CHAr. IX ANCIENT GLACIAL EPOCHS 



171 



occurred surpassing in severity 

 that of which we have such con- 

 vincing proofs, and consisting like 

 it of alternations of cold and 

 warm phases every 10,500 years. 

 The diagram also shows us another 

 long-continued period of high ex- 

 centricity from 1,750,000 to 

 1,950,000 years ago, and yet 

 another almost equal to the maxi- 

 mum 2,500,000 years back. These 

 may perhaps have occurred during 

 the Eocene and Cretaceous epochs 

 respectively, or all may have been 

 included within the limits of the 

 Tertiary period. As two of these 

 high excentricities greatly exceed 

 that which caused our glacial 

 epoch, while the third is almost 

 equal to it and of longer duration, 

 they seem to afford us the means 

 of testing rival theories of the 

 causes of glaciation. If, as Dr. 

 Croll argues, high excentricity is 

 the great and dominating agency 

 in bringing on glacial epochs, geo- 

 graphical changes being subor- 

 dinate, then there must have 

 been glacial epochs of great 

 severity at all these three periods ; 

 while if he is also correct in sup- 

 posing that the alternate phases 

 of precession would inevitably 

 produce glaciation in one hemi- 

 sphere, and a proportionately 

 mild and equable climate in the 

 opposite hemisphere, then we 

 should have to look for evidence 

 of exceptionally warm and excep- 

 tionally cold periods, occurring 



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