CHAP. IX 



ANCIENT GLACIAL EPOCHS 



177 



the rocks and carried out to sea by the glaciers themselves. 

 Moreover, as icebergs float far beyond the limits of the 

 countries which gave them birth, these ice-borne materials 

 would be largely imbedded in deposits forming from the 

 denudation of countries which had never been glaciated, or 

 from which the ice had already disapjDeared. 



But if every period of high excentricity produced a 

 glacial epoch of greater or less extent and severity, then, 

 on account of the frequent occurrence of a high phase of 

 excentricity during the three million years for which we 

 have the tables, these boulder and rock-strewn deposits 

 would be both numerous and extensive. Four hundred 

 thousand years ago the excentricity was almost exactly the 

 same as it is now, and it continually increased from that 

 time up to the glacial epoch. Now if we take double the 

 present excentricity as being sufficient to produce some 

 glaciation in the temperate zone, we find (by drawing out 

 the diagram at p. 171 on a larger scale) that during 1,150,000 

 years out of the 2,400,000 years immediately preceding 

 the last glacial epoch, the excentricity reached or exceeded 

 this amount, consisting of sixteen separate epochs, divided 

 from each other by periods varying from 30,000 to 200,000 

 years. But if the last glacial epoch was at its maximum 

 200,000 years ago, a space of three million years will 

 certainly include much, if not all, of the Tertiary period ; 

 and even if it does not, we have no reason to suppose that 

 the character of the excentricity would suddenly change 

 beyond the three million years. 



It follows, therefore, that if periods of liigh excentricity, 

 like that which appears to have been synchronous with our 

 last glacial epoch and is generally admitted to have been 

 one of its efficient causes, always produced glacial epochs 

 (with or without alternating warm periods), then the whole 

 of the Tertiary deposits in the north temperate and Arctic 

 zones should exhibit frequent alternations of boulder and 

 rock-bearing beds, or coarse rock-strewn gravels analogous 

 to our existing glacial drift, and with some corresponding 

 change of organic remains. Let us then see what 

 evidence can be adduced of the existence of such 

 deposits, and whether it is adequate to support the 



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