﻿Astronomical Theory of Ice Ages and Genial Ages. 549 



If, instead of taking the epoch of greatest eccentricity, we 

 take the epoch of considerable eccentricity which occurred 

 about 100,000 years ago, as calculated by Leverrier, we must 

 take off about one fourth part from the above results. 



Since the whole of CrolPs theory of the stoppage of the 

 Gulf Stream depends on a previous vast cooling of the northern 

 hemisphere (accompanied by a heating of the southern hemi- 

 sphere), it is evident that, unless the temperature results here 

 given be utterly erroneous, they dispose once and for all of 

 the Astronomical theory of the Ice Age. I have, however, 

 prepared a further examination of that theory for the l Geo- 

 logical Magazine ' of January 1895. 



With regard to the Genial Age, the insignificance of the 

 changes effected by the astronomical cause is still more re- 

 markable. When the eccentricity is at its maximum, and 

 the winter occurs in perihelion, it will be seen from the figure 

 that the winter isothermals are shifted 2J or 3 degrees to the 

 north and the summer ones about 5 degrees to the south. 

 For instance, in our coldest 166 days lat. 57°*5 receives 

 just the same sun-heat as lat. 60° in the 166 days of the 

 " genial " winter. To dwell on the inadequacy of this as the 

 cause of the prevalence of tropical vegetation in Greenland 

 and Spitzbergen would be mere loss of time. But Croll has 

 an additional cause to account for a really Genial Age at the 

 pole, namely, increased obliquity of the ecliptic. The maxi- 

 mum effect of this would be to increase by about y 1 ^ part 

 the summer sun-heat received at the Poles. Hence, having 

 said that T T g more ice would be melted annually at the Poles 

 than at present, Croll proceeds to state that " the effects of 

 eccentricity and obliquity thus combined " [i. e., the maximum 

 obliquity synchronizing with the " genial " conditions of the 

 orbit] " would probably completely remove the polar ice-cap 

 from off the latter hemisphere [that enjoying the " genial " 

 conditions] and forest trees might then grow at the Pole." — 

 ' Climate and Time/ 2nd edition, pp. 402-403. It is as easy 

 to make such assertions as this as it is difficult to adduce any 

 shadow of a reason in support of them. That Croll should 

 have been the person to make the astounding statement that 

 the addition of -^g part of the summer heat at the Poles would 

 prevent ice forming there during the winter is the more 

 extraordinary, because in order to get over the difficulty that 

 the summer heat in the supposed glacial epoch was just as 

 much increased as the winter heat was diminished, it was 

 necessary for him, throughout the earlier portion of his book, 

 to insist on the absolute inefficiency of summer heat to melt 

 ice and snow or to mitigate the effects of winter cold (see pp. 

 58-66 and p. 324 of ' Climate and Time'). So far, then, as 



