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imperishable is slowly wasting away, every hill and mountain, every valley and every plain losing some little particle from time to time. If any mass of land lose but one single grain per annum, and that loss be continued for as many years as its bulk contains grains, it must ultimately be altogether destroyed, and its materials strewed over the bed of the sea.

This process is no vain imagination of the brain, nor unnecessary supposition of that which only might take place; it is a literal and certain truth: it is going on around us every day of our lives, and has been going on for a period of time vast beyond all human conception, unceasingly, uninterruptedly, day and night, summer and winter, over the whole surface of the earth.

Our dry lands are slipping and sliding from under our feet more slowly, but as surely, as the glacier slides beneath the feet of those who traverse it, and think it, perhaps, to be not only solid but immovable ice. The life of a man is as transient in the one case as the passage of the traveller in the other; even the duration of nations of men and the existence of the whole human race covers but a small part of the time that has elapsed during some of even the most recent of the great changes that have taken place on the surface of the globe. The geologist is of all men the one who is likely to be most fully impressed with the graphic truth of the comparison which Homer puts into the mouth of Glaucus in his singular colloquy with Diomede, between the succession of the generations of men and the fall and renewal of the leaves of a forest:—

"Like as the generation of leaves, so also of men; For the wind streweth the leaves on the ground, but the forest, Putting forth buds, groweth as the season of spring Cometh on; Thus the generation of men, the one groweth, the other cometh to an end."

To the mind's eye of a geologist, however, not only are the leaves fleeting, but tho trees also and even the forest itself; for be looks back to the time when the very ground on which it stands did not exist, and forward to that when it shall be no more.

Before any firm step can be taken in the study of geology, this enlarged view of time must be clearly gained and fully comprehended as a reality. Without it we cannot understand even the plainest and simplest facts, not so much as the formation of the commonest stone about us, or of the sand and pebbles on which we daily walk. With it we can comprehend how the very slow and gradual action of such operations as have now been described can produce vast changes in the composition and in the structure and position of the component parts of the earth's crust.

Let us briefly sum up these operations. There are good grounds for believing that the earth has a great internal temperature. It appears that molten rock exists beneath the solid external crust, and that small portions of it are now and again ejected on to the surface in the form of lavas and ashes. These ejections, however great their mountainous piles may appear to us, are obviously but the little spittings and boilings over of the great furnace below.

We also believe that while large portions of the crust of the earth remain tranquil and motionless for long spaces of time, other parts are occasionally shaken by vibrations, and are permanently raised or lowered, either with earthquake shocks, or slowly and gradually, so that the movement is not apparent to our senses. These motions affect the whole thickness of the earth's crust to some great but entirely unknown depth, and are obviously those by which the ground that was the bottom of the sea has been raised into dry land, or the dry land depressed so as to become the bottom of the sea; and by which the originally horizontal beds composing the earth's crust have been tilted, and bent, and broken, in the fashion in which we find them in our hills and mountains, and even beneath so many of our plains.

On those parts of the earth's crust which become depressed beneath a certain level, and therefore overflowed by the sea, deposition of materials is always occurring, now in one part and now in another, those materials ultimately forming solid rock. These materials are the result either of the mere mechanical transport of fragmentary solid matter, or of the re-solidification of matter that had been dissolved, the latter taking place chiefly through the instrumentality of the vital action of animals or plants.

Tho ocean, then, is the great producer of stratified rocks, and its bed the womb in which those rocks are generated, while the air and its wind, and the surface of the sea which that wind agitates, is the destructive power under the influence of which all previously formed rock is liable to be ground down.

Here comes in a curious conclusion; supposing that at some long past period of the earth's history all those internal powers, of which earthquakes and volcanoes are external symptoms, had become quiescent and altogether ceased to act, many, if not all, of the dry lands of that period would before this have disappeared, and their materials been strewed beneath the sea. There might then have been no dry land left upon the globe if fresh areas of it had not been elevated. These great internal powers of disturbance then, which sometimes at the moment of their paroxysms produce such temporary ruin and destruction to man and his works, are in reality beneficial and conservative agents, and actually necessary to the life of the globe as a habitable world, in keeping up a proper balance of dry land upon its surface. Not only are the rocks, which have been elaborated beneath the sea-level, from time to time raised above it, now in one part, and now in another, but their deeper parts are indurated and solidified, and often half crystallised by the heat proceeding from the interior, and they are moreover strengthened and supported by means of great masses or large bands of igneous rock thrust up and injected among them. Additions are even made to their surface also by the igneous materials ejected through them, either in a molten state as lava, or as fragmentary ashes.

There is, then, a constant action and reaction in the various physical agencies which are always at work on different parts of the earth's crust. Apparent destruction is constantly taking place in some parts, only to prepare the materials for re-construction in others. Powers which seem to cause nothing but ruin are in fact employed in repairing the ruin caused by agents that seem harmless. The precipices and peaks of the mountain crests, the ravines and abysses that yawn in their flanks, are not, as is often supposed, the result of the convulsive action of internal force, but of the slow and silent influence of the weather through uncounted time. The materials gained from this erosion and partial destruction of the mountains now form the most fertile plains, or will form them when the internal forces lift them from beneath the sea.

The heat and force which are occupied below in hardening and kneading together the materials of future mountain chains, and which ultimately raise them to within the region of atmospheric destruction, are in fact preparing them to resist that destruction, as long as possible, so that they may be able to stand for a time as mountains, in order to condense the atmospheric moisture, and send it down upon the plains in refreshing showers and fertilising streams. The internal movements, which have tilted, or bent, or broken the beds below the surface, have in that way brought them within the reach of man, and facilitated his extracting those which may be of use to him. These fractures also compel much of the atmospheric water which sinks at one part beneath the surface to come back to it as springs in another. They form cavities, also, which become the receptacles of useful minerals that require the more recondite chemistry of nature's laboratory to be put in action for their production. We may reasonably conclude, then, that the advantage of mankind was one of the ends contemplated by the great Author of Nature in the creation of all this wonderful and elaborate machinery, though geology combines with astronomy in rebuking the impertinent presumption which would make Man the be-all or the end-all of the creation of the universe, since all the operations and processes which are here alluded to have been in operation for a time compared to which that of the existence of man is but as a raindrop to the ocean.

CHAPTER X.

ON FOSSILS AND THEIR MODE OF OCCURRENCE.

It has been stated that the crust of the earth is principally composed of stratified rocks, which were formed under water. These were of course chiefly formed in the sea, though some smaller parts were deposited in the beds of lakes or at the mouths of rivers. In whatever water they were formed, it is to be expected that these deposits would from time to time enclose the bodies of some of the animals and plants that lived and died in the water, and occasionally even some of those which were swept into it from the land. Marine limestone is derived from the fragments of animals, and coal is formed by the decomposition of plants. Many sea animals burrow into mud and sand, and live there habitually; and the dead bodies of others must be occasionally buried in such deposits. Some of the animals and plants, or their fragments, that were thus buried in the sediments that afterwards hardened into rock, would doubtless decompose, and leave scarcely any trace of their presence. Of others, however, the form or impression of the body would be retained in the substance of the rock, or even the body itself might bo preserved.

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