The Beginning Of The Earth
( Originally Published Early 1900's )
WONDERFUL are the changes brought about by the many motions of the flying earth ! How complex they are ! Our planet rotates on its axis, which causes day and night. It revolves in its orbit, in a leaning position, which produces the seasons. The pull of the moon and sun on the leaning top causes the axis to rotate conically, and this motion it completes in about 26,000 years, the motion being styled the precession of the equinoxes. The pull of the moon makes it sway back-ward and forward in its orbit; and this pull has to do with the tides. The pull of the other planets alters the eccentricity of its revolution, so that the sun is sometimes much farther from the centre of its orbit than at present; and this variability, combined with the conical motion, causes rhythms of arctic and tropical climate, called glacial periods, which have produced extraordinary changes in the history of living creatures upon the earth.
Turning from these great changes to contemplate the earth's surface, we see a tumult of life, of struggle and conquest, of birth, maturity, and death, of organisms and their parasites, and the parasites of parasites. The microscope shows the seemingly inert soil of the field to be teeming and seething with the most varied life; it exhibits a drop of stagnant water as a world of wonders. The profusion of variety and the complexity of detail seem at first to have no limit. Among insects alone the different kinds or species are numbered by hundreds of thousands. Nor is there only variety due to place, but also variety due to time. Go back a tick of eternity's pendulum, and the life clothing the earth is altogether unlike its present vesture. There are no marks of man's hand upon the earth, for man does not exist. Another tick—the flowering-plants and the mammals are not on the scene; huge reptiles swarm amid the forests of clubmosses, ferns, and horsetails. Still another tick—the sunlight has not begun to use the green coloring matter called chlorophyll to split carbonic acid asunder, and thus there is no vegetation at all. Further back the earth is a great red-hot cinder ; further again, a molten ball surrounded by a dense, flaming, gaseous atmosphere.
Now, let us skip a few millions years and consider the earth as a member of one of the parent systems from which, by impact, our own solar system originated. Per-chance many planets belonged to that old-time system. Almost certainly they did not revolve in a plane, but moved at all angles and in all directions ; in an order beautiful enough of its kind, but not the order of our present system. Imagine an old-time astronomer watching from the then earth the erratic movements of another planet! He is startled by its being out of place; some unknown sister would, perhaps, account for this; but the variation is too sudden and too great. Is it a dead sun, or a small, burnt-out star cluster that draws near? He tells his fellow-observers, and presently every one is at work. By the movements of all the planets it becomes certain that another orb is coming. Will it collide with the earth—with some other planet—may it graze the sun? Presently some of the stars are eclipsed by the dark body. Now, with reflected solar light, it begins to glow, as a distant planet. Now, its orbit can be figured out. It will graze the sun ! It will be nearly a half graze. A new sun will be born—a fiery nebula produced that will envelop the earth. The old order is over, and, as a sum is sponged from a slate, so life is swept from the globe.
What was the nature of the two colliding bodies that gave birth to our solar system? Our imaginary astronomer can give us no information; we can only conjecture. For possibly no cosmic problem offers so fertile a field of inquiry as the impact of celestial bodies. If the paper in the Philosophical Magazine on Cosmic Evolution represents the truth, as a vast mass of evidence seems to show it does, then impact is the Promethean spark that gives life to decaying worlds and systems. The idea of an impact that is a mere graze is that a brilliant spark has been produced by the colliding part of the dead suns as they swept past each other. Such is probably the phenomenon that produces the new stars that occasionally burst forth suddenly with great splendor. But the new star is too hot to be stable; each molecule may have velocity enough to carry it entirely away into space to help in the formation of new and distant universes. The brilliant, flaming mass expands first into a hollow shell of gas called a planetary nebula, and then dissipates altogether. Nova Aurigæ was the first temporary star whose triple constitution was demonstrated. When instead of a mere graze the bodies plunge deeply into one another, then they join and whirl around one another ; and it is to such a whirling collision, it is suggested, that the solar system owes its genesis. Let us imagine the stupendous flash of the grazing collision to have passed; the planets have been swung by centrifugal force into a plane—as a twirled mop disperses its drops of water, or a Catherine wheel its sparks of fire. The planetary bodies fly in curves almost directly from the centre, but the pull of the central mass slowly stops them, just as the pull of the earth stops the upward motion of the ball thrown in triumph at a cricket match. Then they recurve towards the denser portions of the nebula. But countless agencies are at work to alter the curves of their orbits. Let us try to understand one of these.
Suppose a cup and ball with an elastic cord. You throw the ball, and the pull of the cord brings it back. Now you throw so hard that the cord breaks ; the ball does not come back, the attraction is gone. Suppose you throw a cricket-ball upward. Imagine the earth suddenly to disappear—the ball will not return; it will travel straight on in space. Now think of our earth : it was swung off ; it has curved over; it is returning. But suppose the nebula has expanded so much as to be largely outside the earth's orbit ; the part outside will not be pulling it back. If half were outside, the earth would net tend to return; it would revolve in a circle; hence the planets' highly elliptical orbits became approximate circles, and so, by this agency and by many others, the order of the solar system grew up. Our earth is an inner planet. In plunging into the fiery gaseous mass, it loses its light gas and picks up heavy molecules, and, so loaded, it cannot run away. It is a heavy gaseous body, revolving in a nebula. It picks up endless smaller bodies ; presently a larger mass plunges through it and gets entrapped—the earth has caught its moon.
As the solar nebula shrinks it leaves the earth outside, and the earth in its revolution in the surface of the nebula picks up its water and its atmosphere. In the earth's daytime the flaming sun covers its entire sky. Still the nebula shrinks, until Venus emerges. Now the sun is a fire covering the whole area within the orbit of Venus. Another won, and Mercury emerges.
But, while the sun is shrinking, wonderful changes are occurring in the earth itself. The gaseous mass has become liquid, and the liquid cools on the surface and sinks, while the hotter molten material rises up from below to take its place. A circulation is thus set up that tends to cool the liquid rock to its limits. Some of the rock gradually begins to solidify on the surface and to sink. For rock is the reverse of water; water solidifies, expands, and floats; rock matter solidifies, contracts, and sinks. The molecules in the lovely ice crystals are not packed tight like bricks in a box—the crystals are structures to some extent hollow. Pressure tends to fill the spaces, to crush the crystals; in other words, to make the ice into water. But rock, when it solidifies, contracts ; so, when the molecules are rigidly locked into the solid state, pressure tends to keep it solid.
As the rock sinks it is subject to pressure, and may remain solid; but it is also subject to intense heat, and may become more or less soft or plastic, or it may melt in such fervid temperature. The centre of the earth is probably composed of dense metals, like gold, platinum, lead, and mercury. Their density would limit the sinking tendency, so that the crystals of rock would float on the surface of the molten metal and gradually silt up the lava ocean, in places reaching to the surface. The space between the crystals would still be filled with molten matter, and—when the silting reached the surface—this would also begin to solidify. This silting up would be very uneven, and molten lakes would be left which would afterwards cool, solidify, and shrink, producing vast hollows—perchance our present ocean beds. Eventually the crusts would join and coat the earth with a continuous white-hot shell.
In the far back epoch we are thinking of, the carbon of the planet is probably not yet in a solid state. It is possibly all combined with oxygen as carbonic acid gas. The base of the limestone rock is still caustic, not carbonate, the date of the coal measures is still in the distant future. Some of the earth's salts and most of its chlorides are in a state of vapor, gradually condensing on the poles and other cooler parts; falling here and there as molten saline rain, and flowing as glowing lava streams into molten lakes to be boiled off again. Possibly showers of meteorites contribute towards inequalities of temperature. By-and-by, the salt is solidified, and water begins to fall as rain on the poles and other cooler regions, forming boiling lakes; some parts are still too hot for this, and the raindrops fall, to dance up again as quivering spheres buoyed up on their own steam. To boil water requires heat; thus the boiling arctic and ant-arctic seas cool the poles, and thus the rocks shrink and become denser, tending to sink under the increasing weight. As the water stands where the molten saline lakes solidified, it dissolves the saline matter, and the sea becomes salt.
THE MAKING OF THE EARTH'S CRUST
Though it is by no means easy, it is worth trying to gain a living picture of the way in which the surface of the earth came to be what it is. First, the crust cools, shrinks, gets too tight, and splits; then the cool crust becomes too big for the contracting interior, so that it crumples up and breaks. All the while steam explodes, and torrents of boiling water, bearing débris, rush in tumult over the surface. Then, owing to the great world-changes already spoken of, ice accumulates alternately upon either pole and pushes forward upon the polar hemisphere; consequently the centre of gravity of the earth is altered, and the water is dragged to the icy hemisphere, while the opposite hemisphere is left almost dry land, with an equably temperate climate. After a long time, vegetation begins to clothe the surface, modifying all the other agencies. Evidently, while considering such a conflict of forces, we need patience as we try to thread our way through the labyrinth, with its many tortuous twists.
Although, in comparatively early epochs, the poles of the earth would, doubtless, be slightly cooler than the other parts, we must remember how water and carbonic acid oppose the penetrating power of the sun's radiant heat, so that the equator would not be much hotter than the poles. Think of all the water of seas and lakes, and all that is now contained in plants and animals, and in crystals—think of all this water existing as steam in the atmosphere along with the enormous quantities of carbonic acid not yet absorbed or decomposed! An atmosphere surpassing our own many hundred times ! The sunlight would scarcely penetrate such dense clouds as the upper regions of that atmosphere would present; and, even if it did penetrate, it would be "refracted" or curved round the poles, so that polar cold would not be an important factor in producing condensation. Pressure alters the temperature of boiling water; in a perfect vacuum ice-cold water will boil ; so that imagine the high temperature of the steam and rain under the pressure of an atmosphere many hundred times greater than ours now is—equalling tons to the square inch ! All these agencies would tend to produce a general equality of temperature, yet minor irregularities would appear, and in the cooler places the raindrops would spread over the surface, and the water would flow as boiling streams, combining into torrents—possessing the great dissolving power of boiling water at high pressure—bearing rocks, silt, and débris of all kinds. Nor must we forget that to boil water requires much heat (we know how it cools red-hot iron) ; hence, where the boiling water stood its cooling influence would contract the crust and tend to flatten its curve, that is, to bulge in the earth's surface, thereby forming deep depressions, wherein still more water with sedimentary and dissolved material would gather. When a mass is above what is called the "spheroidal point" it takes a long time for water to cool it—the steam keeps the water out of contact. In my boyhood, I saw a huge mass of white-hot iron being carried up the railway on a trolley crane. I followed and saw the men stop at a bridge and lower the mass into the river. There, beneath the water, hung the glowing iron. The surface of the river became a maelstrom; it boiled and bubbled, and at times the water seemed to burst away from the iron. The noise was most remarkable, yet, amidst it all, the block continued to glow. It became tedious to watch its light. The men moved the trolley backward and forward, yet still the mass glowed and grumbled. Then, after an hour or so, it began to blacken and hiss; and so it gradually cooled till it was fit to be handled.
Besides local changes, the whole crust of the earth was cooling and contracting, causing extreme strain and exerting a tremendous. pressure upon the interior, so that immense splits would occur. Clearly these splits did not take place in the thick crust underlying the water areas ; they occurred in the thinner crust of the dry areas, and vast ridges of molten rock poured out and out for the contents of these splits would remain softer than the rest of the crust, and would be, as it were, safety valves allowing escape of molten rock matter. This would go on until the superincumbent mass was so immense that its tremendous weight depressed the white-hot dry crust, which would sink below the level of the lakes, bulging their floors up and causing the water to overflow into the hot areas immediately alongside the great volcanic ridges. Again came the boiling and cooling, again the density of the crust increased; and, as time went on, this alternate action would extend over larger and larger areas. The lakes would become seas, and the seas oceans. The smaller areas of dry land would enlarge to continents, and the continents would sink until the continental areas were oceans and the oceans dry land. Imagine the effect upon the atmosphere when the waters of the ocean rushed in about a white-hot volcanic ridge, thousands of miles long; pouring into the innumerable fissures, becoming high pressure steam, exploding and throwing the rocks scores of miles high ! From the whole ridge would rise an uprush of steam and air. This would be affected by the earth's rotation just as our trade winds are now affected by it, only in a tremendously exaggerated way. Awful tornadoes would occur, and the rainfall would be altogether beyond our conception.
I have seen a storm in the Otira Gorge in New Zealand; but what was that to the possibilities in the early volcanic period of the earth's history ! The rainfall of such storms would be estimable, not in inches, but in hundreds of feet. Yet a few inches produce effects almost incredible. In the Otira storm the rain seemed to fall in sheets. Immense cascades began to gush from the mountain sides where no sign of even a rivulet had appeared before. From higher and still higher points the cascades started forth, shooting out of the dense forest thousands of feet aloft, leaping over the tree-clothed mountain side clear to the gorge below. Great trees trembled, swayed, and fell with a mighty crash upon their comrades. The volume of water in the Otira swelled prodigiously. Huge boulders—big as wagons—in the course of the torrent, became undermined, they trembled and toppled over, releasing other smaller boulders—smaller, yet tons in weight; and these were carried on, to jam, and enclose small lakes; presently to be in motion again, suddenly liberating the water, which rushed forward with the roar of a bursting reservoir. In two days we were able to resume our journey. But how changed the road! The macadam was washed away to the bare rock; in places the very direction of the road had to be altered.
What must have been the gouging and grinding power of these primitive torrents! Think of the rainfall resulting from the steam and vapors in the atmosphere, equivalent to a mile of water covering the entire earth ! What enormous masses of sediment such erosions would send to the bottom of the oceans are shown by the immense deltas of our great rivers. With moisture in the atmosphere so enormously greater than now, the time came when snow could fall on the poles, and tremendous glaciers—smoothing and sculpturing the earth—would be formed. Then, again, owing to the conical motion of the earth's axis, and to the eccentricities in its orbit, alternate torrid and frigid climates would follow each other over and over again from hemisphere to hemisphere. As the earth cooled, more and more water would be deposited, and the alkaline and other metallic oxides, especially the soluble ones, such as lime, would combine with carbonic acid, and be deposited as carbonates. The torrents of hot water, bearing heavy rock and débris of all kinds, would be powerfully erosive, and the eroded matter, as already suggested, would tend gradually to cover the ocean's bed with sedimentary rocks. These, in very thick deposits, would be subject to extreme pressure and to heat from the interior, and would be converted into what are called metamorphic or altered rocks, like quartzite—which seems to have been originally a sandstone.
As the temperature of the surface of the earth sinks, a new action comes into play. The cooling and contraction of the crust becomes slower and slower, until the internal part is cooling by conduction almost at the same rate as the external part by radiation. The atmosphere has greatly decreased, most of the water is deposited, much of the carbonic acid has become fixed; the sun's rays are, therefore, becoming more able to penetrate to the surface of the globe. The sun has diminished exceedingly in size, while its temperature has proportionately increased; for, remarkable as it may appear, the more heat a gaseous world gives out, the hotter it grows. As it shrinks, the pressure resulting from the increased gravitation reduces the internal layers to smaller and smaller bulk, thus causing a tremendous increase in the quantity of heat. A gaseous sun, in becoming compressed to one-half its volume, gives off enormous quantities of heat, yet it is double the temperature when it has shrunk to one-half the diameter.
Heat whose source is at a very high temperature can penetrate gases and vapors much more easily than heat from a comparatively moderate source. Hence from these two causes much more heat reaches the crust of the earth, and retards the lowering of its tropical surface temperature. A time ensues comparatively free from volcanic and earthquake disturbances, and at this stage—during a glacial period—it is probable that the earth be-came cool enough in places to permit plant life to commence at one of the poles. How this may have occurred we will discuss further on. At present we must be con-tent to take a rapid survey of the physics of the earth's crust.
We have traced the molten earth in its process of cooling. We have taken an imaginary glance at the solidifying of the surface, and have seen how, by the hardened rocks sinking down, this solidification would extend to great depths. We have noted that, when solid, the surface would tend to cool more quickly than the interior; and how, shrinking and exerting enormous tension and pressure, it would split, and the interior would be forced out in a molten state; the heat producing this state being largely the energy of the pressure itself. We have traced the oscillations of levels which water and the deposit of sediment would produce, and then the gradual reduction of the temperature of the surface, until solar heat would tend to retard surface cooling, and external and internal contractions would proceed at almost equal rates. A period of comparative quiet would ensue. Then, after a time, the gain of heat from the sun would balance the surface loss, and the crust would cease to cool and contract. Now a new order of events begins to operate. The hot interior loses heat by conduction through the crust, and continues to shrink, while the crust gets to be too big for the contracted contents of the globe ; and, just as the loss of water from a shrivelling apple causes the surface to wrinkle, so the shrinking interior of the earth must cause its surface to crush and wrinkle, and a second period of convulsions ensues. During this time great earthquakes would cause the whole earth to shiver, as the rigid crust crumpled up in its efforts to fit the contracted interior. More and more slowly would the heat pass from the interior until the comparative quiet of the present period was reached.
Thus, then, there are two great agencies producing volcanic action. First, the cooling of the solid crust being more rapid than the internal cooling, the surface shrinks and splits; then, after a pause in the paroxysms, the crust ceases to cool and contract, whilst the continued cooling and contracting of the interior cause it to shrink away from its crust, and the crust begins to crush and crumple to fit its contracted interior.
Probably it was during the interglacial periods occur-ring in the pause between the earlier volcanoes of surface tension and the later volcanoes of surface crumpling that the enormous forests of the carboniferous period clothed wide regions of the earth and formed our chief coal formations; storing up the solar energy of those far-gone ages to supply man in this present period of conflict and unrest.
Thus the earth has passed through its period of tight crust and splitting to a period of quiescence, and now we must consider in detail how the tight crust has to wrinkle and accommodate itself to its shrinking interior. Apparently inextricable sets of agencies are put in operation by the crumpling of the earth's surface. Let us try to disentangle some of them. Owing to the inequality in thickness and strength of the crust, the chief crumpling would take place in the weakest parts; often, probably, the margins of continents. Sometimes the earth's surface would buckle up so much as to form mountain ranges; then, along the tops of these new ridges immense splits would open that would become wider and deeper as the crumpling continued; rain would fall upon the rents and torrents rush down them; so the fissures would become valleys. Along the far-dissevered sides of some of these mountain valleys the rock strata, once continuous, may be traced at the present day for miles. Sometimes the efforts of the crust to fit itself to its shrinking con-tents would produce such tremendous pressure as to heat and fuse the rock; the molten substance would find vent and volcanoes be produced; the hot erupted matter would melt the sides of the outlets and form, in time, circular craters overflowing and forming the slopes of great volcanic cones. As each outrush was exhausted the matter in the vent would solidify and the crater become a reservoir to catch and hold water, which would boil off and so rob the glowing rock of its heat. By-and-by, the pressure would renew itself, again bursting through all obstacles in tremendous eruption; but before the upward pressure exploded through the bottom of the crater, the whole of the original walls of the volcano would be split by fissures radiating from the vent. Into these fissures the fresh molten matter would flow, forming volcanic "dykes."
Many of our mountain chains have been produced by the crumpling and bulging of strata; but, in other cases, the stratified rock material has probably been thrust by lateral pressure up the mountain ridges, produced ages before by the great splits of the primitive volcanic period of tension.
But of the complications of earth-sculpturing agencies there appears to be no end. In addition to the two volcanic periods of tension and pressure and the erosive action of boiling torrents, there came into force—as soon as snow could fall upon the earth—another tremendous factor in modifying surface conditions, namely, glaciadon, or the results of ice. Snow would settle on the poles and accumulate there; it would cap all the higher mountains and gradually spread downwards, advancing from the poles toward the polar circles. An astronomic influence of surpassing potency must here be considered. The orbit of the earth is an ellipse—an ellipse may be seen in a hoop leaning a little away. Generally the earth's orbit is nearly a circle, but sometimes, owing to attractions of the other planets, it becomes a long ellipse, the sun being at one of the foci, that is, near one of the smaller curves of the orbit. In extreme cases the earth may be 13,000,-000 miles nearer the sun at one part of its annual revolution than at another; at present the difference is only 2,000,000 miles. Now, summer is not due to proximity to the sun, for we may be nearest at midwinter. The seasons are caused by the leaning of the axis of the earth. In the summer hemisphere the axis leans toward the sun, and the directness of the rays causes the high temperature. Winter is caused by the axis leaning away from the sun. It is clear that if during, say, the south polar winter, the earth happens to be 13,000,000 miles nearer the sun than it was six months previously, the southern winter will be very mild; but six months later, when the earth has receded 13,000,000 miles from the sun, the win-ter of the northern hemisphere will be very cold indeed. In that cold winter most of the water will fall as snow instead of rain, and the snow will pile up on the pole, while the summer sun—instead of warming the surface of the soil—will be engaged in melting the snow. The heat may not suffice to do this completely, and next winter will increase the mass of snow. If this accumulative process lasts—as it may—for 13,000 years, the polar snow may creep down into temperate regions, and such a vast cap of ice be produced as to alter the centre of gravity of the earth, so that one hemisphere may have nearly all the water as ice and sea, and the other may have an almost entirely land surface. One polar hemisphere, the oceanic and icy one, will be nearly all frigid, while the other, the continental, will be very temperate, the seasons being almost equable, the summers cool and the winters without frosts quite to polar regions. Thus, organic life, both animal and vegetable, will increase prodigiously.
A glacial epoch lasts, roughly, a hundred thousand years, and two such epochs may follow close on one another. But every 13,000 years the tilt of the earth reverses itself, and opposite poles hive mild winters; the ice-cap melting from one pole and forming on the other. There may be several glacial periods in one epoch; for instance, Europe may be for 13,000 years sub-tropical, then, for a like period, arctic; and so on till the time of the epoch is exhausted. In some of these great glacial periods the ice sheet covered Europe. Vast glaciers stretched from the high land of England far out into the Atlantic; in many parts, as the ice was thrust forward hundreds of miles, being lighter than water, it would tend to float; the upward pressure would break it, and huge icebergs would become detached, which, floating farther south, would carry the arctic climate still nearer the tropics.
In our time the southern hemisphere is colder than the northern, but the earth is almost at its least inequality of distance from the sun, and the difference of polar temperature compared to that of a glacial epoch is slight. The winds are now chiefly the result of the uprush of air caused by tropical heat. The area of this uprush travels with the summer alternately into the northern and southern hemispheres ; and the vertical sun in crossing the equator alters its latitude quicker than at any other time; so, by this rapid change of latitude of uprush, we get, in spring and autumn, the equinoctial gales. At mid-summer, in either hemisphere, the vertical sun seems to remain almost stationary; we call this time the solstice, that is, the sun standing still. Then for some weeks the medial line of aërial uprush changes but little, and we have a period comparatively free from violent storms. What will happen to this medial uprush in a glacial epoch l Clearly, it will travel away from the icy regions. Now, ocean currents follow winds, hence the equatorial water will be driven into the warm hemisphere, melting the polar ice, and rendering the earth habitable well within the polar circle; sometimes, possibly, in extreme cases, quite up to the poles. In this way the Gulf Stream of our own period warms northwestern Europe and fits it for human habitation.
The warm equatorial water, being drawn from the chilled hemisphere, causes it to be still more icy; thus many cumulative forces are at work that greatly accentuate the oscillations of climate during a glacial epoch. Many thousands of years of equable temperature clothe the land with vegetation : then, gradually, during long ages, the climate changes, and cold intensifies until ice piles high where plants had luxuriated; and so, as the tilt of the earth's axis reverses itself, vegetation alternates with ice over and over again. In all directions the strata of the earth prove to us the wonderful vicissitudes of climate produced by these epochs; as, for instance, when we find fossil palms and tree-ferns over and under-lying ice-scratched boulders and other remains of glaciation. We may imagine the effect of the encroaching arctic climate on the animal kingdom. How the polar mammals, like the bears, would rejoice in their extending area ! How the tropical reptiles would wriggle towards the equator ! How the gorgeously-plumaged birds and gaily-colored insects would take flight in the same direction ! Then, again, we may picture—with tropical climate slowly creeping back—the pines and arctic plants receding up the mountains, and the woolly rhinoceros and the mammoth retreating toward their polar domain once more ! Think of the melting ice ; the advance of tropical animals and plants to previously temperate regions, of temperate denizens to the arctic world; the entire hemisphere richly verdant—with small oceans, and these in the deeper channels only !
I have already suggested that it was in the pause between the volcanic period of surface tension and that of surface crumpling that the coal measures were deposited. There are many reasons for the belief. The earth was warmer than at present, and the air held more water and more carbonic acid. Geologists, in describing the carbon-iferous age, tell us of monotonous plains, thousands of miles in extent, relieved by scarcely a hill; a continuous swamp with a dank vegetation, and uncouth creatures crawling amidst it. Such conditions exactly correspond with the period of rest. The crumpling of the earth had but just begun. Great part of the early inequalities had been eroded away, leaving immense plains bounded by huge mountain ridges—the volcanic lines of relief-the splits of tension through which rock matter had finally overflowed in the closing ages of the cooling crust. The agency which produced tension had ceased, and the interior was shrinking as fast as the exterior. Nothing disturbed the physical peace of the earth save glaciation. But this is a tremendous agency. The distinguished cosmic geologist, Croll, suggests that coal is the result of the vegetation of interglacial periods. One cannot but wonder that, if this were so, the beds of coal are not much thicker than we find them ; but we must remember that bituminous coal may very largely consist of the spores of the various flowerless plants which chiefly composed the coal forests; that the foliage and trunks of these giant club-mosses and the like decayed, leaving fossil remains only here and there ; while the less perishable, encased, resinous spores, gradually built up our coal deposits. On this view Croll's explanation seems perfect. We have only to imagine limitless plains of vegetation in the polar hemisphere luxuriating for thousands of years without a frost. Then the cold creeps down; ice caps the pole; the oceans deepen, encroaching everywhere upon the land; while clay and other sedimentary material cover the resinous vegetable matter and protect it. Then, again, the axis of the earth reverses its tilt, and for thou-sands of years the ice melts and disappears, and temper ate climate comes once more.
The carboniferous period was rich in limestone deposit. The caustic lime had been slowly turned into carbonate, and this had been dissolved by water and carbonic acid; then, as the warm water flowed toward the pole which was enjoying equable seasons, it would carry with it the young free-swimming or floating stages of corals, stone lilies, and other marine animals which produce limestone rock. As the polar ice melted, the rock thus formed would be elevated into dry land, and would be covered with débris such as wind-swept "loess," becoming the bed upon which new coal forests would grow. Thus was a store of energy laid by, almost entirely in one epoch, for the use of a race that may last 10,000,000 years upon the earth! Ought not this fact to suggest that we should husband our resources for posterity? The cynic may remark that posterity has done nothing for us; but when we realize our indebtedness to the past we must feel our obligations to the future.
Let us now consider the carving and grinding effect of moving ice. As the ice-cap gets thicker and thicker it extends above the lower mountain peaks, and as it slides it grinds them off, forming what the geologists call "hogs' backs." The swollen glaciers carve deep grooves in the sides and bottoms of the valleys, while the ploughing boulders which they bear along are also scratched and ground into glacial silt. From the mountains alongside the sliding glaciers fall immense rocks, which are taken by the glaciers out to sea; and when the glaciers become icebergs and melt, these huge stones are deposited in all sorts of strange declivities in the ocean bed to form gigantic erratic blocks when the sea-bottom becomes elevated into dry land. When, on the other hand, the glacier melts on land the blocks fall and form great ramparts known as terminal moraines. Still the glacier carves the valley deeper and deeper, carrying forward its débris to build still higher and thicker the terminal wall, so that when the glacial period is past and the ice melts, deep lakes are left, sometimes on the plains at the base of the mountain ranges, sometimes far up the mountains themselves.
Let us examine and summarize these ice agencies. The eccentric sun produces epochs of about 100,000 years in duration, sometimes closely recurrent. The dates of these epochs are calculable, and furnish us with a geological clock. The last epoch finished some 80,000 years ago; and about 200,000 years ago a still mightier epoch was concluded, having lasted through two periods of nearly 100,000 years each. We must look back nearly 2,000,000 years before we discern another tremendous epoch, and there the power of the mathematician fails us. Looking forward, we learn that another epoch will arrive in about 800,000 years.
These periods of great eccentricity produce in one polar hemisphere long cold winters and short hot summers, and in the other hemisphere long mild summers and short mild winters. In the glacial hemisphere the summer sun is unable to melt all the snow that fell during the previous long and cold winter, and the snow accumulates. The winter is long as well as cold, because, when the earth is distant from the sun, it has so much further to travel in its orbit and it travels slower. Each year there is more and more snow left unmelted, and fogs are produced that make it difficult for the summer sun to act efficiently; then, as the ice piles up, the tropical uprush of hot air travels away from it, causing winds that carry the warm equatorial water into the warm hemisphere. The cap of ice alters the centre of gravity of the earth, and the mild hemisphere is mainly land, while the frigid one is mainly water. For 13,000 years these agencies act; then, slowly, all is reversed—what belonged to the southern hemisphere now belongs to the northern; another 13,000 years and another reversal happens. The effect of these wonderful changes of environment upon evolution may be pictured by any one who tries to under-stand the subject, and the same study explains many puzzling problems in the geographical distribution of plants and animals.
But we must turn from these fascinating speculations to. the more prosaic problems of the earth's crust. Let us try to answer several questions. Why is nearly all the land in the northern hemisphere? Why do the great peninsulas point to the south? Why are the lofty mountain chains of the temperate regions carved into deep fiords whose precipitous fronts reach deep into the sea? Why are New Zealand northwest winds hot and dry? These problems and some others must be attacked before we are in a position to understand the origin of the lovely verdant vesture that clothes a great part of the earth's surface.
LAND AND SEA
A merely superficial glance at the distribution of land on the globe, shows that it mainly lies north of the equator. One searches in vain for any explanation, save that the great Antarctic ice-cap has altered the centre of gravity of the earth, and drawn most of the water away from the northern hemisphere. The ice-cap has an area probably over 3,000,000 square miles; its thickness it is impossible to estimate. Judged by the size of antarctic icebergs, it may be miles thick at its margin where the bergs break off; and possibly many times thicker at the pole. This mass of ice must attract the water and deepen the ocean. Were the ice to melt, the water would become much shallower, because it would be drawn to the northern hemisphere, and a great part of the southern hemisphere would be laid bare as dry land. Judging by the soundings of the Antarctic Ocean, we believe that an enormous continent would result. At the same time, were the ice piled high at the North Pole, it would, by its gravitating power, deepen the Arctic Ocean. The land in the north would shrink, Scandinavia would become an island, and large peninsulas would stretch along the Rocky Mountains toward Alaska, and along Eastern Siberia—two peninsulas corresponding with the southern peninsulas terminated by Cape Horn and the Cape of Good Hope. Then, from the shallow southern seas would emerge a huge continent; the same continent which, possibly, some ten thousand years ago was peopled by the race who have left on Easter Island—an islet in mid-Pacific—the cyclopean masonry and gigantic statues that amaze every chance visitant.
If, as suggested, the Antarctic ice-cap be the explanation of the singularly unequal distribution of land in the two polar hemispheres, the great glacial epochs must have produced a still more striking discrepancy—the temperate hemisphere would be an almost unbroken continent, with a few deep seas, but no oceans.
Oscillations of land and sea are not, however, entirely due to the piling of ice at the alternate poles; differences of level are also caused by the crumpling of the crust which results from the efforts of the earth's exterior to fit itself to its shrinking interior, notably, for instance, in such alterations as the simultaneous rising and sinking of opposite coasts of large islands. Again, the crumpling of the crust is not merely producing oscillations of land, but is the primary cause of our present volcanoes and earthquakes, the energy of the crumpling being so tremendous as to fuse the rocks into lava, while the action is still further complicated by the explosive and decomposing effects of high pressure steam produced by the inrush of water to heated fissures and vents.
The lateral pressure produced by the crumpling crust acting on strata of different hardness, plasticity, and thickness, results in contortions and "faults" that are almost incredible. Yet recent experiments on confined layers of materials of different hardness and toughness seem to imitate exactly rocks curved into forms as crooked as the letter S. Sometimes strata fold back on themselves ; sometimes the rocks split into vast fissures, whose two surfaces slide thousands of feet over one an-other, until the continuity of the original strata seems lost altogether; this sliding action grinding the inequalities of the moving faces into powder fine enough to give the rocks the polish of cut gems.
The probable explanation of the wonderful fiords of Norway, western Scotland, western Patagonia, and south-west New Zealand is that these coasts all lie in the path of the anti-trades of which our westerly winds are typical. These winds, becoming saturated in their journey over the ocean, are cooled in ascending mountains, and discharge their moisture, partly as rain, but largely as snow. This frozen water packs itself upon the mountain-tops ; then, being forced downward by the weight of the constantly forming ice, it forms glaciers, which, in a cold period, descend to the sea, cutting the western sides of the mountains into more and more precipitous fiords.
When these winds blow over a very high range of mountains a peculiar phenomenon ensues; the air becomes hot and dry, as exemplified in the New Zealand "nor'-westers." Let us try to understand this. In ascending a mountain, air expands and does work in lifting the atmosphere above it. To do work requires heat, and so the air cools in expanding. As it cools, the vapor it contains becomes liquid, and in condensing gives out its so-called latent heat so, when it reaches the mountain top, the air is not cooled so much as it would have been had it contained no vapor. Then it descends the other side, and the air above it compresses and heats it, and, as it does not take up water again, it becomes hot and dry. If it had been dry when it ascended the mountain, and had produced no rain when it cooled, it would have been equally cooled in ascending and heated in descending; but, because it contained vapor it cooled only slightly in rising and was heated much in coming down again. Hence the New Zealand nor'-west winds, that have had to rise over the range of the Southern Alps, are hotter and drier than they were when they reached the western base of the mountains after travelling across the ocean, and similar winds exist in other parts of the earth.
Besides the physical agencies described, we have the effects of solution and chemical change to discuss. These produce a very interesting set of phenomena ; the sculpturing of limestone caves, the growth of stalagmites and stalactites, petrifactions, the disintegration of granite rocks with the formation of clay and fertile soils, the building of silicious terraces such as the beautiful pink and white terraces of New Zealand, the growth of geyser tubes and the marvel of their spouting, and so on. If we grind some chalk to a fine powder, suspend a little of it in water, and pass a stream of carbonic acid gas through it, the chalk dissolves, and the water is what we call hard. If we wash our hands in this water, it curdles the soap : if we boil it, it leaves a deposit of lime in the vessel. The air contains carbonic acid produced by fires and by the combustion which goes on more slowly but not less really in the bodies of living creatures. When rain falls, it absorbs the carbonic acid, and this water, running through the fissures of a limestone district, dissolves the rocks away, and in the course of ages, carves out the extensive limestone caves that are often so very beautiful. Much of their beauty is due to the lime borne by the water having been deposited in translucent masses—called stalactites-that hang from the roof; while corresponding pinnacles—stalagmites—grow by the dropping water leaving its lime on the floor; stalactites and stalagmites frequently lengthening until they meet and thicken into lovely columns. When the water oozes up from the floor of the cave it erects a ring of lime that grows higher into tubes, or widens into basins that form exquisite fonts. When it issues from the cracks in the ceiling it produces curtains of petrified drapery, partitioning one chamber from another. When it flows over plants and through the pores of wood it coats the cells with stone, and makes permanent record of the vegetation of the past. When it passes out to sea, various molluscs, crustacea, stone lilies, corals, zoophytes, and micro-organisms take it into their bodies and use it, making their exquisite shells or skeletons. These, on the decay of the living architects, go to form fresh beds of chalk and limestone, that, under conditions of heat, pressure, etc., may crystallize into marble and other beautiful forms of carbonate of lime.
But water-bearing carbonic acid not merely dissolves some rocks ; it decomposes others, taking the place of flint or silica. Thus felspar, a constituent of granite, is decomposed into materials such as clay and carbonate of potash, that tend to give fertility to soils in granitic districts. I have seen granitic rock so decomposed in this way that a walking stick could be driven some inches into it. The silica that is thus expelled by carbonic acid may remain in solution in hot water, to be deposited on the margin of pools, building the pools larger, and growing into such formations as those of the silicious terraces of the North Island of New Zealand destroyed by the eruption of Tarawera. If such hot water ooze out of a hole in the ground it will deposit the silica in a circle, building up higher and higher until the circle becomes a tube with sloping walls of silica. Then, if the bottom heat increases, we have a geyser which spouts when its water comes to the boiling point. The spouting of a geyser is a very interesting phenomenon. After each discharge the water gradually fills the tube, increasing in temperature the lower its position. Owing to the pressure, the water at the bottom gets to be much above the ordinary boiling point. At last it boils and lifts the water above it, then, the pressure being relieved, the surplus heat produces a volume of steam vast enough to blow the entire contents of the tube scores of feet into the air. Geysers are of many different varieties, but the physical principles are practically the same for all.
These are some of the mechanical, physical, and chemical agencies that have aided in moulding and sculpturing the surface of the earth.