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Show 84 ISLAND LIFE. [PART I. miles. Round the entire Afi·ican coast for example, this depth is reached at distances varying from forty to a hundred and fifty miles (except in the Red Sea and the Straits of Mozambique), the average being about eighty miles. Now the numerous specimens of sea-bottoms collected durincr the voyacre of the Challenger show that true shoredeposits- that is, 0 materials denuded from the lanu and ca~ri~d down as sediment by rivers-are almost always confined withm a distance of 50 or 100 miles of the coast, the finest mud only being sometimes carried 150 or rar?ly ?O? mi!es. As the sediment varies in coarseness and density It IS evident that it will sink to the bottom at unequal distances, the bulk of it sinking comparatively near shore, while only the very fines~ ~nd almost impalpable mud will be carried out to the furthe~t limits. Beyond these limits the only deposits (with few excepti~n.s) are organic, consisting of the shells of minute calcareous or siliceous organisms with some decomposed pumice and volcanic dust which floats out to mid-ocean. It follows, therefore, that by far the larger part of all stratified deposits, especially those which consist of sand or pebbles or any visible fragments of rock, must have been formed within 50 or 100 miles of then existing continents, or if at a greater distance, in shallow inland seas receiving deposits from more sides than one, or in certain exceptional areas where deep ocean currents carry the dib1·is of land to greater distances.1 If we now examine the stratified rocks found in the very centre of all our great continents, we find them to consist of sandstones, limestones, conglomerates, or shales, which must, as we have seen, have been deposited within a comparatively short distance of a sea-shore. Professor Archibald Geikie says :-"Among the I In his P1·elirni1Wry R eport on Oceanic Deposit, Mr. Murray says:-" It has been found that the deposits taking place near continents and islands have received their chief characteristics from the presence of the debris of adjacent lands. In some cases these deposits extend to a distance of over 150 miles from the coast.'' (Proceedi11gs of the Royal Socipty, Vol. XXIV. p. 519.) " The materials in suspension appear to be almost entirely deposited within 200 miles of the land." (Proceedings of the Royal Society of Edinburgh, 1876-77, p. 253.) cHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 85 thickest masses of sedimentary rock-those of the ancient Palreozoic systems-no features recur more continually than the alternations of different sediments, and the recurrence of surfaces covered with well-preserved ripple-marks, trails and burrows of annelides, polygonal and irregular desiccation marks, like the cracks at the bottom of a sun-dried muddy pool. These phenomena unequivocally point to shallow and even littoral waters. They occur from bottom to top of formations, which reach a thickness of several thousand feet. They can be interpreted only in one way, viz., that the formations in question began to be laid down in shallow water; that during their formation the area of deposit gradually subsided for thousands of feet; yet that the rate of accumulation of sediment kept pace on the whole with this . depression; and hence that the original shallow-water character of the deposits remained, even after the original sea-bottom had been buried under a vast mass of sedimentary matter." He goes on to say, that this general statement applies to the more recent as well as to the more ancient formations, and concludes-" In short, the more attentively the stratified rocks of the earth are studied, the more striking becomes the absence of any formations among them, which can legitimately be considered those of a deep sea. They have all been deposited in comparatively shallow water."1 The arrangement and succession of the stratified rocks also indicate the mode and place of their formation. We find them stretching across the country in one general direction, in belts of no great width though often of immense length, just as we should expect in shore deposits; and they often thin out and change from coarse to fine in a definite manner, indicating the position of the adjacent land from the debTis of which they were originally formed. Again quoting Professor Geikie :" The materials carried down to the sea would arrange themselves then as they do still, the coarser portions nearest the shore, the finer silt and mud furthest from it. From the earliest geological times the great area of deposit has been, as it still is, the marginal belt of sea-floor skirting the land. 1 Geographical_Evoluiion. (P1·oceedings of the Royal Geographical Society. 1879, p. 426.) |