CHAP. IX MILD AKCTIC CLIMATES 181 



abundantly apparent. The Oligocene of Northern Ger- 

 many and Belgium, and the Miocene of those countries 

 and of France, have also afforded a rich molluscan fauna, 

 which, Kke that of the Eocene, has as yet presented no 

 indication of the intrusion of anything to interfere with its 

 uniformly sub-tropical character." ^ 



This is sufficiently striking ; but when we consider that 

 this enormous series of deposits, many thousand feet in 

 thickness, consists wholly of alternations of clays, sands, 

 marls, shales, or limestones, with a few beds of pebbles or 

 conglomerate, not one of the Avliole series containing 

 irreo'ular blocks of foreiojn material, boulders or OTavel, such 

 as we have seen to be the essential characteristic of a glacial 

 epoch ; and when we find that this same general character 

 pervades all the extensive Tertiary deposits of temperate 

 North America, we shall, I think, be forced to the con- 

 clusion that no general glacial epochs could have occurred 

 during their formation. It must be remembered that the 

 " imperfection of the geological record " will not help us 

 here, because the series of Tertiary deposits is unusually 

 complete, and we must suppose some destructive agency 

 to have selected all the intercalated glacial beds and to 

 have so completely made away with them that not a 

 fragment remains, while preserving all or almost all the 

 inter glacicd beds ; and to have acted thus capriciously, not 

 in one limited area only, but over the whole northern 

 hemisphere, with the local exceptions on the flanks of great 

 mountain ranges already referred to. 



Tem]jerate Climates in the Arctic Regions. — As we have 

 just seen, the geological evidence of the persistence of sub- 

 tropical or warm climates in the north temperate zone 

 during the greater part of the Tertiary period is almost 

 irresistible, and we have now to consider the still more 

 extraordinary series of observations which demonstrate 

 that this amelioration of climate extended into the Arctic 

 zone, and into countries now almost wholly buried in snow 

 and ice. These warm Arctic climates have been explained 

 by Dr. Croll as due to periods of high excentricity with 

 winter in perihelion, a theory which implies alternating 

 ^ Geological Magazine, 1876, p. 392 



