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Show 344 DARWINISM CIIAP. Now the whole series of marine stratified rocks, from the earliest Palreozoic to the most recent Tertiary beds, consist of materials closely corresponding to the land debris now being deposited within a narrow belt round the. shores of. all .continents · while no rocks have been found whiCh can be 1dent1fied . with the various oozes now forming in the deep abysses of the ocean. It follows, therefore, that all the geological formations lmve been formed in comparatively shallow water, and alway~ adjacent to the continentnJ la~d o~ the .Pe~iod: The great thickness of some of the formatiOns 1s no md1cat10n of a clecp sea but only of slow subsidence during the time that tho de~osition was in. progress. This view is now adopted hy many of the most experienced geologists, especially by Dr. ArchibaJd Geikie, Director of the Geological Survey of Ureat Britain, who, in his lecture on "Geographical Evolution," sa,y~: "From all this evidence we may legitimately conclude that the present land of the globe, though consisting in great measure of marine formations, has never lain under the deep sea; but that its site must always have been nea,r land. E, t'll its thick marine limestones are the deposits of comparati V('ly shallow water." 1 But besides these geological a,nd physical consideration", there i. a mechanical difficulty in the way of repcate<l change of position of oceans and continents which ha not yet received the attention it deserves. According to tho recent careful estimate by Mr. John Murray, the land area of the globe is to the water area as ·28 to ·72. The mean height of the land above sea-level is 2250 feet, while the mean depth of the ocean is 14,640 feet. Hence the lmlh of dry land is 23,450,000 cubic miles, and that of the water~ of the ocean 323,800,000 cubic miles; and it follows that if the whole of the solid matter of the earth's surface were reluced to one level, it would be everywhere covered hy an ocean about two miles deep. The accompanying diagram will serve to render these figures more intelligible. The length of the ·ections of land and ocean are in the proportion of their respective areas, while the mean height of the land ancl the mean depth of the ocean are exhibited on a grca,tly incrca:-;cd 1 I have gh·en a full summary of the eviUence for the perm ruu~ I H'<' of oceanic aml continental areas in my Island L-ife, chap. vi. XII OF ORGANISM 345 vertical scale. If we considered the continents and their adjacent ocea,ns sepa,ratcly they would differ a little, but not very materially, from this diagram; in some cases the proportion of land to ocean would be a little greater, in others a little less. Now, if we try to imagine a process of elevation and depression by which the sea, and land shall completely change places, we shall be met by insuperable difficulties. \V c must, in the first place, assume a general equality between elevation and subsidence during any given period, hcca,use if the elevation over any extensive continental area were not balanced by some subsidence of approximately equal a,mount, Diagram of proportionate meao height of Land and depth of Oceans. Ocean Area. t72 of area of Globe. Fro. 32. an unsupported hollow would be left under the earth' crust. Let us now suppose a continental area to sink, and an adjacent oceanic area to rise, it will be seen that the gre<tter part of the land will disappear long before the new land has approached the surface of the ocean. This difficulty will not be removed by supposing a portion of a continent to subside, and the immediately adjacent portion of the ocean on the other side of the continent to rise, beca,usc in almost every case we find that within a comparatively short distance from the shores of all existing continents, the ocean floor sinks rapidly to a depth of from 2000 to 3000 fathoms, and ma,intains n, similar depth, generally speaking, over a large portion of the ocea,nic area,s. In order, therefore, that any area of continental extent be upraised from the gren,t oceans, there must be a subsidence of a land area five or six times us great, unless it C<Lll be . hown that an extensive elevation of the ocean floor up to and far |