CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 133 



and water ; and though some heat does penetrate the 

 ground and is stored up there, this is so little in proportion 

 to the whole amount received, and the larger part of it is 

 so soon given out from the surface layers, that any surplus 

 heat that may be thus preserved during one summer of the 

 temperate zones rarely or never remains in sufficient 

 quantity to affect the temperature of the succeeding 

 summer, so that there is no such thing as an accumulation 

 of earth-heat from year to year. But, though heat cannot, 

 cold can be stored up to an almost unlimited amount, owing 

 to the peculiar property water possesses of becoming solid 

 at a moderately low temperature ; and as this is a subject 

 of the very greatest importance to our inquiry — the whole 

 question of the possibility of glacial epochs and warm periods 

 depending on it — we must consider it in some detail. 



Effects of Snow on Climate. — Let us then examine the 

 very different effects produced by water falling as a liquid 

 in the form of rain, or as a solid in the form of snow, 

 although the two may not differ from each other more than 

 two or three degrees in temperature. The rain, however 

 much of it may fall, runs off rapidly into streams and rivers, 

 and soon reaches the ocean, a small portion only sinking 

 into the earth and another portion evaporating into the 

 atmosphere. If cold it cools the air and the earth some- 

 what while passing through or over them, but produces no 

 permanent effect on temperature, because a few hoars of 

 sunshine restore to the air or the surface-soil all the heat 

 they had lost. But if snow falls for a long time, the effect, 

 as we all know, is very different, hecciuse it has no mobility . 

 It remains where it fell and becomes compacted into a 

 mass, and it then keej^s the earth below it and the air 

 above, at or near the freezing-point till it is all melted. If 

 the quantity is great it may take days or weeks to melt ; 

 and if snow continues falling it goes on accumulating all 

 over the surface of a country (which water cannot do), and 

 may thus form such a mass that the warmth of the whole 

 succeeding summer may not be able to melt it. It then 

 produces perpetual snow, such as we find above a certain 

 altitude on all the great mountains of the globe ; and when 

 this takes place cold is rendered permanent, no amount of 



