288 On the Cause of Mild Polar Climates. 



the present day. If we, however, adopt this explanation, we 

 shall have to assume that the Alps were suddenly elevated at 

 the time when the bed containing the erratics began to be 

 deposited, and that they were as suddenly lowered when the 

 deposition of the bed came to a close — a conclusion certainly 

 very improbable. Had the lowering of the Alps been effected 

 by the slow processes of denudation, it must have taken a 

 long course of ages to have lowered them to the extent of 

 bringing the glacial state to a close. In this case there 

 ought to be a succession of beds indicating the long continu- 

 ance of cold conditions. Instead of this, however, we have a 

 glacial bed immediately preceded and succeeded by beds 

 indicating an almost tropical condition of climate. When 

 we take this circumstance into consideration, along with the 

 evidence adduced by Mr. J. S. Gardner as to the alternations 

 of warmer and colder conditions in the south of England and 

 other parts of Europe during the Eocene period, the convic- 

 tion is forced upon us that a high state of eccentricity is the 

 most rational explanation of these curious phenomena. 



The greater elevation of the Alps would undoubtedly in- 

 tensify the glacial condition of things, but it would not 

 originate it. The elevated character of the Alps, for example, 

 was no doubt the reason why the plains of Switzerland, during 

 the last glacial epoch, were so much more buried under ice than 

 other parts of Southern Europe; but their elevation was not that 

 which brought about the glaciation. for those plains were free 

 from ice both before and after the glacial epoch, though the 

 Alps were no doubt as high as they were during the ice-period. 



If we adopt the theory that these glacial conditions were 

 due to eccentricity, then we have, as I endeavoured to show 

 many years ago *, a clue to the probable absolute date of the 

 Middle Eocene and the Upper Miocene periods. There were, 

 as we have seen, two epochs during the Tertiary period when 

 the eccentricity was exceptionally high, viz. 2,500,000 years 

 ago and 850,000 years ago. The former might probably be 

 the date of the u Flysch ,} of the Eocene formation, and the 

 latter the date of the period when the Miocene erratics were 

 deposited in the icy sea near Turin. 



Some geologists have maintained that the climatic conditions 

 of the Tertiary period are utterly hostile to the Physical 

 Theory of Secular changes of Climate. The very reverse, 

 however, is the case ; for, as we have seen, several of the 

 facts of Tertiary climate can be explained on no other prin- 

 ciple than that of the theory. 



I shall next consider the facts bearing on Arctic Inter- 

 glacial Periods. 



* Phil. Mag., November 1868; 'Climate and Time/ chap. xxi. 





