CHAP. VIII THE CAUSES OF GLACIAL EPOCHS Ul 



high degree of excentricity and a winter in aphelion. 

 When the glacial epoch is supposed to have been at its 

 maximum, about 210,000 years ago, the excentricity was 

 more than three times as great as it is now, and, according 

 to Dr. Croll's calculations, the mid-winter temperature of 

 the northern hemisphere would have been lowered 36° F., 

 while the winter half of the year would have been twenty- 

 six days longer than the summer half. This would bring 

 the January mean temperature of England and Scotland 

 almost down to zero or about 30° F. of frost, a winter 

 climate corresponding to that of Labrador, or the coast of 

 Greenland on the Arctic circle. But we must remember 

 that the summer would be very much hotter than it is 

 now, and the problem to be solved is, Avhether, suj^posing 

 the geography of the northern hemisphere to have been 

 identical with what it is now, the snow that fell in winter 

 would accumulate to such an extent that it would not be 

 melted in summer, and so go on increasing year by year 

 till it covered the whole of Scotland, Ireland, and Wales, 

 and much of England. Dr. Croll and Dr. Geikie answer 

 that it would. Sir Charles Lyell maintained that it 

 would only do so if geographical conditions were then 

 more favourable than they are now ; while the late Mr. 

 Belt has argued, that excentricity alone would not produce 

 the effect unless aided by increased obliquity of the ecliptic, 

 which, by extending the width of the polar regions, would 

 increase the duration and severity of the winter to such 

 an extent that snoAV and ice would be formed in the 

 Arctic and Antarctic resfions at the same time whether 

 the winter were in yerihelion or aphelion} 



The problem we have now to solve is a very difficult one, 

 because we have no case at all parallel to it from which 

 we can draw direct conclusions. It is, however, clear from 

 the various considerations we have already adduced, that 

 the increased cold of winter when the excentricity was 

 great and the sun in apthelion during that season, would 

 not of itself produce a glacial epoch unless the amount of 



^ I have somewhat modified this whole passage in the endeavour to 

 represent more accurately the difference between the views of Dr. Croll and 

 Sii' Charles Lyell. 



