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Show 192 ISLAND LIFE. [PART I. important alteration of the climates of the tempe~~te and .Arctic zones so long as favourable geographical condition~, such . as have been now sketched out, render the accumulatiOn of Ice impossible. The effect of a high excentricity in producing a glacial epoch was shown to ~e d~e to the capacity of s~ow and ice for storing up cold, and Its smgular power (when In ~arge masses) of preserving itself unmelted under a hot sun by Itself causing the interposition of a protective covering of cloud and vapour. But mobile currents of wann water have no such power of accumulating and storing up heat or cold from one year to another, though they do in a pre-emin~nt degree possess the power of equalising the temperature of wmter and su.mmer and of conveying the superabundant heat of the tropws to ameliorate the rigour of the .Arctic winters. However great was the difference between the amount of heat received from the sun in winter and summer in the .Arctic zon.e during a period of high excentricity and winter in aphelion, the inequality would be greatly diminished by the free ingress of warm currents to the polar area; and if this was sufficient to prevent any accumulation of ice, the summers would be warmed to the full extent of the· powers of the sun during the long polar day, which is such as to give the- pole at midsummer more heat during the twenty-four hours than the equator receives during its day of twelve hours. The only difference, then, that would be directly produced by the changes of excentricity and precession would be, that the summers would be at one period almost tropical, at the other of a more mild and uniform temperate character; while the winters would be at one time somewhat longer and colder, but never, probably, more severe than they are now in the west of Scotland. But though high excentricity would not directly modify the mild climates produced by the state of the northern hemisphere which prevailed during Cretaceous, Eocene, and Miocene times, it might indirectly affect it by increasing the mass of .Antarctic ice, and thus increasing the force of the trade-winds and theresulting northward-flowing warm currents. Now there are many peculiarities in the distribution of plants and of some groups of animals in the southern hemisphere, which render it almost certain CHAP. IX .] MILD ARCTfC CLIMATES. 193 that there has sometimes been a greater extension of the Antarctic lands during Tertiary times; and it is therefore not improbable that a more or less glaciated condition may have been a long persistent feature of the southern hemisphere, due to the peculiar distribution of land and sea which favours the production of ice-fields and glaciers. .And as we have seen that during the last three million years the excentricity has been almost always much higher than it is now, we should expect that the quantity of ice in the southern hemisphere will usually have been greater, and will thus have tended to increase the force of those oceanic currents which produce the mild climates of the northern hemisphere. Evidences of Climate in the Seconda1·y and Palceozoic epochs.W e have already seen, that so far back as the Cretaceous period there is the most conclusive evidence of the prevalence of a very mild climate not only in temperate but also in .Arctic lands, while there is no proof whatever, or even any clear indication, of early glacial epochs at all comparable in extent and severity with that which has so recently occurred; and we have seen reason to connect this state of things with a distribution of land and sea highly favourable to the transference of warm water from equatorial to polar latitudes. So far as we can judge by the plant-remains of our own country, the climate appears to have been almost tropical in the Lower Eocene period ; and as .we go further back we find no clear indications of a higher, but often of a lower temperature, though always warmer or more equable than our present climate. The abundant corals and reptiles of the Oolite and Lias indicate equally tropical conditions ; but further back, in the Trias, the flora and fauna become poorer, and there is nothing incompatible with a climate no warmer than that of the Upper Miocene. This poverty is still more marked in the Permian formation, and it is here that clear indications of ice-action are found in the Lower Permian conglomerates of the west of England. These beds contain abundant fragments· of various rocks, often angular and sometimes weighing half a ton, w bile others are partially rounded, and have polished and striated surfaces, just like the stones of the" till." They lie confusedly bedded in a red unstratified marl, 0 |
