But if this is true, then the reverse change, bringing the sun back into exactly the same position with regard to us as it is in now (all geographical and physical conditions remaining unchanged), would certainly bring back again our present milder climate. The change either way would not probably be very great, but it might be sufficient to bring the snow-line down to 3,000 feet in Scotland, so that all the higher mountains would have their tops covered with perpetual snow. This perpetual snow, down to a fixed line, would be kept up by the needful supply of snow falling during autumn, winter, and spring, and this would, as we have seen, depend mainly on the increased length and greatly increased cold of the winter. As both the duration and the cold of winter decreased the amount of snow would certainly decrease, and of this lesser quantity of snow

a larger proportion would be melted by the longer, though somewhat cooler summer. This would follow because the total amount of sun-heat received during the summer would be the same as before, while it would act on a less quantity of snow; there would thus be a smaller surface to reflect the heat, and a smaller condensing area to produce fogs, while the diminished intensity of the sun would produce a less dense canopy of clouds, which have been shown to be of prime importance in checking the melting of snow by the sun. We have considered this case, for simplicity of reasoning, on the supposition that all geographical and physical causes remained unchanged. But if an alteration of the climate of the whole north temperate and Arctic zones occurred, as here indicated, this would certainly affect both the winds and currents, in the manner already explained (see p. [142]), so as to react upon climate and increase the differences produced by phases of precession. How far that effect would be again increased by corresponding but opposite changes in the southern hemisphere it is impossible to say. It may be that existing geographical and physical conditions are there such potent agents in producing a state of glaciation that no change in the phases of precession would materially affect it. Still, as the climate of the whole southern hemisphere is dominated by the great mass of ice within the Antarctic circle, it seems probable that if the winter were shorter and the summer longer the quantity of ice would slightly diminish; and this would again react on the northern climate as already fully explained.

The Essential Principle of Climatal Change Restated.—The preceding discussion has been somewhat lengthy, owing to the varied nature of the facts and arguments adduced, and the extreme complexity of the subject. But if, as I venture to urge, the principle here laid down is a sound one, it will be of the greatest assistance in clearing away some of the many difficulties that beset the whole question of geological climates. This principle is, briefly, that the great features of climate are determined by a combination of causes, of which geographical conditions and the degree of excentricity of the earth's orbit are by

far the most important; that, when these combine to produce a severe glacial epoch, the changing phases of precession every 10,500 years have very little, if any, effect on the character of the climate, as mild or glacial, though it may modify the seasons; but when the excentricity becomes moderate and the resulting glaciation less severe, then the changing phases of precession bring about a considerable alteration, and even a partial reversal of the glacial conditions.

The reason of this may perhaps be made clearer by considering the stability of either extreme glacial conditions or the entire absence of perpetual ice and snow, and the comparative instability of an intermediate state of climate. When a country is largely covered with ice, we may look upon it as possessing the accumulated or stored-up cold of a long series of preceding winters; and however much heat is poured upon it, its temperature cannot be raised above the freezing point till that store of cold is got rid of—that is, till the ice is all melted. But the ice itself, when extensive, tends to its own preservation, even under the influence of heat; for the chilled atmosphere becomes filled with fog, and this keeps off the sun-heat, and then snow falls even during summer, and the stored-up cold does not diminish during the year. When, however, only a small portion of the surface is covered with ice, the exposed earth becomes heated by the hot sun, this warms the air, and the warm air melts the adjacent ice. It follows, that towards the equatorial limits of a glaciated country alternations of climate may occur during a period of high excentricity, while nearer the pole, where the surface is almost completely ice-clad, no amelioration may take place. The same argument will, to some extent apply, inversely, with mild Arctic climates; but this is a subject which will be discussed in the next chapter.

This view of the character of the last glacial epoch appears to correspond very closely with the facts adduced by geologists. The inter-glacial deposits never exhibit any indication of a climate whose warmth corresponded to the severity of the preceding cold, but rather of a partial amelioration of that cold; while it is only the very latest

of them, which we may suppose to have occurred when the excentricity was considerably diminished, that exhibit any indications of a climate at all warmer than that which now prevails.[[57]]

Probable Date of the Glacial Epoch.—The state of extreme glaciation in the northern hemisphere, of which we gave a general description at the commencement of the preceding chapter, is a fact of which there can be no doubt whatever, and it occurred at a period so recent geologically that all the mollusca were the same as species still living. There is clear geological proof, however, that considerable changes of sea and land, and a large amount of valley denudation, took place during and since the glacial epoch,

while on the other hand the surface markings produced by the ice have been extensively preserved; and taking all these facts into consideration, the period of about 200,000 years since it reached its maximum, and about 80,000 years since it passed away, is generally considered by geologists to be ample. There seems, therefore, to be little doubt that in increased excentricity we have found one of the chief exciting causes of the glacial epoch, and that we are therefore able to fix its date with a considerable probability of being correct. The enormous duration of the glacial epoch itself (including its interglacial mild or warm phases), as compared with the lapse of time since it finally passed away, is a consideration of the greatest importance, and has not yet been taken fully into account in the interpretation given by geologists of the physical and biological changes that were coincident with, and probably dependent on, it.

Changes of the Sea-level Dependent on Glaciation.—It has been pointed out by Dr. Croll, that many of the changes of level of sea and land which occurred about the time of the glacial epoch may be due to an alteration of the sea-level caused by a shifting of the earth's centre of gravity; and physicists have generally admitted that the cause is a real one, and must have produced some effect of the kind indicated. It is evident that if ice-sheets several miles in thickness were removed from one polar area and placed on the other, the centre of gravity of the earth would shift towards the heavier pole, and the sea would necessarily follow it, and would rise accordingly. Extreme glacialists have maintained that during the height of the glacial epoch, an ice-cap extended from about 50° N. Lat. in Europe, and 40° N. Lat. in America, continually increasing in thickness, till it reached at least six miles thick at the pole; but this view is now generally given up. A similar ice-cap is however believed to exist on the Antarctic pole at the present day, and its transference to the northern hemisphere would, it is calculated, produce a rise of the ocean to the extent of 800 or 1,000 feet. We have, however, shown that the production of any such ice-cap is improbable if not impossible, because snow and ice can