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Show 198 ISLAND LIFE. (PART I. condition of the northern hemisphere is the result of the peculiar distribution of land and water upon the globe; and the general permanence of the position of the continental and oceanic areas-which we have shown to be proved by so many distinct lines of evidence-is also implied by the general st,ability of climate throughout long geological periods. The land surface of our earth appears to have always consisted of three great masses in the north temperate zone, narrowing southward, and terminating in three comparatively narrow extremities represented by Southern America, South Africa and Australia. Towards the north these masses have approached each other, and have sometimes become united ; leaving beyond them a considerable area of open polar sea. Towards the south they have never been much further prolonged than at present, but far beyond their extremities an extensive mass of land has occupied the south polar area. This arrangement is such as would cause the northern hemisphere to be always (as it is now) warmer than the southern, and this would lead to the preponderance of northward winds and ocean currents, and would bring about the concentration of the latter in three great streams carrying warmth to the northpolar regions. These streams would, as Dr. Croll has so well shown, be greatly increased in power by the glaciation of the south polar land ; and whenever any considerable portion of this ]and was elevated, such a condition of glaciation would certainly be brought about, and would be heightened whenever a high degree of excentricity prevailed. It appears to be the general opinion of geoloo-ists that the . b gre~t contments have undergone a process of development from ear lie~ to late_r times. Professor Dana says : " The North Amenc~n ~ontment,. whi?h since early time had been gradually expandmg m each directwn from the northern Azoic eastward we~twar~, ~nd southward, and which, after the Pal~ozoic, wa~ fimshed m Its rocky foundation, excepting on the borders of the Atlantic .and Pacific and the area of the Rocky Mountains, had reached Its full expansion at the close of the Tertiary period. The progress from the first was uniform and systematic: the 01-IAl'. IX.) GEOLOGICAL CLIMATES. 199 land was at all times simple in outline; and its enlargement took place with almost the reguln.rity of an exogenous plant." 1 A similar development undoubtedly took place in the European area, which was apparently never so compact and so little interpenetrated by the sea as it is now, whHe Europe and Asia have only become united into one unbroken mass since late Tertiary times. If, however, the greater continents have become more compact and massive from age to age, and have received their chief extensions northward at a coii).paratively recent period, while the antarctic lands had a corresponding but somewhat earlier development, we have all the conditions requisite to explain the persistence, with slight fluctuations, of warm climates far into the nOTth-polar :.1rea throughout Palreozoic, Mesozoic, and Tertiary times. At length, during the latter part of the Tertiary epoch, :.1 considerable elevation tool< place, closing up several of the water passages to the north, and mising up extensive areas in the Arctic regions to become the receptacle of snow and ice-fields. This elevation is indicated by the a.bundance of Miocene and the absence of Pliocene deposits in the Arctic zone and the considerable altitude of many Miocene rocks in Europe and North America ; and the occurrence at this time of a long-continued period of high excentricity necessarily brought on the glacial epoch in the manner already described in our last chapter. We thus see that the last glacial epoch was the climax of a great process of continental development which has been going on throuo-hout long geological ages; and that it was the direct 0 • consequence of the north temperate a.nd polar land havmg attained a great extension and a considerable altitude just at the time when a phase of very high excentricity was coming on. Throughout earlier Tertiary and Secondary times an equally high excentricity often occurred, but it never produced a glacial epoch, because the north temperate and polar :.1reas had less high land, and were more freely open to the influx. of warm oceanic currents. But wherever great plateaux with lofty mountains occurred in the temperate zone a considerable local 1 .Afant,aJ of Gf'nlo,rm, 2nd Eo. p. 52fl. |
