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Show 308 REPR0Dt1CTIVE EFFECTS 01<' TIDES AND Ct1RRENTS. case, the sheet of water is converted into land, wl~ereas, in the other, a shoal only will be raised, overflowed ~t h1gh water, or at least by spring-tides. The only records wh1ch we at pre~ent possess of the gradual shallowing of seas are ~onfined, as mtght be expected, to estuaries, havens, and certam ch~nnels ~f ~o great depth ; and to some inland seas, as the Baluc, Adnatic, a nd Arabian Gulf. It is only of late years that a.c cura.t e sur-veys and soundings have afforde~ data. of c~mpanson m very deep seas, of which fut~re geologist.s will avatl themselves. It appears extraordmary that m some tracts of the sea, adjoining our coast, where we know that currents ~re not ?nly sweeping along rocky masses, ~brown down, from time to .time, from the biO'h cliffs, but scounng out also deep channels m the regular str=ta, thet·e should. exis.t frag~l~ shells and ten~er zoophytes in abundance, whiCh hve umn.JU~ed b~ these VIO· lent movements;. The ocean, however, Is m this respect a counterpart of the land; and as, on our continents, rivers may undermine their bankR, uproot trees, and roll along sand and gravel, while their waters are inhabite~ by .testacea m~d fish, and their alluvial plains are adorned with nch vegetatiOn and forests so the sea may be traversed by rapid currents, and its bed' may suffer great local derangem~n.t, without any interruption of the general order and tranqmll1ty. One important character in the for~ations produced by currents, is the immense extent over w luch they are the means of diffusinD' homoD"eneous mixtures; for these are often coextensive with a gre;t line of coast, and,, by .comrar!so~ with their deposits the deltas of rivers must shnnk mto ms1gmficance .. In the Mediterranean the same current which is rapidly destroymg many parts of the African coast, betw:en .the Straits of Gi?raltar and the Nile, preys also upon the N dotic delta, an.d dnfts the sediment of that great river to the eastward. 'ro this som·c~ the rapid accretions of land on parts of th~ Syria? shores w l~ere n~e~s do not enter, may be attributed. 'lhe rums of anc1ent 'I!te are now far inland, and those of ancient Sidon are two miles distant from the coast, the modern town having been removed towards the sea*'. But the south coast of Asia Minor affords far more striking examples of advances of the land upon the "' HofF, vol. i., p. 253. COAST OF ASIA MINOR, GUJA~A, ETC. 309 sea, where small streams co-operate with the current before mentioned. Captain Beaufort, in his Survey of that coast has pointed out the great alterations effected on these shores ;ince the time ~f Strabo, where havens are filled up, islands joined to the mam land, and where the whole continent has increased many miles in extent. Strabo himself, on comparing the outline of the coast in his time w.ith its ancient state, was convinced, like our countryman, that it had gained very considerably upon the sea. The new-formed strata of Asia Minor consist of stone, not of loose, incoherent materials. Almost all the streamlets and rivers, like many of those in Tuscany and the south of Italy, hold abundance of carbonate of lime in solution, and precipitate travertin, or sometimes bind together the sand and gravel into solid sandstones and conglomerates: every delta and sand-bar thus acquires solidity, which often prevents streams from forcing their way through them, so that their mouths are constantly changing their position *. Among the greatest. deposits now in progress, and of which the distribution is chiefly determined by currents, we may class those between the mouths of the Amazon and the southern coast of North America. It is wcJl known that a great current is formed along the coast of Africa, by the water impelled by the Trade Winds blowing from the south. When this current reaches the head of the Gulf of Guinea, it is opposed by the waters brought to the same spot by the Guinea current, and it then streams off in a westerly direction, and pursues its rap~d course quite across the Atlantic to the cont! nent of South America. Here one portion proceeds along the northern coast of Brazil to the Caribbean Sea and the Gulf of Mexico. Captain Sabine found that this current was running with the astonishing rapidity of four miles an hom· where it crosses the stream of the Amazon, which river preserves part of its original impulse, and its waters not wholly mingled with those of the ocean at the distance of three hundred miles from its mouth t. The sediment of the Amazon is thus constantly carried to the north-west as far as to the mouths of the Ori- • Karnmania, or a brief Description of the Coast of Asia Minol', &c. Loniton, 1817. t Experiments to determine the Figure of the Earth, &c., p. 4.J.5. |