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Show 310 UEPRODUCTIVE EFFECTS OF TlDES AND ClTRRENTS. noco, and an immense tract of swamp is formed along the coast of Guiana, with a long range of muddy shoals bordering the marshes and becoming converted into land#.'. The sediment of the Orinoco is partly detained, and settles near its mouth, causino- the shores of Trinidad to extend rapidly, and is partly swept ~way into the Caribbean Sea by the equatorial current. According to Humboldt, much sediment is carried again out of the Caribbean Sea into the. Gulf of Mexico. Th~ rivers, also, which descend from the high plateau of Mexico, between the mouths of the Norte and Tampico, when they arrive at the edge of the plateau, swollen by tropical rains, bear down an enormous quantity of rock and mud to the sea ; but the current, setting across their mouths, prevents the growth of deltas, and preserves an almost uniform curve in that line of coast f. It must, therefore, exert a great transporting power, and it cannot fail to sweep away part of the matter which is discharged from the mouths of the Norte and the Mississippi. It follows from these observations, that, in certain parts of the globe, continuous formations are now accumulated over immense spaces along the bottom of the; ocean. The materials undoubtedly must vary in different regions, yet for thousands of miles they may often retain some common characters, and be simultaneously in progress throughout a space stretching SOo of ]atitude from south-east to north-: west, from the mouths of the Amazon for example, to those .of the Mississippi-as far as from the Straits of Gibraltar to Iceland. At the same time, great coral reefs are growing around the West ~ndian islan~s; and in some parts, streams of lava are occasiOnally flowmg into the sea, which become covered again, in the intervals between eruptions, with other beds of corals. Th.e v~rio~s rocks, therefore, stratified and unstratified, now formmg m this part of the globe, may occupy, perhaps, far greater areas than any group of our ancient secondary series which has yet been traced through Europe. In regard to the internal arrangement of" pelagian,' fo~ma· tions deposited by currents far from the land, we may mfer that in them, as in deltas, there is usually a division into strata; ~ Lochend's Observations on the Nat. Ilist. of Guiana.. Edin. Trans., vol. iv. t This coast has been recently examined by Captain Vetch.-See also Bauza's new chart of the Gulf of Mexico. REPRODUCTIVE EFFECTS OF TIDES AND CURRENTS. 311 fo 1 r, in both ca~es, the accumulations are successive, and, for t 1e most part, mten·upted. The waste of cliffs on the British coast is almost entirely confined to the winter months . so that running w~ters in. the sea, like those on the land, are pe;iodically char~ed wtth sedtment, and aga~n become pure. It will happ~ n, m many ~as:s, tha~ the ~eltmg of snow will yield an annual tnbute of fl.uvtati~e sediment m spring or summer, while violent gales of wmd ~vill cause the principal dilapidations on the shores to occur m autumn and winter; so that distinct materials may be arranged in alternate strata in deep depressions of the bed of the ocean. Those geologists who are not averse to presume that the course of Nature has been uniform from the earliest ao-es and that causes now in action have produced the for~e; changes of the earth's surface, will consult the ancient strata fot· instruction in regard to the reproductive effects of tides and currents. It will be enough for them to perceive clearly that great effects now annually result from the operations of these agents, in the inaccessible depths of lakes, seas, and the oce~n; and they will then search the ancient lacustrine and marine strata ~or. manifestations of analogous efiects in tiq1es pust . . Nm· Will It be necessary for them to resort to very ancient monuments; for in certain regions where there are active volca?os, and ~here viol~nt earthquakes prevail, we may examme submarme formatiOns many thousand feet in thickness, belonging to our own era, or, at least, to the era of contemporary races of organic beings. |