CHAPTER X. 



THE DATE OF THE GLACIAL PEEIOD. 



Ix approaching the subject of glacial chronology, we 

 are compelled to recognise at the outset the approximate 

 character of all our calculations. Still, we shall find that 

 there are pretty well-defined limits of time beyond which 

 it is not reasonable to place the date of the close of the 

 Glacial period ; and, where exact figures cannot be de- 

 termined, it may yet be of great interest and importance 

 to know something near the limits within which our spec- 

 ulations must range. 



For many years past Mr. CrolPs astronomical theory 

 as to the cause of the Glacial period has been considered 

 in certain circles as so nearly established that it has been 

 adopted by them as a chronological table in which to insert 

 a series of supposed successive Glacial epochs which are 

 thought to have characterised not merely the Quaternary 

 epoch but all preceding geological eras. What we have 

 already said, however, respecting the weakness of Mr. 

 CrolPs theory is probably sufficient to discredit it as a 

 chronological apparatus. We will therefore turn immedi- 

 ately to the more tangible evidences bearing upon the 

 subject. 



The data directly relating to the length of time which 

 separates the present from the Glacial period are mainly 

 connected with two classes of facts : 



1. The amount of erosion which has been accomplished 

 by the river systems since the Glacial period; and 2. The 



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