GROWTH OF GEOLOGY. 265 



now exist in the same localities has forced geologists 

 to admit the intervention of temperate stages, inter- 

 rupting the monotonous tyranny of the cold. Most 

 geologists now recognise at least two glacial epochs, 

 and many find strong evidence of three or even 

 more.* 



Causes. There has been no lack of theories as 

 to the causes of the Ice Age or of the Ice Ages. 

 Some of these theories seem too laborious and others 

 too ingenious, but it seems doubtful if all are not 

 premature. That is to say, we have to discover 

 whether the post-Tertiary Ice Age, so obvious in 

 Europe, was universal or not; and we have also to 

 decide as to the periodicity of the recurrence of 

 glacial conditions in older geological periods, which 

 is almost too difficult a problem. 



Since the days of Agassiz and Charpentier, the 

 causes of the Ice Age have been sought in two direc- 

 tions which were to some extent hinted at by the 

 pioneers. Some have appealed to cosmical or astro- 

 nomical changes, while others have been satisfied 

 with geographical factors. 



Adhemar in 1842 seems to have suggested a 

 theory, which was rehabilitated by James Croll 

 (1875), that a slight alteration in the eccentricity 

 of the earth's orbit might be the essential cause of 

 glacial conditions. 



Lyell may be taken as a representative of the 

 view that geographical changes may have brought 

 about glacial conditions. Depressions allowing the 

 Arctic currents to overflow parts of the continents, 

 elevation of large areas above the snow-line, de- 

 flections of ocean currents, and so on, have been as- 

 sumed as possible causes. 



* See Prof. James Geikie's Great Ice Age and Prehistoric 

 Europe. 



