CHAP. IX.] 



ANCIENT GLACIAL EPOCHS. 



165 



alternations of cold and warm phases 

 every 10,500 years. The diagram 

 also shows us another long-continued 

 period of high excentricity from 

 1,750,000 to 1,950,000 years ago, 

 and yet another almost equal to the 

 maximum 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 excen- 

 tricity is the great and dominating 

 agency in bringing on glacial epochs, 

 geographical 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 supposing that the 

 alternate phases of precession would 

 inevitably produce glaciation in one 

 hemisphere, and a proportionately 

 mild and equable climate in the 

 opposite hemisphere, then we should 

 have to look for evidence of ex- 

 ceptionally warm and exceptionally 

 cold periods, occurring alternately 

 and with several repetitions, with- 

 in a space of time which, geo- 

 logically speaking, is very short 

 indeed. 



