ACTION OF THE SEA ON THE EARTH'S CRUST Of the thousands of earnest students of the physical geography of the globe, but very few have the time or the means to become actually acquainted With the various phe nomena, the origin and functions of which they endeavour to trace. Few are privileged to climb the snow-capped heights of Asia or America; or to gaze in mingled amazement and admiration at the thundering fall of a mighty cataract; or to witness the awe-inspiring sight of a great storm at sea, watch ing with beating heart the vast billows, driven before the wind, and breaking with a deafening roar on a rock-bound coast. By far the larger number of students can never hope to see these and other grand displays in the wonderful pano rama of nature, but, fortunately, actual sight, though a most useful adjunct, is not absolutely necessary to a thorough know ledge. Humboldt, in his Kosmos, truly says, that "every little nook and shaded corner is but a reflection of the whole of nature." Carl Ritter amplifies the thought, and remarks, that "wherever our home is, there lie all the materials we need for the study of the entire globe. The roaring mountain brook is the type of the thundering cataract ; the geological formations of a single little island suggest the broken coast line of a continent ; the study of the boulders which are so thickly scattered, in token of a great primeval deluge from the north, reveals the structure of whole mountain-chains. A small range of hills may be taken as the type of the loftiest Cordillera." From these and other considerations, Ritter concludes that a thorough study of the district where we live is the true key to the understanding of the geography of foreign lands, for the eye may be easily trained to see the greater in the leas. Applying the foregoing considerations to the special subject of this work—the geography of the ocean—it will be evident that comparatively few students will be so circumstanced as to be able to verify the facts given by sight. But those who do not live at the seaside may form true ideas of the forms and phenomena described, by a careful study of their own district, accompanied by a few simple practical experiments, illustrating such points as Nature of Wave motion,' Oceanic Circulation,' and so The work of the rain and rivers' may be practically studied almost everywhere. The gradual wearing away of the superficial strata of the land by aqueous agency is a familiar subject. The rounded pebbles, gravel, and sand found in the bed of a stream, are not only the evidences, but the actual agents of the action of the water. A pond or lake formed by the expansion of a stream, and tem porarily retaining its waters, may be regarded, in many respects, as a miniature of that vastly greater receptacle of all the running waters of the globe,—the ocean. Of the sediment discharged into the lake by the stream, the heavier particles fall to the bottom first ; the finer detritus is carried further away, but, also, ultimately settles down. And the same in the sea. Of the sedimentary matters brought down by rivers, or directly removed by the waves, the coarser particles sub side first, while the finer sand and mud are transported to considerable distances by tidal-waves and currents, but are, also, finally deposited on the ocean-floor.
That the greater may be seen in the infinitely less, will be evident from the fact that the action of the sea on the earth's crust in its threefold aspect—(1) destructive, (2) reproductive, (3) preservative—may be exemplified, to a cer tain extent, by a very simple experiment. For if a small pool be formed where the bank of a stream is moderately level, and then connected with the stream by a miniature canal, having a slight slope, it will be seen that the looser particles in the bed of the little channel are swept down by the water into the pool, where, the water being perfectly quiet, they fall to the bottom. Now, so long as the water in the
pool is at rest, its action is evidently limited to the reception and deposition of the sediment discharged into it by the stream. But if its surface be disturbed—either artificially or naturally—by a current of air, the waves thus formed will, as they break on the banks, be seen to dislodge first the finer and then the coarser particles, which are gradually spread evenly over the bottom,—the coarser close to the bank, while the finer mud settles down further from it. If the air current acting on the water be strong and continuous, a breach may be made in the opposite bank, through which the water will make its escape. Or if a few miniature cliffs be formed at the water's edge, the continuous action of the wavelets at their bases will be seen to result in the formation of little caves or arches, gradually penetrating inwards, until at last one and another are undermined to such an extent that they topple over into the water, forming a line of rubbish along the shore, or here and there forming little low promon tories jutting out into the water. In the latter case, if the artificial air current be directed to force the water against one side of a promontory, it will probably in a short time result in a gradually widening channel being cut across, and thus the projecting part will be transformed into an island. On close examination of our miniature sea, it will most probably be found that its beach or shore line is not " wasted " everywhere alike,—some parts being rapidly pene trated, in others the erosion being scarcely perceptible, while at a few points, instead of being wasted, the land actually encroaches on the water—that is, some of the sand and mud displaced elsewhere, instead of being deposited on the bottom, is thrown ashore again, thus not only preserving the beach at those particular points, but actually reproducing new land. Or some of the heaped-up detritus might be just covered with water, thus forming a shoal or sandbank. And if, after a time, the supply of water be stopped, and the pool drained, the various changes due to the action of the water would be still more apparent. The new strata formed on the bottom would be easily seen by making a careful cutting, and removing the earth on one side. Of course it is only possible on such a small scale to employ two agents, viz.—waves, and a trifling surface drift or current produced by an artificial air-current. But in the ocean, the action of tide-waves and deep-sea cur rents is added to that of wind-waves and surface drifts. These, however, introduce no new and our little pool and streamlet thus serve to illustrate the action of rivers and the sea on the land.
We have thus exemplified in miniature (1) the de structive action of the waves, which are the great agents of marine denudation ; (2) the reproductive action of surface movements of the water—throwing up again on one part of the coast the debris removed from another ; and (3) the preservative action of the great mass of the water—preserving the newly-deposited sediment on the bottom from being wasted or even disturbed. The superficial stratum of the sea only is engaged in wasting or directly adding to the shore ; but the great mass of the ocean overlies constantly-increasing depo sits, which may in time be upheaved, and, being acted on by all the atmospheric and aqueous agencies 1 now at work in levelling the present land, may eventually become habitable. It may therefore be truly said that the present ocean is but a vast workshop, where the materials of future continents are elaborated and preserved.