Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/301

Rh wear; or THE SE..] really greater than that from the action of the breakers. What the sea chieﬂy does is to break down and wash away the rubbish that falls from the cliffs, and thus to leave an ever fresh surface for renewed denudation. ' (5.) Among the erosive operations of the sea must be in- cluded what is performed by ﬂoating ice. Along the margin of arctic lands a good deal of work is done by the broken up ﬂee-ice and ice-foot. These cakes of ice, driven ashore by storms, tear up the soft shallow-water or littoral deposits, rub and scratch the rocks, and push gravel and blocks of rock before them as they strand on the beach. Icebergs also, when they get aground in deep water, must greatly disturb the sediment accumulating there, and must grind down any submarine rock on which they grate as they are driven along. The general result of the erosive action of the sea on the land is the production of a submarine plain. As the sea advances by cutting slice after slice away from the coast, successive lines of beach pass under low-water mark. The whole of the littoral belt, as far down as wave action has inﬂuence, is continually being ground down by the moving detritus. If no change of level between sea and land should take place, the sea might conceivably eat its way slowly far into the land, and produce a gently slop- ing yet almost horizontal selvage of plain covered pern1an— ently by the waves. In such a submarine plain the inﬂu- ence of geological structure, and notably of the relative powers of resistance of diﬂ'erent rocks, would make itself conspicuous. The present promontories caused by the superior hardness of their component rocks would no doubt be represented by ridges on the subaqueous plateau, while the existing bays and creeks worn out of softer rocks would be marked by lines of valleys or hollows. III. TR.&NSPORT.——Tl1e sea by means of its surface-drifts and currents carries sedimentary material to great distances and strews them over its ﬂoor; Near land, where the movements of the water are active, much coarse detritus is transported along shore or swept farther o11t to sea. A prevalent wind, by creating a current in a given direction along a coast—line, will cause the shingle to travel coast- wise, the stones getting more and more rounded and reduced in size as they recede from their sources. The Chesil Bank, which runs as a natural breakwater 16 miles long connecting the Isle of Portland with the mainland of Dorsetshire, consists of rounded shingle which is constantly being driven westwards. On the Moray Firth the reefs of quartz—rock about Cullen furnish abundance of shingle, which moves westwards along the coast for more than 15 miles. The coarser sediment probably seldom goes much beyond the littoral zone. Fine gravel, however, is pushed along the bottom by currents even at 600 fathoms ; for at that depth in the North Atlantic between the Faroe Islands and Scotland small pebbles of volcanic and other rocks are dredged up which have probably been carried by an arctic under-current from the north. At greater depths the force of currents at the bottom must be too.feeble to push along any detritus. But much ﬁne sednnent is carried in suspension by the sea for long dis- tances from land. Some rivers, as the Amazon, pour so n1uch silt into the sea as to discolour its water for several hundred miles away from land. After wet weather the coast—waters round the shores of the British Islands are some- times made turbid from the quantity of mud brought down from the_land. Dr Carpenter found the bottom waters of the Mediterranean to be everywhere permeated by an ex- tremely ﬁne mud, derived no doubt from the rivers and shores of that sea, borne away out far from land, and settling slowly down upon the bottom. He remarks that the cha1'aeteristic blueness of the Mediterranean may be explained, like that of the Lake of Geneva (as shown by Dr GEOLOGY 287 Tyndall), by the diffusion of those exceedingly minute sedimentary particles through the water. But the most startling evidence of the wide extent to which transport takes place in the ocean is that supplied by the observations made during the voyage of the “ Challenger.” From the abysses of the Paciﬁc Ocean, at the furthest distances from land, the dredge brought up bushels of rounded pieces of pumice of all sizes up to blocks a foot in diameter. These fragments were all evi- dently water-worn, and almost certainly were derived from the land. Some small pieces indeed were taken on the surface in the tow-net. Round volcanic islands, and off the coasts of volcanic tracts of the mainland, the sea is sometimes covered with ﬂoating pieces of water-worn pumice swept out by ﬂooded rivcrs. These fragments drift away for hundreds or even thousands of miles until, becoming water-logged, they sink to the bottom. Their universal distribution was one of the most noticeable features in the dredgings of the “ Challenger.” The clay which is found on the bottom of the ocean at the greatest distances from any shore may be partly due to its transport in that condi- tion from land, but more probably to the decomposition of the drifted pumice} Another not unimportant process of marine transport is that performed by ﬂoating ice. Among the arctic glaciers moraine stuff is of rare occurrence ; but occasional blocks of rock and heaps of earth and stones fall from the cliffs which rise above the general waste of snow. Hence on the icebergs that ﬂoat off from these glaciers, rock debris some- times may be observed. It is transported southward for hundreds of miles until, by the shifting or melting of the bergs, it is dropped into deep water. The floor of certain portions of the North Atlantic in the pathway of the bergs must be plentifully strewn with this kind of detritus. By means of the ice-foot also, an enormous quantity of earth and stones is every year borne away from the shore as the ice breaks up, and strewn over the ﬂoor of the sounds, bays, and channels. IV. REPRODUCTION.—Tl1e sea being the receptacle for the material worn away from the land must receive and store up in its depths all that vast amount of detritus by the re- moval of which the level and contours of the land are in the course of time so greatly changed. The deposits which take place within the area covered by the sea may be divided into two groups--the inorganic and organic. It is the former with which we have at present to deal; the latter will be discussed with the other geological functions of plants and animals. The inorganic deposits of the sea- ﬂoor are partly (a) land-derived or terrigenous, partly (L) abyssal. (u.) Lancl-derived or Te;-rz'_r/enous.—These may be con- veniently grouped according to their relative places on the sea-bed. (1.) Shore Deposits.—The most conspicuous and familiar are the layers of gravel and sand which accumulate between tide-marks. As a rule, the coarse materials are thrown up about the upper limit of the beach. They seem to remain stationary there; but if watched and examined from time to time, they will be found to be continually shifted by high tides and storms, so that the bank or bar of shingle retains its place though its component pebbles are being constantly moved. Below the limit of coarse shingle upon the beach lies the zone of ﬁne gravel, and then that of sand. These zones are far from being constant ; yet when they all occur on the same beach, they tend to range themselves according to their relative coarseness, the rougher detritus lying at the upper, and the ﬁner towards the lower edge of the shore. The nature of the littoral accumulations on any 1 Murray, Proc. Roy. Soc. Edi-n., 1876-7, p. ‘.7-17.