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Show 186 ISLAND Lll!'E. [rART J. our doing so. Considering then, that some one or .more of the sea-communications here indicated almost certamly existed during Eocene and Miocene times, let us endeavour to estimate the probable effect such communications would have upon the climate of the northern hemisphere. 1'he Indian Ocean as a source of Heat in Tertiary ti?nes.If we compare the Indian Ocean with the South Atlantic we shall see that the position and outline of the former are very favourable for the accumulation of a large body of warm water moving northwards. Its southern opening between South Africa and Australia is very wide, and the tendency of the trade-winds would be to concentrate the currents towards its north-western extremity, just where the two great channels above described formed an outlet to the northern seas. As will be shown in our nineteenth chapter, there were probably, during the earlier portion of the Tertiary period at least, several large islands in the space between Madagascar and South India; but these had wide and deep channels between them, and their effect would probably have been favourable to the conveyance of heated water northward, by concentrating the currents, .and thus producing massive bodies of moving water analogous to the Gulf Stream of the Atlantic. 1 Less heat would thus be lost by evaporation and radiation in the tropical zone, and an impulse would be acquired which would carry the warm water into the north polar area. About the same period Australia was divided into two islands, separated by a wide channel in a north and south direction, (see Chapter XXII.), and through this another current would almost certainly set northwards, and be directed to the north-west by the southern extension of Malayan Asia. The more insular condition at this period of Australia, India, and North Africa, with the depression and probable fertility of the Central Asiatic plateau, would lead to the Indian Ocean being traversed by regular trade-winds instead of by variable monsoons, and thus 1 By referring to our map of the Indian Ocean showing the submarine banks indicating ancient islands (Chap. XIX.), it will be evident that the south~east trade winds-then exceptionally powerful- would cause a vast body of water Lo cuter the deep Arabian Sea. · CHAP. I'x .J Ml LD AHCTlC CLIMA'l'ES. 187 the constant vis a tergo, which is so efficient in the Atlantic, would keep up a steady and powerful current towards the northern parts of the Indian Ocean, and thence through the midst of the European archipelago to the northern seas. Now it is quite certain that such a condition as we have here sketched out would produce a wonderful effect on the climate of Central Europe and Western and Northern Asia. Owing to the warm currents being concentrated in inland seas, instead of being dispersed over a wide ocean like the North Atlantic, much more heat would be conveyed into the Arctic Ocean, and this would altogether prevent the formation of ice on the northern shores of Asia, which continent did not then extend nearly so far north and was probably deeply interpenetrated by the sea. This open ocean to the north, and the warm currents along all the northern lands, would so equalise temperature, that even the northern parts of Europe might then have enjoyed a climate fully equal to that of the warmer parts of New Zealand at the present day, and might have well supported the luxuriant vegetation of the Miocene period, even without any help from similar changes in the western hemisphere. 1 Condition of North A1ne1·ica during the Te,rtiary Period.-But changes of a somewhat similar character have also taken place in America, and the Pacific. An enormous area west of the 1 In his recently published Lectures on Physical Geography, Professor Haughton calculates, that more than ha1£ the solar heat of the torrid zone is carried to the temperate zones by ocean currents. 'l'he Gulf Stream itself carries one-twelfth of the total amount, but it is probable that a very small fraction of this quantity of heat reaches the polar seas owing to the wide area over which the current spreads in the North Atlantic. The corresponding stream of the Indian Ocean in Miocene times would have been fully equal to the Gulf Stream in heating· power, while, owing to its being so much more concentrated, a large proportion of its heat may have reached the polar area. But the Arctic Ocean occupies less than one-tentl• of the area of the tropical seas; so that, whatever proportion of the heat o.E the tropical zone was conveyed to it, would, by being concentrated into one-tenth of the surface, produce nn enormously increased effect. Taking this into consideration, we can hardly doubt that the opening of a sufficient passage from the Tnclinn Ocean to the Arctic seas would produce the effects above indicated. |