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Show 254 IMPERFECTION OF THE [CHAP. IX. the superimposed formations ; so it is in North Ame~ica, ·and in many other parts of the world. Th~ most sk~lful geoloo-ist if his attention had been exclusively confined to th~e iarge territories, would never have suspecte~ th~t durino- the periods which were blank and barre~ In h1s own gountry, great p~les of sediment, charged With new and peculiar fonns of hfe, ha~ elsewhere been a?CUinulated. And if in each separate territory, hardly any 1dea can be formed of the length of time which has elapsed between the consecutive formations, we may infer that this could nowhere be ascertained. The frequent and great changes in the mineralogical composition of consecutive formations, generally implying great changes ~n the geography o~ the surroundil;g lands, w~ence the s~diment has b~en deriv.ed, accords with the behef' of vast Intervals of time having elapsed between each formation. But we can, I think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence. Scarcely any fact struck me more _when examini:r~.g many hundred Iniles of the South American coasts, whwh have been upraised several hundred feet within the recent period, than the absence of any recent deposits sufficiently extensive to last for even a short geological period. Along the whole west coast, \vhich is inhabited by a peculiar marine fauna, tertiary beds are so scantily developed, that no record of several successive and peculiar marine faunas will probably be preserved to a distant age. A little reflection will explain why along the rising coast of the western side of South America, no extensive formations with recent or tBrtiary remains can anywhere be found, though the supply of sediment n1ust for ages have been great, from the enormous degradation of the coast-rocks and from muddy strea1ns entering the sea. The explanation, no doubt, is, that the littoral and sub-littoral deposits are continually worn away, as soon as they are brought up by the slow and gradual rising of the land within the grinding action of the coast-waves. We may, I think, safely conclude that sediment m~st oe accumulated in extremely thick, solid, or extensive CHAP. IX.] GEOLOGICAL ItEOORD. 255 1nasses, in order to withstand the incessant action of the waves, when first upraised and during subsequent oscillations of level. Such thick and extensive accumulations of sediment may be formed in two ways; either, in profound depths of the sea, in which case, judging from the researches of E.] orbes, we may conclude that the bottom will be inhabited by extrernely fevv animals, and the mass when upraised will give a n1ost imperfect record of the forms of life which then existed ; or, sediment may be accumulated to any thickness and extent over a shallow bottom, if it continue slowly to subside. In this latter case, as long as the rate of subsidence and supply of sediment nearly balance each other, the sea will remain shallow and favourable for life, and thus a fossiliferous formation thick enough, when upraised, to resist any amount of · rlegradation, n1ay be formed. I arn convinced that all our ancient fonnations, which are rich in fossils, have thus been formed during subsidence. Since publishing my views on this subject in 1845, I have watched the progresa of Geology, and have been surprised to note how author after author, in treating of this or that great formation, has come to the conclusion that it was accumulated during subsidence. I may add, that the only ancient tertiary forination on the west coast of South America, which has been bulky enough to resist such degradation as it has as yet suffered, but which will har~ly last _to a distant geologi?al ~ge, was certainly deposited dur1ng a downward osCillation of level, and thus gained considerable thickness. All geological facts tell us plainly that each area has undergone nurnerous slow oscillations of level, and apparently these oscillations have affected wide spaces. Consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation, may have been formed over wide spaces during periods of subsid~ nce, but only where the supply of sediment was suffiCient to keep the sea shallow and to embed and preserve t~e remains before they had time to decay. On the other lia~d, as lol?g as the bed of the sea remained stationary, th~ck depos1ts could not have been accumulated in the |