wide- spread warping and changes of level in western United States, the rocks were folded along a belt from Mexico to Alaska, and also apparently the entire length of South America. This marks the beginning of the present Rocky Mountains FIG. 450. Gentle folds characteristic of the Rocky Mountains in Wyoming. and the Andes, although the present height of those moun- tains is due chiefly to later movements. The folding in the Rockies at this time was by no means so intense as it was in the Appalachians at the close of the Permian. The folds are chiefly broad arches with troughs between (Fig. 450). Near the Canadian boundary the lateral compression was relieved not only by folding, but by a profound dislocation, FIG. 451. The great overthrust in the Rocky Mountains of Montana. The Proterozoic rocks on the left have been pushed up over the Mesozoic rocks on the right. the older rocks having been pushed up over the Mesozoic strata along a great thrust plane. At one point the Algon- kian rocks have been thrust out over the Cretaceous beds to a distance of at least seven miles (Fig. 451). While the folding and warping were in progress, volcanoes came into existence in many parts of western America. Volcanic mountains comparable to the modern cones of Vesuvius and Fujiyama were built up in Colorado, Montana, THE CRETACEOUS PERIOD 429 and other western states, as well as in South America. These were, however, only the first of a series of volcanoes which grew up during the Tertiary period (Fig. 452). The earlier ones became ex- tinct so long ago that they have been worn down, but the latest of them we still see in such , , _ FIG. 452. Section of a Tertiary volcanic cone the great peaks as IVlt. upper part of which has been removed by erosion. The lava flows are vertically shaded, interbedded with fragmental deposits. They are Rainier and Mt. Shasta. Rapid changes in the animal life. We naturally expect to find that the exclusion of the shallow seas which over- lapped the continent in the Cretaceous period, the growth of mountain ranges where there had been lowlands before, and the accompanying changes of climate had a marked effect upon the living things. Thus at the close of the Cretaceous period, the ammonites, which had long been abundant in the seas, died out in a comparatively short time, leaving no descendants. Among the reptiles the change was quite as marked, although not as complete. Almost all the great reptiles which were so characteristic of the Mesozoic era became extinct, and only the smaller forms which we have to-day, such as the snakes, lizards, and turtles, lived on. The crocodiles seem to be the only remaining represen- tatives of the large Mesozoic reptiles. ' At the same time the mammals began a rise which in the next period became extraordinarily rapid. The appearance of the higher mammals in North America seems to have been sudden, as if they had immigrated from some other locality in which they had slowly developed from simpler forms. If this is true, the place of their origin is not yet known. Whether or not the rapid spread and growth of the mammals was responsible for the disappearance of the great reptiles is an open question, but the suggestion is at least plausible. B. & B. GEOL. 25 430 HISTORICAL GEOLOGY THE MESOZOIC ERA IN NORTH AMERICA Changes in the form of the continent. At the close of the Paleozoic era the continental platform of North America had been left largely above the sea. Only on the Pacific coast did the ocean come farther inland than now. On the east side of this extensive continent stood the rugged moun- tains of the Appalachian system, perhaps not unlike the Andes of to-day. In the West lay broad, arid plains with occasional salt lakes, but it is improbable that high mountains stood there at that time. As the era continued, the sea tended more and more to overspread the land. Late in the Jurassic period a long gulf came in across the depressed lowland which is now occupied by the great mountains of western Canada. A little later the Atlantic Ocean began to encroach upon the eastern and southern border of the continent, and along its shores were deposited the earliest sediments of the present coastal plain. Finally, in the Cretaceous period, the depres- sion of the central western part of the continent allowed the sea to submerge a broad strip extending from the Arctic Ocean to the Gulf of Mexico, thus leaving North America divided into two smaller land masses. The scene of earth movements, like that of sedimentation, was shifted to the West in the Mesozoic era. Not being resurrected by further warping, the Appalachian Mountains in the East had been gradually worn down to low hills with broad valleys between. They remained in this condition through the later part of the era. The crumpling of the Pacific coast strip had doubtless produced a series of great mountain ranges, among the descendants of which are the Sierra Nevada, Cascade, and Alaskan ranges of to-day. After a long period of comparative quiet during the Comanchean and Cretaceous periods the level strata of the western in- terior were arched and locally complexly folded, thus estab- lishing the third great North American mountain system, THE CRETACEOUS PERIOD 431 the Rocky Mountains. By the retreat of the inland sea at about the same time, the continent was left more nearly in its present condition than ever before. Climatic conditions. In the Mesozoic, as in earlier eras, the climatic conditions left but scanty records from which we may now draw inferences. Extensive red beds give evi- dence of an arid climate over large areas in western United States, Europe, and China; but such conditions may have been due to the same local factors which produce deserts to-day. It is thought that the growth of abundant corals and other tropical animals in northern Europe in the Jurassic period indicates a much warmer general climate than the present. There are also differences in the faunas of northern, middle, and southern Europe and North America which may be due to climatic zones. That such zones have been in existence throughout geologic history can hardly be doubted, but, as already said, evidence of their presence in the earlier periods is scanty. It is quite probable, moreover, that the zones have been more distinct at certain times than at others. Evolution of higher types of animals and plants. When the Mesozoic era began, the old Paleozoic ferns and seed ferns were sinking into a subordinate place, as the conifers, cycads, and other naked-seed plants came to the front. Before the end of the era, however, even these were super- seded in large measure by the modern flowering trees, shrubs, and grasses. By this change the landscapes doubtless came to look much more like those which we now see. Early in the Mesozoic era the reptiles were in the youth of their race, rapidly developing and rising to their zenith before the Comanchean period. Having mastered the life of the dry lands, the shallow seas, the air, and even the open oceans, they kept their dominant place until the end of the era. Their later years were marked by inability to com- pete with the rising mammals, and it is perhaps for this reason that they were soon relegated to the background. Other groups of animals underwent corresponding, if per- 432 HISTORICAL GEOLOGY haps less striking, changes ; and when the next era opened, all the large groups, except the birds and the mammals, had nearly reached their modern condition. QUESTIONS 1. In the drier parts of South Dakota the Cretaceous shales contain many little lenses of limestone. These now stand out as conical hills, known as "tepee buttes" from their resemblance to an Indian tent. If the climate of this region were moist and the surface densely forested these buttes would probably not be formed. Why? 2. The Dakota sandstone is exposed along the flanks of the Rocky Mountains in sharp ridges, locally known as "hogbacks." What does this tell about the character of the formation? 3. Some of the Cretaceous chalk is interbedded with layers of sandstone. What does this indicate about the depth of water in which the chalk was formed ? 4. In some of the Cretaceous beds sticks of wood with charred ends have been found. What inference is suggested ? 5. Chalk consists largely of the shells of protozoans. What are the habits of these animals ? Under what conditions do they suc- ceed in forming a deposit of chalk ? 6. About the Black Hills of South Dakota the Laramie beds appear to contain no material derived from the Paleozoic group, while the Tertiary beds which lie upon the Laramie are largely composed of such debris. How may this be explained ? 7. Bees, butterflies, and many other insects of like habits have not been found in rocks older than Cretaceous. How may this fact be related to evolution among the plants ? CHAPTER XXIV THE TERTIARY PERIOD Results of the warping and folding. The crustal dis- turbances which brought the Mesozoic era to a close wrought great changes in the land forms on the continent of North America. In the Appalachian region a broad swelling or upwarp of the Cretaceous peneplain 1 had raised its surface some two or three thousand feet, and the streams, thus reju- venated, were already engaged in etching out the softer strata, leaving the harder ones protruding as mountain ridges. The great central portion of the country had been raised very little, but in the Cordilleran region of the West, the comparatively low-lying Mesozoic surface had been con- verted into mountains of considerable height with interven- ing basins and valleys. There, as in the Appalachians, the hills and mountains were being worn down and the resultant sediments were filling up the lowlands. Divisions of the period. The Tertiary period, while per- haps no longer than many that preceded it, is of course much better known, because it is nearer the present. It is usually divided into several epochs 2 : (3) Pliocene (more recent). (2) Miocene (less recent). (1) Eocene (dawn of the recent). Additions to the Atlantic and Gulf coastal plain. From New England south to Florida, and almost encircling the Gulf of Mexico, the Tertiary sediments are found lying upon the Comanchean and Cretaceous deposits which had formed 1 See page 414. 2 Of these, the Eocene is probably much longer than either of the others, and is often divided into Eocene (proper) and Oligocene. 433 434 HISTORICAL GEOLOGY FIG. 453. Supposed outline of North America early in the Tertiary period. The dotted pattern represents deposits made on land. (Modi- fied after Willis.) the beginning of the coastal plain. Some of these Tertiary rocks are sand, peat, clay, and marl, and some are soft lime- stone or chalk. They are interrupted by a number of slight unconformities. In these deposits and unconformities we find recorded the fact that the eastern and southern margin THE TERTIARY PERIOD 435 of the continent was sometimes submerged, and was thus the site of deposition ; and that at other times it was out of water, and was liable to erosion. The changes of level, whether of the land or of the sea, were not great in any in- stance. One result of the slight warping to which eastern United States was subjected during the Tertiary period was the formation of an island within the present confines of Florida, and later the addition of this island to the mainland in the form of a peninsula. No deposits older than the Ter- tiary limestones are exposed in that state. At several places in Texas, Louisiana, and California wells drilled down into the Tertiary sediments have yielded petroleum. This oil, and the natural gas which usually accompanies it, is prob- ably produced by the slow decomposition of animal and vegetable matter which was mixed with the sediments at the time they were deposited. Certain sandy beds become saturated with the gas and liquid and when one of these is pierced by the drill a flowing well may result. In some cases the gas pressure is so great that the oil is blown out in a jet. These "gushers" often wreck the buildings and derricks over the wells, and much oil is wasted before the fountain can be controlled. Only a part of the oil produced in the United States comes from Tertiary beds. That of Ohio and Indiana is in the Paleozoic rocks, and the Kansas oil is only a little younger. Doubtless the condi- tions for the formation of gas and oil have been present somewhere in all- the geologic periods. Local sedimentation on the Pacific coast. In the Ter- tiary period, the Pacific coast was apparently somewhat abrupt and rugged, although perhaps less so than it is to-day. Erosion was the chief activity along the western slope. Here and there, however, deposits of Tertiary age have been found, those in the coast ranges of California being largely of marine origin, while farther north, near Puget Sound and in Alaska, early Tertiary beds containing coal seams are known. The latter were evidently laid down in swampy lowlands near the coast, but not submerged by the sea. Alluvial deposits in the Great Plains. Throughout most of the Tertiary period the Great Plains were much nearer 436 HISTORICAL GEOLOGY sea level than now, and less intrenched by valleys. Many streams which issued from the newly made mountains on the west were spreading their loads of gravel, sand, and mud far and wide over the low-lying surface. Here and there lakes and marshes doubtless existed temporarily, and the location of these shifted from time to time, so that the de- posits which now represent the Tertiary in the Great Plains are partly such as are laid down in lakes, and partly those which rivers and even winds make. In the Tertiary epochs the climate of the Great Plains region was on the average moister than it is to-day. Coaly layers in the Tertiary strata indicate the existence of swamps, where now only dry prairies are to be found. The Tertiary deposits, which have since been elevated and subjected to a drier climate, are now being rapidly dissected by the growth of ramifying valleys and tributary gullies. In parts of Dakota and Montana the result is an extremely rugged complex of ridges, mesas, and buttes, over which travel is very difficult, and which are therefore known as " Bad Lands " (Fig. 454). Changes affecting the western mountains. The young Rocky Mountains and others farther west were being rapidly worn down by the activities of wind, rain, and streams. Some of the material thus furnished found lodgment in the interior basins between the mountain ranges, and there accumulated to great thickness. As in the plains, these deposits were made partly in lakes, but are to be ascribed in large measure to the work of streams which built alluvial fans in front of the valleys they had cut in the mountain slopes. Coalescing with each other, these fans came to form alluvial plains. In addition to the sand, gravel, and silt, beds of volcanic ash and sometimes of coarser tuff are found included in these Tertiary strata. They record the eruptions which took place at intervals from the volcanoes in Colorado, Montana, and many other western states while the Tertiary sediments were being laid down. The old volcanic cones have been THE TERTIARY PERIOD 437 -' FIG. 454. Bad Land topography in South Dakota. (Darton, U.S. Geol. Sun.) slowly worn down, but their cores and remnants of the old lava flows may still be recognized. Most of the ore deposits which have given the western states their renown as mining districts are connected with the volcanic intrusives of Tertiary times. The gold, the discovery of which caused the rush of immigrants to California in 1849 and succeeding years, was found partly in gravels in the valleys of Tertiary rivers. The famous gold mines of Cripple Creek, Colorado, and the copper mines of Butte, Montana, and parts of Utah, all get their ores from veins adjacent to bodies of porphyry and other igneous rocks which were forced into the older formations in the Tertiary period. In this respect there is a contrast between the western and eastern mountains of the United States. The climate of the mountain region could not have been as arid as it 'is to-day, for the luxuriant vegetation which flourished there in the Tertiary period shows that the rain- fall was plentiful In some of the driest parts of Utah and 438 HISTORICAL GEOLOGY Wyoming the Tertiary strata have preserved abundant leaves of palms, figs, and magnolias. The present dryness is doubtless to be ascribed in part to the later uplifting of the present mountain ranges which shut off the moist winds from the Pacific Ocean. Mountain growth. In the West, the Eocene epoch was occupied largely in the wearing down of the highlands which had been produced at the close of the Cretaceous period, and in the filling of the lowlands. Warping and volcanic activity, although they had not ceased, were of minor im- portance. It was an epoch of quiescence. FIG. 455. Trend lines of folds made during the middle Tertiary epoch of mountain-building. Near the middle of the Tertiary (Miocene epoch), how- ever, the disturbances were renewed on quite as grand a scale as before, but in part along different lines. One of the greatest results of this deformation is now seen in the series of high mountain chains which partially encircles the globe north of the equator (Fig. 455). In our own hemisphere it THE TERTIARY PERIOD 439 is represented in the mountains of Cuba, Porto Rico, and southern Mexico (Antillean system). (See Fig. 44.) These ranges are less conspicuous than some of the mountains on the land, only because they are largely submerged. The highest peaks of Cuba rise more than twenty-five thousand feet above the floor of the adjacent Caribbean Sea. In the old world the eastward trending mountains, from the Pyre- FIG. 456. Geography of the world as it is thought to have been at a time early in the Tertiary period. Note the continuous land in the northern hemisphere, with isolated continents in the south. nees in Spain, through the Alps, Caucasus, and many other ranges, to the Himalayas and far beyond, belong to this great belt of Tertiary mountains. Hitherto most of these regions had been beneath the sea; on the site of even the great Himalayas there was, up to the early part of the Ter- tiary period, a broad sea, not unlike the Mediterranean (Fig. 456). In this sea limestone was being quietly formed. But in the Tertiary disturbance these and all older rocks 440 HISTORICAL GEOLOGY of this locality were folded, compressed, and raised into lofty ridges which are now being carved by erosion into rugged mountains. A little later, the Sierras, Rockies, and other western ranges began a renewed epoch of growth; this time not chiefly through folding, as at the close of the Cretaceous period, but by mere warping and faulting. The rise of the Sierra range and its northward continuations consisted of an arching of the surface; but locally, as along the east base of the Sierra, the arch cracked (Fig. 457), or, in other words, was faulted, and that side is now much steeper than the slope toward the Pacific. There is good evidence that the slow uplifting of the Sierra is still going on, for as recently as 1872 a slip of nearly twenty-five feet occurred along this fault plane. FIG. 457. Stereogram of a low fold broken on one side. Tertiary volcanoes. In this western region volcanic eruptions continued, but with somewhat decreasing activity. They have only very recently ceased, and it is in fact by no means certain that the present is anything more than a temporary period of quiet in that respect. Near the middle of the Tertiary period, eruptions of lava from fissures as well as from volcanic craters took place over a vast area in the northwestern part of the United States, particularly in Idaho, Washington, and Oregon. Flow after flow of liquid lava welled up through cracks in the earth and poured out over the surface, leveled up its inequalities, and finally pro- duced a plateau more than a thousand feet in height and equal in extent to several good-sized states (Figs. 25, 458). Simi- lar eruptions have occurred occasionally in earlier periods, but nothing quite like them has been observed in historic times. THE TERTIARY PERIOD 441 The renewal of the uplifts late in the Tertiary and con- tinuing into the next period brought on the conditions which we now think of as characteristic of the region. It is to these later movements that the present elevation of our FIG. 458. Stereogram of a part of the Columbia River lava plateau, showing flows with interbedded layers of sand and gravel. high ranges is due ; and, as has been said, in some of them the growth is still in progress. During the uplifts, the streams sank their valleys deeper and deeper into the lands, so that the West is now characterized, not only by moun- tain ranges, but by high plateaus deeply cut by canons, FIG. 459. Young fault block mountains in southern Oregon. (Modified after Davis.) Why are the depressed spaces between the blocks flat? such as those of the Colorado and the Snake rivers. The growth of the mountains also deprived the winds from the Pacific of a large part of their moisture, and thus condemned the interior basins and the Great Plains to a much drier climate than they had before. 442 HISTORICAL GEOLOGY LIFE or THE TERTIARY PERIOD Modern aspect of the lower forms of life. Before the Tertiary period, all the important types of plants had made their appearance, and the flowering group had taken the place it now holds in the lead. The lower groups of animals had likewise become much like those we have to-day. The trilobites, brachiopods, ammonites, and other ancient divi- sions had given way to modern groups of crustaceans, bi- valves, cuttlefish, and others. The fishes, amphibians, and reptiles had passed their prime and were represented in the Tertiary period only by species resembling those now living. Only the birds and mammals, then, claim our interest, because they alone are still progressing. Of these, the mammals are much the more important, and have left us the better-preserved fossils. They are now the highest and most powerful of the animal groups. Generalized mammals of the Eocene epoch. Among the beds of sand and clay which were laid down in the broad Eocene valleys of our western mountain region and certain other parts of the world, abundant skeletons of mammals have been found. They show that many kinds were even then in existence, that they differed considerably in their habits of life, and that they were already the leading ani- mals of their time. At the present day we have no difficulty in distinguishing the several large groups of mammals from each other. Thus we have the flesh eaters (Carnivores), such as the tiger, bear, and wolf ; the hoofed mammals (Un- gulates), such as the horse, buffalo, and deer; the gnawers (Rodents), such as the squirrel and rat; the whales and dolphins (Cetaceans), which are swimmers exclusively; and still others. It is difficult, however, to place the early Ter- tiary mammals in these familiar divisions. Instead, we find varieties which seem to have combined the characteristics of several later groups. For example, it is possible to trace THE TERTIARY PERIOD 443 the horse, deer, and rhinoceros families, with their specialized hoofs and grinding teeth, back to a peculiar five-toed animal which had a full set of rather simple teeth, and was no larger than a dog (Fig. 460). This Eocene form seems to be an ancestral or generalized type from which the later hoofed ani- mals diverged and ascended. Furthermore, it resembles in many respects the equally generalized ancestors of the dogs, bears, and cats, although cats and horses, for example, to-day seem to have little in common. FIG. 460. A generalized hoofed mammal (Phenacodus) which lived in North America near the beginning of the Tertiary period. (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copyright by the Amer. Mus. of Nat. Hist.) Rapid evolution of the mammals. The progress of these generalized mammals of the early Eocene was aston- ishingly rapid. In each later series of deposits the bones of new and more modern varieties are found. Thus, before the middle of the Tertiary period (Miocene), the main divisions of the mammals became entirely distinct and we may easily recognize cats, horses, monkeys, whales, bats, elephants, and 444 HISTORIC -L GEOLOGY many other kinds. True, they were not the same species which exist to-day ; some of the horses, for example, had three toes instead of one, as they now have ; but the types were un- mistakable. Before the close of the Tertiary, the older and more primitive mammals had been exterminated from the northern continents, and the whole animal kingdom had taken on very largely its present aspect. FIG. 461. Ancestral Eocene horses (Eohippus) with three and four toes on the feet. (Painted by C. R. Knight, under the direction of Professor H. F. Osborn. Copyright by Amer. Mus. of Nat. Hist.) The mammals adopt many modes of life. As mentioned in Chapters on the Mesozoic era, the reptiles when in their prime had occupied the forests, the plains, the marshes, the seas, and all other situations in which animals could well exist. In the Tertiary period we find the mammals stepping into the places relinquished by the reptiles, perhaps after having actu- ally displaced them by sheer victory in competition. Thus THE TERTIARY PERIOD 445 we have mammals of the forest (for example, squirrels), of the plains (antelopes), of the marshes (beavers), of the air (bats), of the ocean (whales), and many more. Interestingly enough, as the mammals adopted these modes of life, they often took on in a degree the form and appearance of their reptilian predecessors. To appreciate this one has only to compare the bat with the pterosaur (Fig. 435), the porpoise with the fish reptiles (Fig. 421), and the rhinoceros with the heavy dinosaurs (Fig. 433). Migrations of the Tertiary mammals. There are to-day some very peculiar things about the distribution of certain animals which are explained only when we study the fossils from the Tertiary formations. The camels are now found in Asia and Africa, and also in the Andes Mountains of South America. In Tertiary times, as the fossils show us, they roamed widely over western North America as well, and it seems probable that they migrated thence to Eurasia by way of Alaska at a time when that peninsula was less submerged than now and enjoyed a warmer climate. Later they died out in North America. This is but an instance of many migrations by which the mammals of Eurasia and America mingled during the Tertiary period. Some islands were so isolated by water that they could not be reached by the mammals which originated in the larger continents. Australia is a case in point. There we find almost none of our familiar higher mammals, but instead a host of peculiar marsupials, among which are kangaroos, wombats, and opossums. It is known that these marsupials are most closely related to animals that lived in Europe in the Mesozoic era, but died out there earty in the Tertiary period. The inference is that Australia has been isolated from the other lands since perhaps the Creta- ceous period, and that during the Tertiary period her peculiar mammals evolved along their own lines without that inter- ference which comes from sharp competition with the more progressive higher animals. B. & B. GEOL. - 26 446 HISTORICAL GEOLOGY South America and Africa were similarly isolated at certain times, but later in the Tertiary period they were linked with the northern lands and thence received the tide of immigrants belonging to the more advanced mammals. By studying the present distribution of animals and work- ing out the paths of their earlier migrations, we can learn much about the changes which have taken place in geography during the later periods. The map (Fig. 456) shows roughly how the continents and seas are thought to have been ar- ranged in early Tertiary times, as compared with the present. QUESTIONS 1. The Eocene coal in the vicinity of Seattle is bituminous and locally even anthracitic ; that in Mississippi is soft lignite. With- out further information, what predictions would you venture as to the geological conditions in the two regions ? 2. It has been suggested that the climatic changes known to have taken place in the Tertiary period may have been caused partly by changes in the ocean currents. What would happen to-day if Florida joined Cuba and the Bahama Islands, while at the same time Central America were submerged deeply ? 3. Can you suggest why the known deposits of Eocene age are largely those which were made on the surface of the land ? 4 - Tne accompanying diagram (Fig. 462) represents a mountain range in the West. Show how the depth of erosion early in the Tertiary may be estimated FIG. 462. -Section of moun- from Such a Section ' tain range. The folded 5. The Eocene strata of Wyoming beds are Paleozoic and include beds of limestone with fossil Mesozoic. The horizontal fishes. From your knowledge of Tertiary ^^^ what do you SUS P ect Was * he origin of these beds ? 6. What events or conditions are recorded in the following sec- tions (Figs. 463, 464, and 465) selected from the Tertiary strata in different parts of the United States? 7. The pyramids of Egypt were built of Eocene limestone. What change must have taken place in this deposit of shells p IG 453 Limestone since it was formed ? (Florida). > '; THE TERTIARY PERIOD 447 8. In Thibet, marine limestone of Eocene age has been found at altitudes of 20,000 feet. How much of the history of Thibet may be inferred from this fact? 9. On the west- ern slope of the Sierra Nevada range there are flat-topped ridges capped with sheets of lava (Fig. 466). Beneath the lava gold-bearing gravels have been found. Can you suggest how the present conditions FIG. 465. Conglomerate, sandstone, and shale, in contact with a mass of granite (Oregon). FIG. 464. Alternate shale, sandstone, and conglomerate (Colo- we re brought about ? rado). What would be the FIG. 466. Diagram of a lava-capped ridge in Cali- fornia. best method of mining the gold in these deposits ? CHAPTER XXV THE QUATERNARY PERIOD The great ice sheets. During the later part of the Ter- tiary period the climate of the northern regions was becom- ing somewhat colder, so that palms no longer flourished in Greenland, nor corals off the coast of Scotland, as they had in the early Tertiary. In the Quaternary period, from causes not yet understood, the temperature of the northern regions had been lowered to such a degree that the snows of winter were not melted off in summer. Thus glaciers came into existence, not only in high mountains and polar regions where we have them to-day, but over large regions which are now free from them. Through the long accumulation of snows, thick ice sheets, or continental glaciers, grew up in North America and in Scandinavia and spread outward in all directions until they covered Canada and much of Europe. In North America the ice sheets extended into the United States as far south as southern Illinois and New Jersey. Singularly enough, they did not cover much of Alaska, in spite of the fact that it is farther north than some of the coun- tries which were glaciated (Fig. 467). The fact that ice sheets did not cover Alaska and Siberia, two of the coldest parts of the world, shows that low tempera- ture was not the only condition needed to bring on glaciation. Plenty of snow is likewise essential, and so in rather dry regions or where there are short, hot summers, even where there is great cold, we find no glaciers. Successive advances and retreats of the ice. The Glacial epoch was marked by the growth and eventual melting off of not merely one ice sheet but of several, one after the other. This is true of both Europe and North America. In the 448 (449) 450 HISTORICAL GEOLOGY Mississippi Basin evidence of several advances and retreats has been discovered. The greatest extension was reached by the second ice sheet, which spread southward almost to the mouth of the Ohio River ; but later ones fell only a little short of that. Between the several advances, the ice sheets seem to have entirely disappeared or to have been reduced to much smaller size. That these disappearances were caused by periods of warmer climate is shown by the finding of leaves of southern plants in clay beds between two layers of glacial till as far north as Toronto in Canada. Trees now charac- teristic of the Ohio Valley then lived abundantly north of Lake Erie. In view of the fact that ice sheets grind down the surface over which they slowly creep, we need not wonder that the later ice sheets removed much of the deposits left during pre- ceding glaciations. Even where the earlier sheets of drift were not destroyed, they are now largely buried by deposits of the later ice sheets. We therefore know the older drift best around the edges of the newer. Its greater age is indicated clearly by the fact that it has been deeply trenched by branching systems of valleys which have been growing and extending themselves through all the time since the early deposits of drift were laid down. The last drift sheet was made so re- cently that the streams have barely begun this work of trench- ing, and its usually rough surface is still dotted with undrained lakes and marshes. Estimates of the length of the Quaternary period. Many attempts have been made to estimate the number of years represented by the glacial advances and retreats. At present the cliff at Niagara Falls is being cut back several feet per year. It has been calculated that at some such rate it would take from 7,000 to 50,000 years to cut the entire gorge below the falls. Since the falls could not have begun until after the last ice sheet had retreated to Lake Ontario, a some- what longer time would be required to take us back to the beginning of the retreat of the latest glaciers. By compar- THE QUATERNARY PERIOD 451 ing the effects of weathering and erosion on the older and younger sheets of drift it is possible to gain a rough idea as to their relative ages. Estimates thus made of the length of time since glaciation began range from 500,000 to 1,500,000 years. It is impossible to make a much closer calculation than this because there are so many factors which vary from time to time and in a way which cannot be predicted. But the fact is clear that the period was many times as long as the known part of human history. How the ice sheets changed the land surface. The work of glaciers has already been discussed in Chapter VI. There it was shown that the effects wrought by glaciers are very differ- ent in different places. Thus the last Canadian ice sheets produced varied changes according as the country they in- vaded was flat, hilly, or mountainous. In the mountains of New York the ice scoured off the slopes of the hills, and removed the crags and talus slopes, but did not greatly change the general forms (Fig. 468). Preexisting valleys were scoured out and deepened where they ran parallel to the ice movement, and were par- *> 46 8--Lw mountains which ; have been scoured by an ice sheet, tially filled With drift where leaving the summits smooth and their courses lay across the J?J"J dec ! l ai \ d . * he valleys partly filled with drift. line of glacial movement. The so-called finger lakes of western New York are in valleys thus deepened and locally blockaded. Where the hills were lower and the ice thicker in proportion, the effects of erosion by the ice sheet were more pronounced. Not only was a vast amount of soil and rock ground from the hills, but many of the preglacial valleys were completely buried (Fig. 469). In such regions the present hills and hol- lows are simply the irregularities of the drift itself, as it was deposited. The older topography has thus been obliterated over large areas of Illinois, Minnesota, and other northern states. 452 HISTORICAL GEOLOGY Disturbance of river courses. Before the ice covered the northern region the many rivers had become in large meas- FlG. 469. Preglacial hills and valleys obliterated by the deposition of glacial till. (After Tarr.) ure adjusted to the hard and soft rocks in which they were excavating their valleys. As the ice overspread their basins many such valleys, with their rivers, were wholly destroyed, and the new streams which arose after the ice melted pursue courses quite unrelated to those of their predeces- sors. The Rock River in Illinois and Wisconsin ex- emplifies this (Fig. 470). Other streams, espe- cially those located near the margin of the ice sheet, were merely crowded to one side and forced to make new val- leys. Thus the Missouri River appears to have been displaced by one of FIG. 470. A portion of northwestern the earlier ice sheets. It Illinois, showing the course of the Rock River before and after glaciation. (After Leverett.) cut a new channel along the front of the glacier THE QUATERNARY PERIOD 453 (B, Fig. 471), and even after the ice melted back again the river held its new course. Marginal lakes of the retreat stage. Like all ice sheets, those of the Glacial epoch pushed out lobes or tongues along the valleys near their margins. The ice sheet thus came to have scal- loped edges. During the last retreat the great gla- cial lobes which had oc- cupied such depressions ponded the waters be- tween the moraines they had left and the front of the ice, thus producing a series of lakes (Fig. 473). The overflow water from these lakes ran south- ward, largely into tribu- taries of the Mississippi River, Lake Superior draining out past Duluth and Lake Michigan past Chicago. As the ice re- treated slowly northward the lakes grew in size and some joined those next to them to form larger lakes ; while others, having lost the ice wall on one side, disappeared entirely. Our present Great Lakes began as marginal waters of this kind, and it was only after the ice had retreated into Canada that they were all connected and found the St. Lawrence Valley the lowest point of outflow. As the ice retreated from Minnesota, the Dakotas, and Manitoba, it left a shallow basin in which another great lake came into existence. Lake Agassiz, as it is called, was once five times as large as Lake Superior, but when the ice sheet which blocked its northern edge finally melted away, the FIG. 471. A and B. Diagrammatic maps of South Dakota, showing how the Mis- souri River was displaced by the invasion of an ice sheet. (Modified after Todd.) 454 HISTORICAL GEOLOGY waters of the lake were drained off, leaving only much smaller lakes, as Winnipeg, in the deepest parts of its basin. Its exist- ence is now known from the many terraces and sandy beaches made by its waves, and by the broad, flat bottom built of fine silts which were deposi- ted in the lake. This alluvial plain is now one of the richest wheat- growing districts in the world. Features of the latest drift sheet. We have already said that the older sheets of drift have been trenched by many valleys, so that the origi- nal moraines and other purely glacial features are no longer easily rec- ognized. The last ice sheet (called the Wiscon- sin) disappeared so re- cently that, in general, erosive agencies have not had time to mar the surface of the deposits which it left. Where the edge of the ice sheet lingered we now find terminal moraines. There the drift is usually thicker than elsewhere, and rough hills alter- nating with undrained hollows are characteristic. Many of the hills are composed of rudely stratified gravel heaped up in conical form. These kames are often excavated for road material and railroad ballast. On account of the roughness and bowldery soil of the terminal moraines, they are not com- monly cultivated, but are left as woodland and pastures. FIG. 472. A modern glacier on the coast of Alaska, showing a marginal lake in- closed by a terminal moraine which is in turn fringed by an outwash plain. (Mod- ified after Maddren, U.S. Geol. Surv.) Is the glacier retreating or advancing ? THE QUATERNARY PERIOD 455 Lakes are especially abundant in the terminal moraines. In Minnesota and Wisconsin thousands of them mark the posi- tions of these .belts. Stretching southward from the moraines, gently sloping plains mark the outwash deposits which were built by the overloaded glacial streams. Owing to the porous, well- FIG. 473. Lobate edge of the American ice sheet with marginal lakes left during its retreat. (Modified after Taylor and Leverett, U.S. Geol. Surv.) drained soil, some of these plains make excellent farming land, although others are too sandy. Down every valley leading away from the moraines, gravel and silt were strewn, forming a flood plain. When the glaciers disappeared and the streams became relatively free from detritus they were able to cut down into these valley trains and have left portions of them as terraces. 456 HISTORICAL GEOLOGY Back of the terminal moraines, over wide areas, the ground moraine prevails, an undulating plain with gentle slopes. Lakes and marshes strung on crooked, aimless streams are of common occurrence (Fig. 474). Where the drift is thin, rock hills may protrude, their rounded forms and polished, grooved surfaces showing plainly the wear of the ice sheet upon them. Elsewhere the entire surface is molded from the glacial bowlder clay. In such districts there may be drumlins, smooth, elliptical hills of till all trending parallel to the direc- tion in which the ice was moving. The successive advances and retreats of the ice made the distribution of these several features less simple than might be FIG. 474. Aimless drainage of a glaciated region, eastern Wisconsin. . FIG. 475. Tree-shaped drain- age systems in an unglaciated region, northeastern Iowa. expected. Readvancing ice plowed over and defaced drumlins and moraines which had been left at an earlier stage. Out- wash deposits of stratified drift made in front of such an advancing glacier were often worked over and buried under a sheet of till. Later, outwash sands and gravels were spread over moraines as the ice retreated. The escaping water ponded behind terminal moraines cut channels in them here and there. Some of the effects of glaciation on human affairs. Men- tion has already Ipeen made of the excellent soils usually found iipdh glacial lake floors and outwash plains. The finely pulverized rock material left generally over the glaciated THE QUATERNARY PERIOD 457 regions is on the average a better soil than the residual sandy clays which are produced by the ordinary weathering of many rocks. In some places, notably in parts of New England and eastern Canada, however, the till contains so many bowlders that cultivation of the soil is very laborious. Among the best harbors in the United States are the glacial fiords and bays of the New England coast. These facilities early helped to lead the people of the region to engage in fishing and to become the best seamen and shipbuilders of the country. The general derangement of rivers by the ice sheets hindered inland navigation in a measure, but at the same time it con- ferred large benefits in the form of available water power from the many falls and rapids. The abundance of these falls near the centers of trade in northeastern United States has assisted in making that region a great manufacturing district. As the progress of invention makes it possible to transmit electric power over longer and longer distances, these falls will be used more extensively ; and as the fuel resources of the country are gradually depleted, more and more depend- ence will be placed on electricity from water power. The glaciated regions are thus likely to retain their interest and importance in the manufacturing industry. THE GLACIAL EPOCH OUTSIDE OF THE ICE SHEETS In the rest of the United States and in other continents the events of the Quaternary period were much like those of the preceding Tertiary. By the erosion of running water, pla- teaus were being cut into hills and mountains, winds were carv- ing out the softer rocks in the deserts, and waves were eating back the rocky coasts. Along low-lying plains and river bot- toms, gravel, sand, and mud were strewn ; while the waves and winds built barriers and sand dunes along the edges of the shallow seas. There is a decided contrast between the conditions and ap- pearance of the recently glaciated and the unglaciated parts of the land (Figs. 474 and 475). Over much of the region where 458 HISTORICAL GEOLOGY the last ice sheet left its deposits there are lakes, marshes, aimless rivers, waterfalls, and scattered bowlders. Elsewhere, lakes and marshes are confined largely to the river bottoms and seashores ; waterfalls are few ; the rivers are grouped in branching, treelike systems ; bowlders from distant regions are not to be found ; and the hill soils are chiefly residual. Valley glaciers in the mountains. In the mountains to-day there are small valley glaciers wherever there is suffi- cient cold and snowfall. In the Glacial epoch these were larger than now and vastly more numerous. Only the lower ranges in western United States were free from them. It is easy to identify the places where these alpine glaciers have been at work, long after they have passed away, for they not only scoured and striated the valley floors, but made the original valleys U-shaped, sharpened the mountain peaks into crags and pinnacles, and built loop-shaped mo- rainic ridges farther down the valleys. Along the aban- doned valleys many lakes now testify to the work of the ice. The wild scenery of the high, snowy ranges to-day is due largely to the sculpturing by Quaternary glaciers. Great Quaternary lakes of Utah and Nevada. The basin which lies between the Rocky Mountains and the Sierra Nevada is now arid, and most of the rivers flowing into it dwindle away in the desert soils, or feed salt lakes from which no streams flow out. During the Glacial epoch, all of these lakes were much larger than now. Great Salt Lake in Utah is only a remnant of a lake, called Bonneville (Fig. 476), which was two thirds as large as Lake Superior and one thousand feet deep. The former existence of this great lake is shown plainly by the series of cliffs and terraces which parallel the slopes of the adjacent mountains (Fig. 477). These terraces were made by the waves on the lake. At that time, Lake Bonneville overflowed northward into the Snake River. In lakes with outlets the water is continually being changed and so is not allowed to become salty. Bonne- ville was therefore a fresh lake. In western Nevada a series THE QUATERNARY PERIOD 459 of valleys was filled at that time by a most irregular lake which has been named Lahontan. In the drier recent epoch FIG. 476. Quaternary lakes of western United States. FIG. 477. An abandoned shore line of Lake Bonneville. (After U.S. GeoL Suro.) 460 HISTORICAL GEOLOGY its water, like that of Lake Bonneville, has largely evaporated, leaving several small, salty lakes and dry, flat-bottomed de- pressions covered with sand and crusts of lime. These deserts were fatal to some of the early emigrants to California who came overland from the East. Decline in volcanic activity. While volcanoes were less numerous in the West during this period than in the Tertiary they were still fairly common, as is attested by numbers of small cinder cones among the western mountains and high plateaus. In the bed of old Lake Bonneville several little craters were formed after the lake shrank to nearly its pres- ent size. In northern California there is another little cone surrounded by a recent lava flow and a layer of ashes in which the stumps of trees killed by the last eruption are still standing. The great cones of Mts. Shasta, Rainier, and others along the Pacific slope, which were built largely during the Tertiary period, probably increased somewhat in size in the course of the Quaternary period. Some, indeed, are thought to have had eruptions within the last few centuries. Except in Alaska and Mexico, however, the present is not a time of notable volcanic activity in western North America. ANIMALS OF THE GLACIAL EPOCH Mammals attain their modern state. The Glacial epoch is so recent geologically that the animals of that time differ but little from those which exist to-day. Add to the mam- mals of to-day certain large forms which were common then, but have since been exterminated, and we have essentially the Quaternary fauna. Migrations caused by glacial fluctuations. Much more striking peculiarities are found when we compare, from the standpoint of their distribution, the animals of the present day with those of the Glacial epoch just preceding. Obvi- ously the effect of the slow expansion of the ice sheets was to crowd all animals and plants away from the glaciated region, THE QUATERNARY PERIOD 461 and for the United States that meant in general southward. On the other hand, as the ice retreated during the milder times between glaciations, the same forms of life would be invited by the amelioration of conditions to press northward. In this way probably several backward and forward migrations were induced. Southward advance of Arctic life. In the strictly glacial times musk-oxen, such as now live in the Arctic regions, came as far south as Kentucky, and herds of reindeer ranged over the treeless hills of France. Elephants, such as the mammoth FIG. 478. The American mastodon. (Painted by Gleeson, in the U.S. Nat. Mus.) and the mastodon (Fig. 478), and rhinoceroses, both covered with long, woolly hair, were among these Arctic types. Even their bodies with flesh and hide intact have been found pre- served in the frozen gravels of certain Siberian rivers. Southern forms come north. During the genial intervals in which the glaciers disappeared, many southern animals B. & B. GEOL. 27 462 HISTORICAL GEOLOGY lived farther north than now. In Europe, hyenas, lions, hippopotami, and other African mammals reached England and Belgium. In the United States, at some such time, sloths and armadillos, related to South American types, frequented the southern states, coming as far north as Pennsylvania; while horses were abundant in Alaska, along with buffa- loes (bisons) and elephants. First appearance of man. In the caves of France and some other parts of Europe, human bones and implements have been dug from beneath the hard layers of lime carbon- ate which incrust the floors of caves generally. With them FIG. 479. Drawing of a reindeer on a piece of bone, from a cave in southern Europe. (U.S. Nat. Mus.) FIG. 480. Seals carved on a piece of bone found in southern France. (U.S.Nat. Mus.) Are seals found in that region to-day ? are mingled the bones of the mammoth, reindeer, hyena, and hippopotamus, none of which have lived in central Europe in historic times, but which were plentiful there during the Glacial or Interglacial epochs. Doubtless these earliest human beings of which we have knowledge lived in the caves, and brought thither the bones of these animals, which they had killed with the rude stone-tipped spears and arrows now found with their skeletons. They have even left us fairly correct pictures of the reindeer, mammoth, bison, and other animals of this time, drawn on ivory and slate. It is uncertain whether man had reached America as early as the last glacial advance, for neither human bones nor implements have been found with THE QUATERNARY PERIOD 463 remains of the extinct animals of the glacial times. There is, however, no proof that he was not then on the scene. THE RECENT EPOCH Little change since the glaciers passed away. By the departure of the last ice sheet northern North America was left in very much its present condition. Streams have cut small valleys in the glacial drift, many lakes have been filled by the accumulation of silt and vegetable matter, and some have been drained; but aside from such minor changes the aspect given the land by the glaciers has been preserved through the few thousands of years which have elapsed since the ice retreated. The Champlain submergence. About the shores of Lakes Champlain and Ontario marine shells and the bones of whales have been discovered in beds of clay high above the present lakes. In order that the sea should have extended in so far, the land must have been several hundred feet lower than now. At this time the salt water probably spread up the Hudson River to Lake Champlain, and also up many other valleys in the East. That changes of level are still in progress is known from the fact that old beaches all along the Great Lakes are no longer level, as of course they must have been when made. In general, they are now higher on the north and northeast and are tilted southwestward. Those of Lake Superior gradually sink from an elevation of four hun- dred feet at the east end of the lake to lake level and even pass beneath the water before reaching Duluth. Other slight risings and sinkings of the land have been in progress recently in many parts of the world. Indeed, there is scarcely a coast anywhere which does not reveal either raised beaches and sea-cut cliffs, or else drowned valleys and archipelagos. The former are conspicuous at many points in California and Alaska, while the latter are especially characteristic of the Atlantic coast of Maine and Britain. 464 HISTORICAL GEOLOGY Final readjustments in the living world. Since the last retreat of the ice, only slight changes have been wrought in the living world. The animals and plants we have to-day are similar to those of the Glacial epoch. True, certain species have migrated from one region to another. Thus the reindeer has moved north to Lapland and Siberia. Such animals as the mammoth and the cave bear have become extinct. Few, if any, newer types, however, have appeared. The event of chief importance, not only to us as human beings, but from a purely geological viewpoint, was the rapid spread and advancement of the races of men. Long before the dawn of historic times man had pushed outward from the place of his origin (itself yet unknown) and had colonized all the larger lands, and eventually even such remote islands as New Zealand and Hawaii, whither no other mammal except bats had ever gone. So long ago were the principal migra- tions made that the inhabitants of different continents have become distinct races through long isolation. Some of these races have since made comparatively little progress, while others have increased and developed with astonishing rapidity. The geologic effects of human activities. Probably no other land animal, certainly no other mammal, has equaled the human species in its effect upon the earth and its many living things. By digging canals he has connected seas and lakes hitherto separated. By cutting down the forests he has exposed to rain and wind the soils formerly held firmly upon the hills. Clear streams have thus become muddy, and permanent streams intermittent, while springs have disappeared and shifting sands have buried plains once fertile. In an even more striking way man has produced changes in the animal and plant world. Certain kinds he has protected and domesticated, so that they have become abundant in many countries. Others he has hunted almost or quite to extinction. Among the former are the cat, dog, and cattle ; while the auk, the passenger pigeon, and the bison may serve as examples of the latter. A full list of either would be long. THE QUATERNARY PERIOD 465 Some he has relegated to remote regions ; thus the wild turkey was formerly common throughout eastern United States, but is now to be seen only in certain mountainous portions of the southern states. Still others he has transported all over the world; for example, the brown rat, originally a native of northwestern Europe, is now found in every continent and island in the habitable zones. In more recent times man has even been instrumental in producing entirely new varieties of animals, and especially of plants, by the method of con- trolled breeding, which is now so successfully practiced. These are but a few examples of the many changes of which the human races have been the cause ; but they are enough to show how very important the geologic and biologic influ- ence of this highest of the mammals has become. QUESTIONS 1. In the shape of the edge of the last ice sheet, what evidence is there of the former existence of a large valley where Lake Michigan now lies ? 2. Part of this valley is now below sea level. To what extent do rivers erode their valleys below sea level ? What other factors may have been important here ? 3. In Indiana and Illinois the large bowlders in the drift are chiefly igneous and metamorphic rocks, such as granite, gneiss, gabbro, and quartzite, while bowlders of the sandstone and lime- stone which underlie the drift are less common. Why should this be true? 4. Have the uplands of Figure 477 been glaciated ? The evidence ? 5. At a point on the edge of the Wasatch Mountains in Utah a glacial moraine has been found dislocated, as shown in Figure 481. What events are indicated? 6. In northeastern California trees have been found associated with a bed of fine volcanic ash in the relations shown in Figure 482. What inferences may be drawn from this ? 7. Why should the skele- tons of mastodons and other large animals of the Glacial epoch be found in peat bogs ? FIG. 481. Dislocated terminal moraine. 66 HISTORICAL GEOLOGY 8. Elephants are at present confined to the tropical region. Do the bones of elephants in Alaska and northern Si- beria therefore indicate a tropical climate there in re- cent times ? 9. Of the caves with rela- tions as indicated in Figures 483 and 484, which affords the better evidence of the FIG 482. -Relations of trees to a bed of Q1 { ^ f ^ ^ volcanic ash near Lassen Peak, California. species, and why ? 10. Applying the principles already learned from the past history of animals, how can you account for the fact that the FIG. 483. Section of a cave in FIG. 484. Section of a cave in lime- limestone, showing earth (A) con- taining bones of men and extinct mammals, overlain by a crust of stalagmite (B), and the mouth of the cave closed by a deposit of till. Australian race of man has made less advancement than any other? 11. In California the Quater- nary glaciers were abundant on mountains eight to ten thousand feet high, while in Nevada the only peaks which had even small glaciers were more than eleven thousand feet high. Why should there be this contrast? stone, the floor of which is covered with earth (A) containing human bones and implements. FIG. 485. Map of the distribution of bowlders scraped off from an out- crop of igneous rock by a glacier. 12. In a locality in England bowlders derived from a small volcanic plug of peculiar rock are found distributed in the glacial drift as shown in Figure 485. Explain the fan-shaped distribution INDEX Abrasion, 88. Acidic rock, 24. Agents, geologic, 10. Aggradational processes, 11. Algae, 293. Algonkian period, 322. Alluvial fan, 172, 175, 176. Alluvial plain, 176. Alluvial soil, 17. Alluvial terraces, 183. Ammonite, 399, 426. Amoeba, 295. Amphibians, 301, 374, 386. Amygdules, 32. Angiosperm leaves, 420. Angio sperms, 294. Animals, 294. Ordovician, 346. Animal groups, Ordovician, 343. Animikean system, 323. Anorthosite, 30. Anthracite, 380. Anticlines, 66. Appalachia, 332, 398. Appalachian Mountains, 398. Appalachian trough, 390. Archaean system, 317. Archseopteryx, 412. Archseozoic era, 317. Arizona, 326. Artesian wells, 112. Arthropods, 299, 356. Atmosphere, 13. work of, 86. Atoll, 260. Augite, 21. Bacteria, 293. Bad Lands, 436. Bar, 248. Barrier island, 248. Barrier reef, 260. Basal unconformity, 33. Basalt, 29. Basalt-porphyry, 29. Base level, 140. Basic rocks, 24. Basin, drainage, 142. Basins, glaciated, 219. Batholiths, 51. Bed, 54. Bedding plane, 18, 54. Beds, limestone, 64. Beheaded stream, 169. Belt, cementation, 80. weathering, 80. Bergschrund, 200. Birds, 301. Cretaceous, 426. Jurassic, 412. Bituminous coal, 380. Bivalves, 298. Blastoid, 297, 373. Block mountains, 276. Blowhole, 243. Bombs, 33. Rrachiopods, 298, 336, 343, 354, 364, 377, 385, 399. progress of, 343. Braided rivers, 178. Breakers, 236. Breccia, 38. volcanic, 33. Bridge, natural, 118, 244. Bryophytes, 293. Buttes, 167. Bysmalith, 51. Calcareous springs, 111. Calcite, 22. 467 468 INDEX Cambrian period, 331. Cambrian strata, 331. Canons, 165. Carbonation, 103. Carboniferous period, 369. Carboniferous swamp, 384. Carnivores, 442. Cave, sea, 243, 245. Cementation, belt of, 80. zone of, 119. Cenozoic era, 306. Cephalopods, 299, 346, 355, 365. Cetaceans, 442. Chalk, 39. Chambered mollusks, 299. Champlain submergence, 463. Chemical elements, 19. Chemung formation, 361. Chert, 39. Chimney island, 244. Cinders, 33. Circumdenudation, 281. Circumerosion, 281. Cirques, 221. Clastic sediment, 369. Clay, red, 260. Cleavage, 20. Cliff, sea, 241, 243. undercut, 265. Climate, Cretaceous, 424. Pennsylvanian, 386. Permian, 393. Quaternary, 448. Tertiary, 437. Clinton formation, 350. Coal fields, 381. Coal, origin of, 378. Coal Measures, 377. in Europe, 382. in United States, 380. Coastal plain, 414. Coelenterates, 296. Columnar structure, 53. Comanchean period, 414-416. Competent strata, 67. Concretions, 121. Cones, 173. volcanic, 46. Conglomerate, 38. Consequent falls, 159. Consolidation, 37. Continental glacier, 207. Contour interval, 93. Copper, 120, 325. Copper vein, 80. Coral, 259, 296, 345, 353, 366. Cordaites, 385. Corrasion, 125, 133. Cracks, mud, 56. sun, 56. Creep, 114. Cretaceous period, 418. birds of, 426. climate of, 424. deposits of, 420, 423. inundation of, 419. mammals of, 431. sea animals of, 424. Crevasse, 199. Crinoids, 297, 344, 372. Critical temperature, 81. Cross-bedded sandstone, 54 ; 89. Cross-bedding, 54, 55. Crumpling, 65. in Jurassic period, 408. in Permian period, 390. Crust, earth, 61. Crustaceans, 300. Crustal disturbances, Cretaceous, 427. Ordovician, 346. Current, littoral, 246. shore, 246. Cut-off, 180. Cuttlefish, 299. Cycad, 403. Cycle, of erosion, 150. metamorphic, 83. Cystid, 297. Dead Sea, 356. Deep, 233. Degradational processes, 11. Dells, 162. Delta, 184, 228. Deposition, 91. INDEX 469 Deposition, causes of, 171. by glaciers, 208. by ground water, 119. Deposits, coral, 259. character of, 256. Cretaceous, 420, 423. Jurassic, 407. in lakes, 268. land-derived, 255. organic, 258. Tertiary, 433, 435. Deserts in Triassic period, 397. Devonian period, fishes, 365, 367. land life, 368. mollusks, 364. in the East, 359. sea life, 361. in the West, 359. Diastrophism, 11. Diatom ooze, 259. Dike, porphyry, 50. Dikes, diagram of, 58. Dinosaur, 409, 410, 425. Diorite, 29, 30. Dip, 66. Distributaries, 173. Divide, 141. shifting, 156. Dolerite, 29. Dolomite, 39. Downthrow, 72. Drainage basin, 142. Drift, 17, 204, 217. unstratified, 205. Drumlin, 212. Dunes, 95, 97, 98. Dust, volcanic, 33 ; Dust wells, 201. Earth, crust of, 61. history of, 289. origin of, 308. Echinoderms, 29 7 . Echinoids, 297, 423. Effect of ice sheets on land, 451. Elements, chemical, 19. Eocene epoch, 433. Eohippus, 444. Eolian sandstone, 98. Eolian soil, 17. Epicontinental sea, 339. Erosion, 90, 125, 155, 239. cycle of, 150. glacial, 217. rate of, 239. stream, 135. Esker, 228. Exfoliate weathering, 101. Exfoliation, 100. Expansion of Ordovician sea, 339. Falls, 157, 161. consequent, 159, subsequent, 159. Fan, 187. alluvial, 176. Faulted mountains, 276. Faults, 70. horizontal, 74. normal, 71, 73. reversed, 71, 72, 73. Feeding grounds, 196. Feldspathic rocks, 29. Feldspars, 21. Felsite, 29. Ferns, 293, 383. Fiord, 224. Fishes, 300. Devonian, 365, 367. Fissility, 70, 72. Fissures, 47. Flint, 39. Flood plain, 176, 182. Flood-plain lakes, 179. Flood tide, 236. Flowage, zone of, 78. Folded mountains, 153, 277. Folded strata, 67. Folds, 66-70. Formation, 54. Niagara, 351. Fossiliferous slate, 44. Fossils, 10, 291, 302. of Proterozoic era, 329. Silurian, 354. Fractures, 70-75. 470 INDEX Fractures, zone of, 78. Fragmental rocks, 39. Fringing reef, 259. Fungi, 293. Fusulina limestone, 382. Gabbro, 29, 82. Gastropods, 298, 337, 345, 354 423. Generalized type, 393, 443. Geodes, 122. Geologic divisions, 306. Geologic processes, 10. Geology, 9. Geysers, 113. Glacial conditions in tropics, 394, Glacial trough, 221. Glaciated hills, 219. Glaciation, effects of, 456. Glaciers, 191-230. Glass, volcanic, 31. Glassy rocks, 26. Glauconite, 258. Globigerina ooze, 258. Gneiss, 18, 42, 43. Gold, 120, 437. Goniatites, 365, 373. Gorges, 162. Gradation, 11. Grade, 135. Gradient, 134. Granite, 23, 28. Granite rocks, 130. Granitoids, 25. Graptolites, 296, 344. Great Plains, 419, 421. Great Salt Lake, 356. Greensand, 258. Ground water, 107. Gully, 138. Gymnosperms, 294. Gypsum, 22. Hamilton shales, 360. Hanging valley, 222, 225. Hardpan, 209. Helderberg limestone, 359. Hematite, 22, 350, Hesperornis, 427. Hills, glaciated, 219. Hogbacks, 166, 432. Hook, 247. Horizons, 331. Horizontal fault, 74. Hornblende, 21. Huronian system, 323. Hydration, 104. Hydroids, 296. Hydrosphere, 14, 15, 109. Ice cap, 194, 449. Ice field, 191. Ice pillars, 200. Ice sheet, 194. Greenland, 203. latest, 455. south polar, 202. Ice sheets, effect of, 450^52. Ichthyosaur, 401. Igneous rocks, 23, 37, 46, 49. [ncompetent folds, 68. tncompetent strata, 67. [nlet, 250. [nterior sea, 351. [ntermediate rocks, 24. [ntermittent streams, 139. [ntrenched meanders, 152. intrusion, laccolithic, 51. inundations, Cretaceous, 419. invertebrates, Triassic, 399. Iron, 120, 323. sland, chimney, 244. land-tied, 248. slands, barrier, 248. ellyfish, 296. oints, 53, 70, 71. urassic period, 405-411. Kame, 228, 455. kaolin, 22. arst topography, 118. ^eweenawan system, 324. jaccolithic intrusion, 51. -/accoliths, 50. jagoon, 250. INDEX 471 Lahontan, 459. Lake, 233, 261. Lake Bonneville, 459. Lakes, deposits in, 268. extinct, 267. fate of, 266. flood-plain, 179. function of, 264. marginal, 454. ox-bow, 180. permanent, 263. processes in, 264. Quaternary, 458. salt, 268. Lake Superior region, 322. Laminae, 54. Lamination, oblique, 55. Lamp shells, 298. Land animals, Pennsylvanian, 385. Land, in Jurassic period, 405. of Permian period, 390. Land life, Devonian, 368. Landslide, 115, 116, 117. Land-tied islands, 248. Lapilli, 33. Laramie formation, 423. Lava, 25. Lava flow, 49. Lava plateaus, 47. Lava sheet, 58. Lead, 120. Lead deposits, Ordovician, 342. Lee slopes, 219. Lepidodendron, 383. Levee, natural, 177. Limestone, 39. beds of, 64. Mississippian, 370. Limonite, 22. Lithosphere, 14, 16. Littoral current, 246. Load of stream, 132. Loess, 98. Loop, 248. Magnetite, 22. Mammals, 301, 402 Mammals, Cretaceous, 431. evolution of, 444. hoofed, 443. Jurassic, 411. migration of, 445. Man, appearance of, 462. Mantle rock, 17. Marble, 44. Marginal lakes, 453. Marine strata, 397. Marl, 269. Massive structure, 52. Mastodon, 461. Mauch Chunk shale, 370. Meanders, 152, 178, 179. Medina sandstone, 350. Medusas, 296. Mesas, 52, 167. Meta-igneous series, 45. Metamorphic cycle, 83. Metamorphic rock, 19, 41. Metamorphism, 11. Meta-sedimentary series, 44. Mica, 22. Migration of dunes, 94. Migrations in glacial epoch, 460. of Tertiary mammals, 445. Millstone grit, 387. Mineralizers, 26. Mineralogy, 9. Minerals, 19, 20. Mineral vein, 79. Miocene epoch, 433. Mississippian crinoids, 372. Mississippian limestone, 378. Mississippian period, 369. Mississippian seas, 371. Mississippian sedimentation, 370. Mollusks, 298, 406. Devonian, 364. Monadnocks, 150. Monoclines, 66. Monroe strata, 357. Moraines, 209. Mosasaur, 425. Moss, 293. Mountain growth of Tertiary period, 438, 472 INDEX Mountain range, 275. Mountain system, 275. Mountain, volcanic, 52, 280. Mountains, 274. block, 276. combination, 281. destruction of, 282. distribution of, 275. faulted, 276. folded, 277; of circumdenudation, 281. of circumerosion, 281. Movement of glaciers, 197. Movements of sea, 235. Mud, 258. Mud cracks, 56. Narrows, 162. Natural bridge, 118, 244. Natural levee, 177. Nautilus, 299. Nebula, spiral, 312. Nebular theory, 309. Necks, volcanic, 52, 53. Neutral rocks, 24. Newark formation, 398. Niagara fauna, 352. Niagara formation, 351. Normal faults, 71, 73. Nunataks, 203. Oblique lamination, 55. Obsidian, 25, 31, 32. Ocean deposits, 256. Ocean, offices of, 234. shores of, 237. Oceans, 233. Olivine, 22. Onondaga limestone, 360. Oolite, 260. Ooze, 37, 258. Ordovician deposits, 342. Ordovician period, 339, 341. Ore vein, 80. . Organic deposit, 258. Oriskany sandstone, 360. Ostracoderms, 365, 366, Outcrop, 18, 69. Outliers, 357. Out wash plains, 228. Overthrusts, 73, 74. Overturned folds, 67. Ox-bow lake, 180. Oxidation, 103. Oxides, 19. Oyster shell, 406. Paleontology, 9. Paleozoic Alps, 371. Palisades, 53. Peat, 269, 380. Pelecypods, 298, 345, 364, 399, 406, 423. Peneplains, 147, 154. Pennsylvanian period, 376. Pennsylvanian plants, 382-384. Pennsylvanian land animals, 385. Peridotite, 29, 30. Permian period, 389-392. Petrification, 122. Petroleum, 435. Petrology, 9. Phenacodus, 443. Physical geology, 10. Piedmont glaciers, 194, 202. Piedmont plateau, 414. Pitch, 70. Pitchstone, 32. Pitchstone-porphyry, 33. Piracy of streams, 169. Plains, 285. alluvial, 176. base-level, 147. classes of, 286. flood, 176, 182. origin of, 286. outwash, 228. Planes, bedding, 54. Planetesimal theory, 311. Plants, 293. Comanchean, 416. and animals, Ordovician, 346c Pennsylvanian, 382. Plateaus, 284. erosion of, 285. INDEX 473 Plateau, lava, 47. origin of, 285. Playas, 56. Plesiosaur, 401. Pliocene epoch, 433. Plucking, 217. Plugs, 53. Pocono sandstone, 370. Polyps, 296. Porphyritic texture, 24. Porphyry, 28. Porphyry dike, 50. Pothole, 161. ' Potomac series,, 415. Processes, geologic, 10. Profile, interrupted, 153. Proterozoic era, 322. fossils of, 329. Proterozoic group, unconformi- ties in, 327. Proterozoic rocks, 325. Proterozoic strata in Arizona, 326. in Lake Superior region, 322. Pteridospermae, 293, Pteridophytes, 293, 383. Pteropod, 337. Pterosaur, 411. Pumice, 31. Quaternary lakes, 458. Quaternary period, 448. Quaternary volcanoes, 460. Quartz, 21. Quartzite, 38, 79. Radiolarian ooze, 259. Rainbow Falls, 158. Range, mountain, 275. Rapids, 157. Ravine, 138. Reef, barrier, 260. fringing, 259. Reindeer, 462. Rejuvenated area, 153. Relief, 274. Relief model of North America, 62. Reptiles, 301, 393, 400. Residual soil, 17. Reversed faults, 71, 72, 73. Ripple marks, 55. River system, 142. Rivers, braided, 178. Rock structure, 46, 167. Rock terraces, 166. Rock texture, 24. Rock waste, 17. Rocks, 20. acidic, 24. basic, 24. classes of, 17. feldspathic, 29. fragmental, 39. glassy, 26, 31. granite, 130. igneous, 19, 23, 37, 42, 49. intermediate, 24. mantle, 17. metamorphic, 19, 41. neutral, 24. relation of, 44. secondary, 41. sedimentary, 19, 37, 40. stratified, 19. Rodents, 442. Run-off, 107. Salamander, 374. Salina beds, 356. Salt lakes, 268. Sand dune, 92, 93, Sandstone, 38. cross-bedded, 89. Eolian, 98. Sapping, 160. Scallop shell, 385. Schist, 42, 82. Scoria, 31. Scoriaceous texture, 32. Sea, epicontinental, 339. movements of, 235. Sea animals, Cretaceous, 424. Sea cave, 243, 245. Sea cliff, 241, 243. Sea erosion, 239. Sea expansion, Ordovician, 339. 474 INDEX Sea level, 254. Sea life, Permian, 392. Sea urchins, 297, 423. Seals, 462. Seas, Mississippian, 371. Secondary rocks, 41. Secretions, 122. Sedimentary rock, 19, 37, 40. structure, 54. Sedimentation, Mississippian, 370. Ordovician, 340. Sediments, formation of, 37. Seed ferns, 293. Seed plants, 294. Seepage, 110. Shale, 39. Shark, 367, 374. Shelves, continental, 256. Shore current, 246. Shore lines, 238, 250. Shores, ocean, 237. Silicates, 20. Sills, 50. Silurian period, 349. Silver, 120. Sinks, 118. Siphuncle, 299. Slate, 43, 82. fossiliferous, 44. Slickensides, 71. Slump, 115. Snail group, 298. Snow line, 191. Snow field, formation of, 191. Soils, 16. Solution by ground water, 116. zone of greatest, 117. Source of ground water, 107. Spermatophytes, 294, 416. Spit, 247. Sponges, 295. Spores, 293. Springs, 110. calcareous, 111. deep-seated, 113. ferruginous, 111. hillside, 111. medicinal, 110. Stack, 244. Stalactites, 120. Stalagmites, 120. Starfish, 297. Stock, 51. Stoss slope, 219. Strata, competent, 67. folded, 67. incompetent, 67. Stratification, 37, 54. in a sand dune, 93. Stratified rocks, 19. Stratum, 54. Stream, beheaded, 169. degrading, 134. graded, 135. intermittent, 139. Stream piracy, 169. Streams, work of, 125. Striate, 217. Strike, 69. Structural valleys, 138. Structure, columnar, 53. massive, 52. Structures of sedimentary rocks, 54. Submergence, Champlain, 463. Subsequent falls, 159. Subsoil, 16. Sun cracks, 56. Suture, 299. Syenite, 29, 30. Synclines, 66. System, mountain, 275. Taconic revolution, 348. Talus, 90, 102. Teleosts, 416. Temperature, 100. critical, 81. Tepee buttes, 432. Terrace, 188. alluvial, 183. river, 152. wave-built, 241. wave-cut, 241. Terrigenous deposits, 256. Tertiary climate, 437. INDEX 475 Tertiary deposits, 433, 435. Tertiary mountains, 439. Tertiary period, 433. Tertiary volcanoes, 440. Texture, porphyritie, 24. rock, 24. scoriaceous, 32. vesicular, 31. Thallophytes, 293. Throw, 71. Tides, 236. Tools of a river, 133. Topographic divisions of North America, 63. Topography, karst, 118. mature, 146. old, 146. serrate mountain, 102. Transportation, 86, 125. by glaciers, 208. by streams, 130. Transported soil, 17. Triassic invertebrates, 399. Triassic period, 397. Appalachian Mountains in, 398. deserts in, 397. marine strata in, 397. volcanic eruptions in, 398. Trilobite, 336, 343, 352, 365, 373. Tropics, glacial condition, 394. Trough, glacial, 221. Tuff, 33, 120. Type, generalized, 393, 443. Unconformities, in Proterozoic group, 327. value of, 304. Unconformity, 75, 76. basal, 322. of Comanchean period, 415. Undercut cliff, 265. Undertow, 235. Unglaciated regions, 457. Ungulates, 442. United States coal fields, 381. Upthrow, 71. Valley deepening, 139. Valley flat, 140. Valley glaciers, 194, 211, 458. surface of, 199. Valley lengthening, 141. Valley system, 142. Valley trains, 227. Valleys, 137. hanging, 222, 225. mature, 143. old, 143. structural, 138. tributary, 142. young, 143. Vein, mineral, 79. Velocity of stream, 134. Vertebrates, 300. Vesicular texture, 31. Volcanic ash, 33. Volcanic breccia, 33. Volcanic cones, 46. Volcanic dust, 33. Volcanic eruptions, Triassic, 398. Volcanic glass, 31. Volcanic mountain, 52, 280. Volcanic necks, 52, 53. Volcanoes, Quaternary, 460. Tertiary, 440. Vulcanism, 11, 77. Warping, 64. Waterfall, 160. Water gaps, 162. Water table, 107. Waters, ground, work of, 107-123. Weathering, 125. belt of, 80. exfoliate, 101. processes of, 126. rate of, 127. Wells, artesian, 112. Worms, 297. Zinc, 120. Zinc deposits, Ordovician, 342. Zone, of flowage, 78. of fracture, 78. of greatest cementation, 119. of greatest solution, 117. THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE." THE PENALTY WILL INCREASE TO SO CENTS ON THE FOURTH DAY AND TO $i.OO ON THE SEVENTH DAY OVERDUE. FEB 2 1955 DEC 31956 FB 2 8 1958 LD 21-100m-8,'34 LIBRARY G THE UNIVERSITY OF CALIFORNIA LIBRARY