CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 



159 



far the most important ; that, when these combine to pro- 

 duce a severe glacial epoch, the changing phases of pre- 

 cession 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 be- 

 comes moderate and the resulting glaciation less severe, 

 then the changing phases of precession bring about a con- 

 siderable alteration, and even a partial reversal of the 

 glacial conditions. 



The reason of this may perhaps be made clearer by con- 

 sidering 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, Avhen 

 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 ex- 

 posed earth becomes heated by the hot sun, this warms 

 the air, and the warm air melts the adjacent ice. It fol- 

 lows, 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 sur- 

 face 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 ap- 

 pears 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 



