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Show 184 ISLAND LIFE. [PAU'l' 1. penetr~tes within the Arctic circ~e, .is th:oug~ Behring's Straits; but this is both shallow and limited m w1dth, an~ the consequence is that the larcrer part of the warm currents of the Pacific turns back along 5 the shores of the Aleuti~n Islands and North-west America, while a very small quantity enters the icy ocean. . . But if there were other and wider opemngs mto the Arctic Ocean, a vast quantity of the heated water which is now. tur~ed backward wop.ld enter it, and would produce an amehoratwn of the climate of which we can hardly form a conception. A great amelioration of climate would also. be . caused by the breaking up or the lowering of sue? Arcti~ highlands ~snow favour the accumulation of ice; while the mterpenetratwn of the sea into any part of the great continents in the tropical or -temperate zones would ag~in tend to rail'e the winter te,mperature, and render any long continuance of snow in their vicinity almost impossible. Now geologists have proved, quite independently of any suoh questions as we ·are here discussing, that changes of the very kinds above referred to have occurred during the Tertiary period ; and that there has been, speaking broadly, a steady change from a comparatively fragmentary and insular condition of the great north temperate lands in early Tertiary times, to that more compact and continental condition which now prevails. It is, no doubt, difficult and often impossible to determine bow long any particular geographical condition lasted, or whether the changes in one country were exactly coincident with those in another; but it will be sufficient for our purpose briefly to indicate those more important changes of ]and and sea during the Tertiary period, which must have produced a decided effect on the climate of the northern hemisphere. Geographical Changes · favouTing mild Northe1·n Olim.ates in Tertiary times.-The distribution of the Eocene and Miocene formations shows, that during a considerable portion of the Tertiary period, an inland sea, more or less occupied by an archipelago of islands, extended across Central Europe between the Baltic and the Black and Caspian Seas, and thence by narrower channels south-eastward to the valley of the Euphrates CHAP. IX.) MILD ARCTIC CLIMATES. 185 and the Persian Gulf, thus opening a communication between the North Atlantic and the Indian Ocean. From the Caspian also a wide arm of the sea extended during some part of the Tertiary epoch northwards to the Arctic Ocean, and there is nothing to show that this sea may not have been in existence during the whole Tertiary period. Another channel probably existed over Egypt 1 into the eastern basin of the Mediterranean and the Black Sea; while it is probable that there was a communication between the Baltic and the White Sea, leaving Scandinavia as an extensive island. Turning to India, we find that an arm of the sea of great width and depth extended from the Bay of Bengal to the mouths of the Indus; while the enormous depression indicated by the presence of marine fossils of Eocene age at a height of 16,500 feet in Western Tibet, renders it not improbable that a more direct channel across Afghanistan may have opened a communication between the West Asiatic and Polar seas. It may be said that the changes here indicated are not warranted by an actual knowledge of continuous Tertiary deposits over the situations of the alleged marine channels; but it is no less certain that the seas in which any particular strata were deposited were always more extensive than the fragments of those strata now existing, and often immensely more extensive. The Eocene deposits of Europe, for example, have certainly undergone · enormous denudation both marine and subaerial, and may have once covered areas where we now find older deposits (as the chalk once covered the weald), while they certainly exist concealed under some Miocene, Pliocene, or recent beds. We find them widely scattered over Europe and Asia, and often elevated into lofty mountain ranges; and we should certainly err far more seriously in confining the Eocene seas to the exact areas where we now find Eocene rocks, than in liberally extending them, so. as to connect the several detached portions of t~e formation whenever there is no valid argument against 1 Mr. S. B. J. Skertchley informs me that he has himself observed thick Tertiary deposits, consisting of clays and anhydrous gypsum, at Berenice on the borders of Egypt and Nubia, at a height of about 600 feet above the sea-level; but these may have been of fresh-water origin. |