GLACIAL EPOCHS AND POLAR CLLMATES. 31 



details of the causes of Glacial Epochs, or of the 

 various theories propounded to explain them. To 

 such of my readers who may desire to acquaint 

 themselves further with this portion of the subject, 

 I would refer to Dr. Croll's Climate and Time in 

 their Geological Relations, and to Wallace's Island 

 Life, in which latter work especially the whole 

 phenomenon is treated in an original and very 

 masterly manner. Briefly stated, Glacial Epochs 

 are caused by the slow and irregular periods of 

 great eccentricity in the earth's orbit, combined 

 with the precession of the equinoxes, and initiated 

 by high land and unusual amount of moisture 

 (which are favourable to the accumulation and 

 storage of ice and snow) at the glaciated pole. It 

 appears pretty certain that periods of high eccen- 

 tricity cannot 'produce glaciation unless the land 

 area of the pole with its winter in aphelion is 

 favourably adapted, and the warm ocean currents 

 have been diverted from the polar area. All the 

 geological evidence hitherto collected is against 

 the supposition that any one or more Glacial 

 Epochs occurred during the Secondary and Ter- 

 tiary Periods. As Wallace clearly demonstrates, 

 this vast expanse of time seems to have been char- 

 acterized by a uniform warm or temperate climate, 

 admitting of a luxuriant vegetable growth up to 

 the highest latitudes at present reached by man. 

 The evidence points unquestionably to the fact 

 that the Post-Pliocene Glacial Epoch was an ex- 

 ceptional phenomenon — no such vast and terrible 



