OUTLINES OF THE HALF-COURSE IN Natural History WITH REFERENCES TO DANA’S MANUAL OF GEOLOGY, AND NOTES. COPYRIGHT, i88s- (S^ambribgc, Pass.: Wm. H. Wheeler, Publisher. 1885. Digitized by the Internet Archive in 2017 with funding from University of Illinois Urbana-Champaign Alternates https://archive.org/details/outlinesofhalfcoOOunse OUTLINES OF NATURAL HISTORY 4. INTRODUCTORY. The Earth. How REGARDED. Dana^ ff, 1-2. Earth is to be treated as a mechanism which has a history ; as a theatre (i) of the application of force, (2) of the development of life. It is as truly as is the steam-engine, a mechanism which is applying solar force. Kepler’s idea of the earth as a living, breathing crea- ture is nearer right, than that of an utterly inanimate mass. Relation to Universe. Dana, ;p. j. Three proofs of Earth’s kinship to pkwirts : — (1) Telescopic — spherical form, continents and volcanos. (2) Meteoric — elements and laws of crystallography same. (3) Spectroscopic — elements of sun, etc., same as those of earth. Aim of Geology. Dana,ff. 4.-6. I. Physiographic Geology. Earth’s Contour and Surface Divisions. Its form. Dana, f. p. Flattening of earth is correlated with its rotation, being just what would result from earth’s present angular veloc- ity in a liquid globe of same size and density. This gives proof of the present or former molten state of the earth. OUTLINES OF NATURAL HISTORY 4. 4 Land and Water. Dana ^ j >^, lo , ij . From the cooling and development of an original nebu- lous mass, there results a solid surrounded by two oceans, of water and air, respectively. Much depends on the interaction of these two oceans, one partially, the other entirely covering the earth’s surface. We see on surface of globe a great land-mass about No. Pole from which three triangles project southward into mass of water about So. Pole. Note that oceans keep practically same level, while lands rise and fall. Two great geologic forces are constantly at work on land, those above and at sea-level being acted on by erosion^ the destructive force, and those below sea-level being subjected to depo- sition^ the constructive force. Depth of Oceans. Dana^ ff. //, 12. Note that this is on the average much greater than the elevation of land-masses. Surface Reliefs. Dana^ fp, 15-23. Substitute for definition of mountain as follows : — A mountain is a ridge of the earth’s surface in which at least a part of the relief is due to the folding of strata. A hill is a relief caused by the cutting away of parts of strata by erosive action, leaving remainder as an elevation. A volcanic cone is an elevation produced b}^ a deposit of substance ejected from a crater. System in Earth’s Reliefs and Feature-lines. Dana^ fp. 23-38. Dana has been chief student of this subject and has given it undue prominence here. Read especially “Law of the System,” p. 23, and “Recapitulation,” p. 37. Great exception to general N. W. and N. E. trends is the Indo- European mountain system which trends E. and W. Atmospheric Currents. Dana ^ p . pj . 3 . These give us key to oceanic currents and are therefore taken up first. If we had no atmosphere of air, we should have one of steam as a result of the sun’s heat PHYSIOGRAPHIC GEOLOGY. s which is sufficient to melt 8000 cu. mi. of ice, daily. Were there no transportation of heat, the temperature of the tropics would be perhaps 150° and that of the poles — 50° F. This would leave but a narrow belt where life could exist and it would probably be kept off there by furious winds. But since air is easily penetrated by direct heat and holds that which has entered it, lower air at Equator becomes very much warmed, and rises, causing an inflow of cooler air from poleward regions, to balance which an outflow towards poles takes place in upper air. This would give north and south winds toward the Equa- tor in the tropics. But as earth’s rotation causes an effect like that of a turn-table, throwing moving bodies on its surface to the right of a direct course, these currents of air are thrown to right, and N. E. and S. E. wind pro- duced, known as the Trades. But air is a very poor conductor of heat and acts to equalize temperature only in a secondary way. Oceanic Currents. Dana^ p. j8. Trade winds blowing steadily over the oceans within the tropics toward N. W. and S. W. cause, by their con- stant brushing over the surface, strong currents in the same directions, which meet near the equator and flow out on a westerly course. If earth’s surface were wholly water this current would simply follow the equator, but being broken up by continents, it is turned N. and S. and carries into Temperate and Polar regions vast quan- tities of heat. Thus in Atlantic the equatorial current splits on C. St. Roque, and losing influence of trades, a great current is carried by its own inertia up the western shore and across the ocean to its northeastern shore, and on in part, into the Arctic Zone. A part of this great "Gulf Stream” returns along the coast of Europe, but much the greater part returns along the bottom of the ocean, as a slow, cold current. Dr. Croll has shown that the Arctic Zone gets OUTLINES OF NATURAL HISTORY 4. 6 more from Gulf Stream than from sun directly, and many parts of the earth are made habitable by warm cur- rents. Since Cape St. Roque is some distance south of equator, the No. Atlantic O. gets more than its share of the equatorial current. To this fact is due the climate of England and her history. Great Japan current of the Pacific does not send any branch into Arctic O. If Alaska were below the surface this would happen ; and this may perhaps explain the former mild climate of Greenland. Gulf Stream had long been known in a general way, but was first definitely studied by Benj. Franklin, though he never clearly published his results. First careful study of ocean currents was by Maury, who plotted them from study of a large series of ship’s logs. He believed them caused by greater height (17 ft.) of water at equa- tor than at poles, due to its expansion from greater tem- perature at equator. Dr. Croll has shown that this gradient is incompetent to produce the known results. Waves are also caused by winds, but are only undulations not implying motion of translation. Their effect is to strike a blow. Climate. Dana^f.4.^, Land climate is result of interrelations of water and heat, and furnishes greatest extremes. Sea climate is much less variable. As an example of effect of absence of water, moon’s surface is dead, without atmosphere or life. Only changes there are due to expansion and con- traction caused by enormous daily extremes of temper- ature (perhaps 1000° to — 200° F.) At the critical point of 32° F. the influence of water is wholly changed. Above, it is a liquid, readily absorb- ed and essential to life ; below, it is a ver}^ slowly moving solid, fatal to life. Distribution of Fertile and Sterile Lands. Dana, f. 44. LITFIOLOGICAL GEOLOGY. 7 II. Lithological Geology. Rocks. Dana^ -pf. ^7, 62. This term is used to include all the materials of the earth’s mass, excepting imbedded ice. Rocks give us the chief points in the study of the earth’s physical history, while their contained fossils furnish much of the history of life. They may be divided into two great groups, according to their origin. Mineral Rocks are those which have originated solely from physical and mechanical causes, including by far the greater mass of rocks, the sandstones, granites, etc. Ac- cording to structure they may be divided into two groups : Fragmental rocks, formed of broken fragments of various rocks, are produced wholly by action of celestial forces working through water. These include sand- stones, shales, etc. As soon as laid down these rocks become subject to action of terrestrial and celestial forces, for as new strata are laid down over them, the effect is to raise the temperature in lower strata in the same way as if a series of blankets were piled up over them, each new blanket serving to raise the temperature at any given point. This blanketing thus causes the lines of equal earth temperature (isogeothermal lines) to rise, and as a result of increased temperature and pressure the fragmen- tal rocks become slowly changed to the. second or crystal- line form which is the state of rest of rock material. Crystalline rocks include granite quartz, schists, etc. The effect of erosion on these rocks is to carry them back to the fragmental form, thus completing the cycle. Organic Rocks owe their origin to physical and organic causes combined, and consist almost wholly of compacted remains of organisms. They include the limestones. Elements in Rocks. Dana, f. ^8. Add to those enumerated by Dana, phosphorus, the re- lation of which to life will be noticed later. OUTLINES OF NATURAL HISTORY 4. 8 ? Distinctions in Study of Rocks. Dana^f. 6j. Note that specific gravity of rock is necessarily greater than that of water ; otherwise the earth’s machinery must be stopped. Note carefully descriptive terms, p. 66. Conditions and Structure of Rock Masses. Stratified Condition. Dana, f, /p. Stratified rocks are those arranged in layers, in whose de- position there has been an element of gravity. They may be formed in three ways : (i) by falling of matter through water to bottom ; (2) by falling of matter through air, as from a volcano; (3) by lava flow from fissure or crater. A layer (p. 81) is the structural unit in stratification ; it indicates the the continuation of certain forces for a time, followed by a change. A formation consists of several or many strata of many layers each. It is less clearly marked than a stratum, which is not so readily distinguishable as a layer. Structure of Layers. Dana, p, 82. Of structures originating in the act of deposition, the laminated and shaly, separated by Dana, are practically the same. Compound structure is due to the action of winds, waves, or currents. Strata formed by the wind are of uniform, fine material ; those formed by water may be of various materials. Strata formed by regularly flowing currents are of an uniform inclination, while those formed by tide currents or waves may be very uneven and broken. Ripple marks {p. 84.) indicate the presence, at time of their formation, of shallow water and a current, but not necessarily of a beach, since they are known to be formed in water 200 ft. deep. They are like marks formed by wind on sand or snow. How this action starts and what determines height and distance of ridges is unknown, though it may be a rolling motion of air or water. Both measurements are greater in water than in air. LITHOLOGICAL GEOLOGY. 9 Mud-Cracks are such as are commonly formed under a hot sun on muddy shores, and are often and easily fos- silized by being filled with blowing sand or some other substance, and then buried under fresh mud. Rain Drof prints are well preserved on some old beaches ; from them we can often tell the direction of the wind during the shower, by the splash on the leeward side of the print. Of structures of origin subsequent to la 3 fing down of strata : — the concretionary structure ( -p. 8^) somewhat resembles the crystalline, but differs in that crystallization takes place in accordance with mineral species, at high temperatures, and results in definite geometrical masses, while concretion takes place more with reference to sub- stance of matter deposited, at lower temperatures, and re- sults in amorphous masses. Concretions often begin about an organic nucleus. Sometimes they are hollow or with interior filled with crystals. They often simulate animal forms, as in case of a form found in very old limestones in Canada and named Eozoon Canadense^ whose true nature was for 20 years a subject of much dispute, many believing it to be the fossil remains of a very simple animal. Concretion and crystallization are the two great causes of metamorphosis in rocks, and are greatl}^ aided by the presence and circulation through the rocks of waters having mineral matters in solution. A similar action is the taking on, by the constituents of rocks, of more of their own material from solutions, thus more firmly cementing the rock together. This is called in- terstitial metamorphosis. All but the newest rocks are penetrated by breakage - lines (^. 88^ usually in different directions, and irregu- larly placed. These various planes cut up the rock and facilitate quarrying, where not so numerous as to render the rock worthless. The cause of formation of these joint- OUTLINES OF NATURAL HISTORY 4. lO -planes^^ is probably successive enormous pressures to which the rock has been subjected, each set of joint-planes being the index of a pressure at right angles to its direc- tion. Slaty cleavage {p. 8g) is also caused by pressure, but differs from jointed structure in having its planes close together and in a single direction only. The rocks in which this structure occurs were formerly clayey mud which contains much mica in tiny particles. Pressure on this mud would bring the bits of mica more nearly par- allel and start fracture-lines. Movements of rocks on each other cause groovings, polishing, etc. Positions of Strata. Dana, p, gi. Natural position of strata is horizontal, the tendency being to level irregularities, and, on an inclined surface, to form an incline of less angle. River-deltas, however, form beds which, at their further ends, have steep slopes. Dislocations of strata caused by powerful forces are common, and of several forms. The simplest is the fold, which may be simple or modified by subsequent greater folds, or may be collapsed, that is, pressed together till there is no longer resistance to pressure and its sides meet, the whole fold falling over sidewise. The line of direction of the fold is called its axis. An upfold, with sides sloping away from a ridge is called an anti- clinal; a downfold with sides sloping into a trough, a synclinal. The compass course of the axis of a fold is its strike, while the slope of its sides is the dip. These folds, etc., are negative results of the cooling of the earth’s interior and consequent shrinkage and packing of the crust to accommodate itself to a smaller nucleus. Faults are usually results of similar forces to those causing folds, and occur where rocks from their structure or from being thick-bedded, do not readily bend under pressure, but rend apart. Faults very commonly follow I LITHOLOGICAL GEOLOGY. 1 1 joint planes, and may be of any size. In most cases there has been, in addition to rending, a slipping of rocks on opposite sides of the fault upon each other and this sometimes amounts to thousands of feet, bringing strata of very different age on to same level. Fault-planes may be located, like folds, by dip and strike. The portion of rock faulted out of place is usually wedge-shaped, and in a large majority of cases the strata have been pushed in the direction indicated by the point of the wedge. Since a joint-plane is commonly a warped surface the result of slipping of rocks is often to form open spaces where sides fail to fit closely, and many mineral veins have originated in this way. Another form of fault is caused by splitting of rock mass from shrinkage, forming “Gash-cracks.*’ Even the hard rocks are fluid to sufficiently great pressure and may be shown to have been compressed so as to be increased many times in length, by distorted fossils found in them. These can be recognized when extended to three or four times their length, but beyond this become mere blurs. One of the hardest tasks of the Palaeontologist is the reconstruction of distorted fossils. Great sources of error in study and practical estimation of strata are folding and faulting, by which a single stratum may be brought to surface at several points and so made to seem like so many strata. The geologist avoids error of this sort by carefully noting the thickness of seemingly different strata as well as the character and thickness of beds immediately above and below them. Similarity in these data indicates pretty surely the iden- tity of what seem to be distinct beds. Effects of denudation and conformable and unconfor- mable strata are sufficiently treated in text g6, loo). Arrangement of Strata. Dana, f. loi. Means of determining the order of arrangement of strata OUTLINES OF NATURAL HISTORY 4. 12 are several, but the most important is the stud}’ of their fossils. It was suggested 200 years ago by Dr. Hooke that it might be possible to "raise a chronology” from the study of fossils, but nothing was done until the begin- ning of the present century, since which time it has been shown by careful study of fossils that there have been many nearly complete changes in the organic life of the earth’s, surface. This was long believed to be due to the sudden extinction of life and the subsequent creation of new forms ; but it is now believed that at various times new forms have arisen from existing forms by rather rapid changes and modifications, though we cannot say what forces have determined these changes. The study of these organic remains enables us to identify layers or beds of the same approximate age, to study ancient climates from the distribution and limits of species, and to study the currents of the ancient seas, and from them continental lines, chiefly from remains of reef-corals which require food-bearing currents for their growth. From these studies, materials are accumulating for a complete history of the ancient conditions and changes of the earth, since organic life, on account of its delicacy and impressibility, preserves for us in its remains, records of many important facts of which the rocks give no hint. But the old idea that similarity of species of fossils in two layers indicates identity of age must not be too much trusted since a given region of the globe may have had at one time a very similar fauna to that of another region at a very different geologic period. For instance, as we go away from Europe we find life differing more and more from the life of that continent, and find that the present fauna of N. A. much more closely resembles the Middle Tertiary fauna of Europe than that of to-day, while the present Australian fauna is very like that of Europe in the Jurassic period. It should be noted that fossils “of the same age” may LITHOLOGICAL GEOLOGY. 13 represent organisms which lived thousands of years apart, but under similar terrestrial conditions, that is, in the same geologic period. Fossils of the same age or geologic period are said to belong to the same ^'‘horizony Unstratified Rocks. Dana^ f, loj. These rocks may be divided into three classes; (i) lavas which have flowed from craters ; these sometimes present a stratified appearance from the superposition of successive flows, and on the other hand some bedded rocks have been so metamorphosed by heat as to resem- ble lavas; (2) lavas which have flowed between bound- ing walls of rifts, forming dikes ; and (3) deposits from solutions in water, such as those of mineral matter in veins. To these ma}^ be added for convenience, glacial drift which owes its origin to cold, while those properly belonging here are the result of heat-action. Dikes and Veins. Dana^ -pf. 108-1 ij. Both these formations originate as rifts or cracks in the rock, caused by faulting or otherwise, and usually follow- ing joint-planes which become filled by diflerent mineral substance. Dikes are formed of full width from the first. When a series of beds, with a rift, becomes greatly heated by the blanketing eflect of strata lying above, and one bed, being more fusible than the rest, becomes melted, the effect is to at once force this molten material upward into the rift, with or without volcanic action, filling the rift and forming a dike, which usually does not extend to the sur- face and is not exposed until the overlying strata have been planed away, ages later. Dikes are usually verti- cal and often very irregular ; they frequently intersect, when their relative ages can be made out. Fissures not filled by dike-material are sure to be sooner or later, by slow process, filled with deposits from per- colating water, forming veins. These fissures may be formed at first of their full width, but oftener they are OUTLINES OF NATURAL HISTORY 4 . 14 gradually spread by the force of deposition and crystalli- zation of substances within ; this is called interstitial growth. All substances are found in sea-water in some quantity, it is supposed. These are taken up by plants and animals, as for instance, lime salts by corals, mol- lusks and others ; soda and potash b}^ sea-weeds ; and nearly all elements in minute quantity by various forms. When these plants and animals die and decompose, these elements become a part of the rocks. This may be termed the first concentration of the elements. When deeply buried in the rocks, where the water has a solvent power equal to that of the strongest acids, due to its great temperature and pressure and large proportion of CO'S these substances are dissolved and the percolating water becomes charged with them. Here we have the second concentration. If now we have a series of beds with one or more rifts or cracks running through them, we shall find this saturated water working downward through the permeable strata, which are bordered by impervious ones, until it reaches the rift, through which it will rise rapidly, depositing on the walls the substances held in solution, in the order of least solubility, and so forming veins of metals or other substances. The forces which cause con- stant percolation of water through the rocks are: (i) Gravitation ; (2) Osmosis ; and (3) its tendency to.pene- trate towards a centre of heat. DYNAMICAL GEOLOGY. {Dana, Part IV, j>. 605.') Energy from the celestial spaces as applied to the earth’s surface may be divided into two classes : — the general- ized, physical forms, the winds, currents, etc. ; and the localized form, seen in organic life. DYNAMICAL GEOLOGY. 15 Life. Dana^ f. 606. An organic form may be defined as a form of matter which has the power of taking into itself matter from without, and of utilizing its material and force. The work of earth-building depends on the interaction of or- ganic and inorganic forces. The tendency has been to belittle the influence of organic life in this matter, but it is true that hardly any existing rock would be in just ils present condition, but for this influence. In crossing North America we are for more than half the distance on rocks which are largely the result of organic life, and it may even be said that our continent owes its present form to this really powerful agent. Its Protective Effects. Dana^f. 6oy. The most important protective effect of vegetation is to protect soil and roots from the washing effect of rains. A ploughed field loses much of its loose surface-soil with every rainfall. As the vegetation depends for its nour- ishment largely on the soil, so it protects itself by hold- ing the soil, and a given piece of ground depends for its soil on its vegetation. Thus the two are interdependent. Sea-weeds protect the rocks on which they grow by pre- venting the water left in the crevices from freezing at low tide and rending the rocks. Our salt marshes of the New England coast are not merely protected, but have been won from the sea by maritime plants, chiefly grasses, rushes, and sedges. Its Transporting Effects. Dana, p. Soy. Its Destructive Effects. Dana, p. 6oy. These are better called Metaniorphic effects of life, in harmony with the modern idea of our world, that of change, rather than of destruction and death. Decay effectually attacks only exposed surfaces of rocks. Roots in their growth split rocks and thus expose more surface for erosive action. A cubic foot of rock broken up into cubic inches has more than ten times as much OUTLINES OF NATURx\L HISTORY 4. 1 6 surface as if in a single cubical block. Thus plants help to make the soil they need. If the Colorado canons were grown with vegetation, their vertical walls and sharp angles would be lost, from the rounding and eroding action due to the leverage of plant-growth. Earthworms do a great work in changing the surface of the ground. Mr. Darwin has shown that they average 40,000 to each acre of arable soil ; and these pass through their digestive systems for the sake of the organic matter it may contain, man}^ tons of earth per year which is left at the surface of the ground, so that thus the surface is often complete!}^ changed. Thus, also, stones and small objects soon become buried and Mr. D. believed that even some of the rural churches of England had been undermined and caused to sink noticeably by earth- worms. A similar action is going on, by worms, on the sea-floor. Ants also do a similar, but less extensive work. After life is extinct, many organic forms have much effect. Many growing trees shed during their lives woody matter of twenty times their own bulk, yet there remain at the base, but a thin layer of vegetable mould, whose constituents are constantly being absorbed by row- ing plants, while the great bulk of the constituents of the original wood has passed into the air and been more or less absorbed by plants, in the gaseous form. Thus the dust to which we return is but a transient passing stage be- tween life and life. Its Contribution to Rock-Formations. Dana^ -pf. 608- 626. These take place chiefly under water where decomposi- tion is slow and imperfect. Here plant and animal remains become enclosed in the rocks as stored-up carbon, in form of coal, limestone, etc. On land, one of the chief forms of deposit is the peat- bog. These are formed in small ponds which become gradually filled up by the growth of peat mosses. DYNAMICAL GEOLOGY. 17 Starting from the edge of the pond, these mosses, chiefly of the genus Sfhagniim^ grow out slowly into the pond, growing upward and dying below, and forming a tangled, closely matted mass, which holds water like a sponge. This water admits of but very slow decay of the dead moss, which gradually falls to the bottom and forms a layer of black mould, between which and the sheet of moss is a layer of water. But the growth of the moss increases the thickness and weight of the sheet, and Anally it reaches the bottom, filling the basin. As the moss is very yielding, the growth always forms a veiy treacherous bog. In some bogs in Northern N. A. the Sphagnum may grow up a slope for a few feet, but in Europe are bogs which have spread hundreds of feet from the original edge of the pond. These are called clhnhing bogs. They sometimes become so filled with water in heavy rains as to burst and flood the surrounding country with black mud. A peat-bog must have one of two fates. It may be quickly lowered beneath the sea and then buried under rock strata, to form a coal bed under influence of heat and pressure. If slowly sunk, it will be destroyed by wave-action. Or it may remain above the sea-level, and be finally drained by cutting down of surrounding land. Then the dried peat will decay and return its stored carbon to the air. As temperature of tropics causes more rapid decay, no important bogs or coal deposits are found there. Peat bogs and coal-deposits are often underlaid by thin strata of so-called ^Hfifusorial earth which consists of remains of microscopic plants and animals which lived in the pond that preceded the bog. In the sea, the less complete decomposition of plants and their different composition result in the deposition of carbonaceous shales. A large deposit of this sort in the Ohio valley represents an old Sargasso sea. It is very rich in coal-oil products and was quarried for distillation, before the introduction of bored wells. OUTLINES OF NATURAL HISTORY 4. 1 8 Animals leave deposits of mineral matters taken up by them during life, especially carbonate and phosphate of lime, and silica. Phosphorus, though existing in such small quantity, is essential to the animal organism : it enters into the constitution of bone, and is intimately connected with the working of the nervous system ; it is secreted especially by the King Crab {Limuhis) and the Brachiopods. Carbonate of lime is secreted by many forms. It is deposited on the floor of the deep sea chief- ly by the tiny Foraminifera which live at the surface and whose dead shells and tests are constantly raining down upon the bottom to add to the slowly growing deposit. But shallow water deposits are due to fixed animals, and especially to corals. Rising from the sea-floor in the So. Pacific O. are many mountain-like masses with steep slopes, coming above the surface as ridges enclosing shallow basins of water. These are solid masses of carbonate of lime in the forms of coral skeletons, with living corals at the top, and are called coral islands or atolls. The sea is constantly wearing away these masses by its beating on the outside, while the basin or lagoon within is quiet, though con- nected with the ocean by one or more openings, usually on the leeward side. On the ridge above water is found a beach of white calcareous sand, and above a wind- piled heap of this sand covered with a vegetation chiefly of palms. On the outside of the reef where the sea beats strongest are found the dome-shaped corals which can withstand the waves (Maeandrina, Astraea, etc.) ; in more protected situations are the branched forms (Madre- pores), while in the inner basins are many very delicate corals and other animals. The crevices and tide pools everywhere swarm with various forms of animal life suited to them. Along the N. E. coast of Australia is a great wall of coral over looo mi. long and several thousand feet high, known as the Australian barrier reef. DYNAMICAL GEOLOGY. 19 These coral reefs and islands depend on a sufficiently rapid growth to counteract and to gain on the wearing action of the sea. Their existence depends on the possi- bility of the association of numbers of individuals in colonies and on the presence of currents bearing suffi- cient food and warmth (70 — 75"^ F.) to support these col- onies in their growth and to supply the immense energy expended by them. Atolls were at first believed to be formed by masses of coral growing on sunken craters of old volcanos, since corals cannot live at a depth greater than 100 feet. But dredging showed the whole mass of the cones to be com- posed of coral, and Mr. Darwin began to collect data concerning them. He found that, while most of the atolls enclose open lagoons, some of them have islands of volcanic rock in the centre, and in other cases the island is a mass of rock closely fringed by coral, without water between. From these facts he deduced the now accepted theory of their origin, that they are coral-reefs which started about the sides of volcanos when they became extinct. As the sea floor slowly sunk carrying down the volcano, the corals grew upward until at last in many cases they alone remain to show the site of the old volcano, while in other cases its top remains as the island in the lagoon. This would indicate a slow and steady subsidence of the sea-floor, as a rapid one would quickly have carried the corals beyond their depth ; and we have a good illustra- tion of the normal subsidence of the sea-floor caused by the cooling of the earth’s interior, and resulting in a de- crease in the earth’s diameter of one to three feet in one thousand years. From old coral-reefs we can determine the set of ancient currents and from the latter, the conformation of land masses ; so that fossil coral-reefs give us valuable aid in plotting the early geography of the earth. OUTLINES OF NATURAL HISTORY 4 . 20 Cohesive, Capillary and Gravitative Attraction. Cohesion and Crystallization. Dana^j>. 62^, Cohesion is very variable in rocks, and each rock mass has two elements of cohesion, that of the whole mass, and that of the units constituting the mass. But in a given rock, the cohesion is constant, though sometimes increased or decreased by the effect of crystallizing action. Crystallization acts only till its work is accomplished, a comparatively short time. Some rock are very elastic and this quality usually in- creases with density. A species of rock known as Ita columite and resembling sandstone, is frequently asso- ciated with gold-bearing rock. It is so ffexible that the ends of a strip supported at the middle will drop, of their own weight, to a certain point, beyond which, however, they are rigid. All rocks contract and expand under cold and heat, the amount varying for different rocks. This has to be taken into account in the construction of fortresses and is of much importance in Geology, when we consider the great extremes of temperature through which many rocks have passed. Capillarity. Dana^f. 62g. This has an important effect. On a beach the waves grind up the sand and pebbles much less than they would, were it not for the thin layers of water between the grains, which are held there by capillary action. During long and severe droughts, on salt marshes which have been reclaimed and brought under cultivation, there may be seen efflorescences of salt, brought up from considerable depths by capillary action. Roofing slates are artificially separated by pouring water upon the edge of o block, when it is drawn into the cleavage planes, which were before invisible. mm DYNAMICAL GEOLOGY. 21 Gravitation. Dana^ p. 6jo. This force appears in some way in nearly all motion on the earth’s surface, though always in conjunction with other forces. The soil on sloping hillsides tends con- stantly to slip down. But the chief effect of gravity is to pull down rocks from cliffs or elevations. The “Diab- lerets” is an amphitheatre near the upper Rhone, on top of whose high vertical walls formerly stood five mountain peaks. The surface on which they rest is soft and slopes inward toward the amphitheatre. Two of these peaks have already leaped off and another will soon do so, having slipped down the slope to the edge. Several destructive falls of this sort have taken place in the Alps. Earth- quakes greatly assist in starting an action which gravi- tation can continue and complete. An important work of gravity is the breaking up of rock masses and so assisting in soil-formation. The Atmosphere. Dana^ p. 630. The atmosphere is interesting, because it is the mechan- ism through which celestial energy is applied to the earth’s surface. All life is absolutely dependent on the atmosphere, aquatic forms depending on the air contain- ed in the water. In keeping the water aerated, storms, which stir it up and cause breakers, are very effective. The atmosphere is to be considered (i) as a reservoir and (2) asin motion. It is a reservoir of oxygen, hydrogen and CO2 . Oxygen is taken from the air by all living things and by the oxidation of various substances ; a small amount is returned by the reduction of oxides, but how the great bulk gets back is not clear. CO^ is taken by all plants and is returned by decay and combustion. The atmosphere in motion is the wind. Its greatest authenticated velocity is about 100 miles per hour, though its force much exceeds this in volcanic explosions or rock- falls. The light weight of the air makes its rending power very small ; but on soft rocks, sand, driven by the OUTLINES OF NATURAL HISTORY 4. 22 wind, often has considerable cutting power. Loose sand not held in place by vegetation, as on sea-shores and de- serts, is often heaped into hillocks or "dunes.” These start like ripple-marks