the work of disintegration and redisposition at a very rapid rate. A few hours' study of such a beach will be of more advantage than pages of description. Here are cliffs, terraces, deltas, and every other form of destruction and reconstruction, effected by the streams running down the gentle slope to the sea.

Having thus briefly glanced at a few instances of the local action of running water, we need only picture to ourselves the mechanical power of the rainfall of the whole world, in order to form some conception of its aggregate effect as it is poured down from higher to lower levels in countless streams, each bearing its burden of soil to some place of temporary or final rest. We may picture the effect of the enormous rainfall about the tropical belt, where frequently 150 inches of rain are precipitated every year upon the parched surface-an amount equal to 15,000 tons of water to every acre. On the mountains high above tropical plains, snow and ice are doing their part, and perpetual frost is splitting fragments from the rocks, to be borne away into the valleys by impetuous torrents from the melting snow-fields. Within the polar circles ice on a more magnificent scale is levelling down the land. Glaciers covering thousands of square miles are slowly sliding down the valleys, grinding them deeper, forming sands, clays, and gravels, and pushing these down to the shore. Icebergs, many miles in circumference, are carried by currents along coasts and against cliffs, scooping channels by their immense. weight-gigantic ploughs, which leave their furrows in the bed of the sea.

No one can contemplate the majestic spectacle of a storm by the sea-shore without feeling profoundly impressed by the power displayed in this form of moving water. The mind, however, is more apt to be led away by the poetical aspects of the subject than to contemplate it in its scientific light. If we wish to know what those rolling billows are doing, let us watch them carefully for a few minutes when the tide is nearly high. Every wave, in its onward sweep, is carrying up shingle, gravel, and rock-material of all kinds, large and small, angular and rounded, and hurling it against the base of the cliff, which is cut under, drilled into holes, and battered by the unceasing assault. At the top of the cliff indentations are made by rain trickling down from above. Horizontally and perpendicularly the rock is being disintegrated and worn away. Presently portions of the overhanging mass will fall, and so much more of the barrier against the sea's encroachment is cast into the mill below, where it will be ground down, and in its turn contribute to a new formation. Two agents combine in distributing the soil thus torn from the coastline; first there is the daily ebb and flow of the tide, which acts with much greater energy on the shallow bottom near land than far at sea; and secondly, innumerable currents, both constant and variable. These rapidly carry the materials to and fro, and transport them long distances. Any fixed projecting object on the bottom, such as a wreck, for instance, will sometimes form a gathering-point, and be covered in a short time. Just as in the case of the river, the heavier stones will

form beaches in shallow water, and the lighter settle in more tranquil localities, the former to become conglomerates, the latter densely impacted sedimentary rocks.

Hitherto our attention has been confined to the land and its immediate neighbourhood, where, no doubt, the forces which determine the collection and distribution of the elements of new formations have the greatest intensity. The profound depths of ocean, however, are continually receiving deposits—a fact suspected, but not until quite recently proved. Geologists had arrived at the conviction that some stratified rocks must have been laid down in very deep water, as certain of the clay slates and chalks. In the first place, water was evidently the agent of deposition; and in the second, the immense accumulation and finely divided state of the materials precluded the idea of any conditions but those characteristic of deep and undisturbed water; the remains of animal life, too, included in these rocks pointed to their marine origin.

Owing to the progress recently made in the science of oceanic meteorology, wonderful revelations have taken place in the secrets of the great deep. The most recent of the deep-sea exploring expeditions in the Challenger has enabled us to survey, as it were, the floor of the ocean by means of the dredge. The results must be touched upon lightly here, for they will be more fully detailed in the chapter on the " organic rocks." In many parts of the Pacific Ocean far from land the depth is over 18,000 feet; in the Atlantic,

frequently 12,000 feet. These are submarine valleys, and they are being filled with new deposits of two kinds a variety of clay and a variety of chalk; the former being invariably found at a greater depth than the latter. This clay or ooze appears to be formed to a large extent of decomposed pumice-stone (the froth of lava ejected from volcanoes either on land or beneath the sea), great quantities of which are found floating on the sea. The chalk mud is very different in composition, being almost entirely carbonate of lime, derived from the shells of minute animals, countless millions of which are living and dying in these oceans. But the important fact is that we find similar shells in chalk at present on the surface of the earth; whence it is inferred that the chalks now above ground were once laid down in ocean depths. There is no possibility of mistake here; for the microscope and a few simple chemical tests disclose the nature of these clays and chalks brought up by the dredge and submitted to the closest examination. The water must be absolutely motionless in these submarine valleys to allow so fine a sediment to accumulate. This process, it must be remembered, is going on in all the deeper parts of the Atlantic and Pacific Oceans, and we cannot doubt that these are the clay slates and limestones of the distant future, just as those with which we roof our houses are similar representatives of a remote past.

Though not stratified rocks in any of the above senses, coal and like accumulations of fossilized vegetation may be so classed for convenience. They are

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at all events very distinct from unstratified rocks, and, besides, frequently present the appearance of layers. Moreover, they are so intimately associated with limestones, sandstones, shales, etc.—all water-formed rocksand in some cases appear to be merely vast masses of drift vegetation from rivers, subsequently overlaid by sand and mud, that they possess, mechanically speaking, a stratified character. Further, the plants composing many of the beds grew in swamps subject to inundation and even occasional incursions of the sea, and were mixed with various sediments. Consideration of these organic "rocks must be deferred for the present.

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Every stratified rock is of course laid down approximately horizontally; but we shall find that a great alteration has taken place in many instances. Abrupt dislocations and fractures, displacements and contortions, occur in every part of the world. A stratum once horizontal may since have assumed the form of a series of curves or angles; it may have been bent so sharply as to have broken; its edges may have separated, and the interval become filled with a more recent rock. Whenever this is the case, the rock will lie at different angles with the horizon, from the gentlest incline to the perpendicular. How, then, has this been brought about? Simply by the continued action of those internal forces of the earth which crumpled and twisted the earlier unstratified formations, and may be seen in operation at this day on a minor, but still unmistakeable scale. Veins of metallic ores have been pressed into fractures and crevices, and molten volcanic rocks