THE SEA AS A REPRODUCTIVE AND PRESERVATIVE AGENT Besides being an agent of destruction, the sea is also an agent of reproduction—directly engaged in forming new land, which may in time be reclaimed and cultivated. The material removed from one part is sometimes partly thrown ashore again on the adjacent coast, and thus the land may actu ally encroach on the sea. We have an instance of this on the shores of Lincolnshire, where large tracts formerly covered by water have been reclaimed. In several other places the land is protected from the sea by deltas and bars, formed from the accumulated debris brought down by large rivers, such as those of the Mississippi; the Nile, the Ganges, and others.
The sea, by means of its waves and currents, acts generally on a limited portion of the land only—viz., that be tween the high and low-water mark. During violent storms its action may extend as high as 200 or 300 feet or more above the sea level, and downwards to a corresponding depth. Its action between these limits is of course greatly assisted by atmospheric agencies—such as the winds, chemical action of the air, rain, &c. These, however, cannot be properly treated of in a work devoted exclusively to the ocean ; but most works on physiography, er physical geography, contain a more or less full account of their action. Suffice it for us now to state that all the various agents of denudation are unceasingly in opera tion; and of the resulting debris carried into the sea, the coarser detritus falls to the bottom close to the shore, while the finer particles are swept out into the open sea, but finally also settle down on the floor of the ocean. But the sea itself teems with various forms of animal and vegetable life—the larger and higher organisms inhabiting the surface stratum, while at great depths myriads of the foraminifera and other minute forms abound ; so that, amongst the sedimentary matters eroded from the land, there are deposited on the ocean floor enormous quantities of the remains of marine forms, which must in time be deeply imbedded in the new strata thus formed. The present dry land is, as is well known,
formed of rocks, in many of which fossilized remains of marine animals are frequently found. What is now land, therefore, must at one time have been covered by the sea, otherwise the remains of marine animals could not have been thus im bedded. In fact, the whole series of stratified rocks must have been laid under water, in exactly the same manner that the debris of the land is now deposited on the floor of the ocean.
The mechanical action of the .largest waves ceases at a depth of 100 fathoms, and even the motion of the deeper submarine currents is scarcely perceptible, so that the greater part of the bed of the ocean is covered by a cushion of water, nearly if not absolutely still—incapable, indeed, of disturbing the fleecy-like deposits of tiny shells and other matters that cover so large a part of the ocean floor. The sea is, therefore, eminently preservative, its all but absolute repose at great depths permitting the uninterrupted and even deposition of the materials of future continents. And if we may judge of the future by the past, the various agents of change—igneous, aqueous, and atmospheric—must ultimately alter the present distribution of land and water, and what is now dry land may again be covered by the sea, and the present ocean floor may be upheaved, and become habitable. But the process is slow—so slow, indeed, that the six thousand years of history are as nothing compared with the vast periods which must elapse before a radical change can be effected by natural agencies. Professor Huxley has estimated that a million years must elapse before the basin of the Thames will be worn down to the sea-level ; while the reduction of the British Isles to a flat plane at sea-level would occupy about five and a-half million years !