CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 



153 



remarkable in how many cases geologists have independ- 

 ently deduced some alternations of climate as probable. 

 Such are the interglacial deposits indicating a mild climate, 

 both in Europe and America ; an early phase of very 

 severe glaciation when the " till " was deposited, with 

 later less extensive glaciation when moraines were left in 

 the valleys ; several successive periods of submergence and 

 elevation, the later ones becoming less and less in amount, 

 as indicated by the raised beaches slightly elevated above 

 our present coast line ; and lastly, the occurrence in the 

 same deposits of animal remains indicating both a warm 

 and a cold climate, and especially the existence of the 

 hippopotamus in Yorkshire not long after the period of 

 extreme glaciation. 



But although the evidence of so/?ic alternations of climate 

 seems indisputable, and no suggestion of any adequate 

 cause for them other than the alternating phases of 

 precession during high excentricity has been made, it by 

 no means follows that these changes were always very 

 great — that is to say, that the ice completely disappeared 

 and a warm climate prevailed throughout the whole year. 

 It is quite evident that during the height of the glacial 

 epoch there was a combination of causes at work which led 

 to a large portion of North-western Europe and Eastern 

 America being buried in ice to a greater extent even than 

 Greenland is now, since it certainly extended beyond the 

 land and filled up all the shallow seas between our islands 

 and Scandinavia. Among these causes we must reckon a 

 diminution of the force of the Gulf Stream, or its beingf 

 diverted from the north-western coasts of Europe; and 

 what we have to consider is, whether the alteration from 

 a long cold winter and short hot summer to a short 

 mild winter and long cool summer would greatly affect 

 the amount of ice if the ocean currents remained the 

 same. The force of these currents is, it is true, by our 

 hypothesis, modified by the increase or diminution of the 

 ice in the two hemispheres alternately, and they then 

 react upon climate ; but they cannot be thus changed till 

 after the ice-accumulation has been considerably affected 

 by other causes. Their direction may, indeed, be greatly 



