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Show 188 ISLAND LIFE. [rART I. Mississippi, extending over much of the Rocky ~oun~ai~s, c?nsists of marine Cretaceous beds 10,000 feet thwk, mdwatmg great and long-continued subsidence, and an in~ular condition of Western America with a sea probably extendmg northwards to the Arctic Ocean. As marine Tertiary deposits are found conformably overlying these Cretaceous strata, Professor D~na is of opinion that the great elevation of this part of. Amenca did not begin till early Tertiary times. Other :rerti~ry beds in California, Alaska, Kamschatka, the Mackenzie River, the Parry Islands, and Greenland, indicate partial submergence of all these lands with the possible influx of warm water from the Pacific · and the considerable elevation of some of the Miocene beds in Greenland and Spitzbergen renders it probable that these countries were then much less elevated, in which case only their higher summits would be covered with perpetual snow, and no glaciers would descend to the sea. In the Pacific there was probably an elevation of land coun-terbalancing, to some extent, the great depression of so much of the northern continents. Our map in Chapter XV. shows the islan<ls that would be produced by an elevation of the great shoals under a thousand fathoms deep, and it is seen that these all trend in a south-east and north-west direction, and would thus facilitate the production of definite currents impelled by the south-east trades towards the north-west Pacific, where they would gain access to the polar seas through Behring's Straits, which were, perhaps, sometimes both wider and deeper than at preaent. Effect of these Changes on the Clirnctte of the A 1·ctic Regions.- These various changes of sea and land, all tending towards a transference of heat from the equator to the north temperate zone, were not improbably still further augmented by the existence of a great inland South American sea occupying what are now the extensive valleys of the Amaz;on and Orinoco, and forming an additional :reservoir qf s~p~r-heate(j. water to add to the supply poured into the North Atlantic. · It is not of course supposed that all the modifications here indicated co-existed at the same time. We have good reason to believe, from the known distribution of animals in the Tertiary CHAP. IX.] MILD ARCTIC CLIMATES. 189 period, that land-communications have at times existed between Europe or Asia and North America, either by way of Behring's Straits, or by Iceland, Greenland, and Labrador. But the same evidence shows that these land-communications were the excep~ ion rather than the rule, and that they occurred only at long mterv~ls a~d for short periods, so as at no time to bring about anythmg hke a complete interchange of the productions of the two continents.1 We may therefore admit that the communication between the tropical and Arctic oceans was occasionally interrupted in one or other direction; but if we look at a globe instead of a Mercator's chart of the world, we shall see that the disproportion between the extent of the polar and tropical seas is so enormous that a single wide opening, with an adequate impulse to carry in a considerable stream of warm water, would be amply sufficient for the complete abolition of polar snow and ice, when aided by the absence of any great areas of high land within the polar circle, such high land being, as we have seen, essential to the production or perpetual snow even at the present time. Those who wish to understand the effect of oceanic currents in conveying heat to the north temperate and polar regions, should study the papers of Dr. Croll already referred to. But the same thing is equally well shown by the facts of the actual distribution of heat due to the Gulf Stream. The· difference between the mean annual temperatures of the opposite coasts of Europe and America is well known and has been already q~oted, but the difference of their mean winter temperature is still more striking, and it is this which concerns us as more especially affecting the . distribution of vegetable and animal life. Our mean winter temperature in the west of England is the same as that of the Southern United States, as well as that of Shanghae in China, both about twenty degrees of latitude further south; and as we go northward the difference increases, so that the winter climate of Nova Scotia in Lat. 45° is found within the Arctic circle on the coast of Norway; and if the latter 1 For an account of the resemblances and differences of the mammalia of the two continents during the Tertiary epoch, see my Geographical Di.qtribution of Anirnals, Vol. I. pp. 140-156. |