﻿Astronomical Theory of Ice Ayes and Genial Ages. 545 



When, therefore, there is such a complete absence of pro- 

 portion between the summer and winter temperatures and the 

 summer and winter sun-heats, what ground can be assigned 

 for the assumption of a rough proportion between the changes 

 in the summer or winter temperatures and the changes in the 

 corresponding sun-heats due to eccentricity? Even if the 

 temperatures themselves did show a certain rough proportion 

 to the sun heats, we could not expect as near an approach to 

 the proportion in their changes, because the disturbing ele- 

 ment might be large compared to the changes in the tempera- 

 tures, though small compared to the temperatures themselves. 

 As it is, the argument breaks down completely ; and the 

 assumption on which the supporters of the Astronomical 

 theory of the Ice Ages and Genial Ages rely is found to be 

 nothing but a vague speculation, not only unsupported by 

 the physical evidence bearing on the subject, but in direct 

 opposition to it. The weakness of the foundation, however, 

 has not deterred the astronomical geologists from erecting 

 on it an edifice of vast proportions. They affirm that there 

 have been a number of successive glaciations, alternating 

 with genial ages at intervals of about 10,500 years, that 

 glaciation must have been simultaneous all round the pole, 

 tliat a glacial age in the northern hemisphere must have been 

 accompanied by a genial one in the southern, and vice versa. 

 And as M. Leverrier's calculations show that the last period 

 of considerable eccentricity was about 100,000 years ago, the 

 last Ice Age must have occurred about that date. Dr. Croll 

 indeed fixes all the possible dates for three million years of 

 past and one million of future time ; but Sir Robert Ball 

 declines to fix the dates, not from any want of implicit con- 

 fidence in the theory, but merely from a probnble want of 

 accuracy in the determination of the astronomical constants 

 involved. I imagine, however, that Sir Robert Ball would 

 accept as fairly reliable Leverrier's calculations for this com- 

 paratively short period, though he does not accept those of 

 Croll for the longer period. Hence we may safely assume 

 with Croll, that the last Ice Ago must have terminated 

 80,000 years ago, if his theory be correct. 



When CrolPs theory was originally published, the general 

 opinion of geologists was that the date of the Ice Age as 

 fixed by the theory was far too recent. But the uniformi- 

 tarian school of those days has now been replaced by the 

 modern school, which, relying on calculations of the rate of 

 denudation, tends to fix the glaciation of North America as 

 hardly more than ten or twenty thousand years ago. Hence 

 geologists are now disposed to welcome any criticism of 



