THE CAUSE OF THE GLACIAL PERIOD. 



323 



Summarily stated, this theory is, that the passage from 

 the Tertiary to the Quaternary or Glacial period was char- 

 acterised by remarkable oscillations of land-level, and by 

 corresponding changes of climate, and of ice accumula- 

 tion in northern regions ; that the northern elevation was 

 connected with subsidence in the equatorial regions ; that 

 these changes of land-level were both initiated and, in the 

 main, continued by the interior geological forces of the 

 globe ; but that the very continental elevation which 

 mainly brought on the Glacial period added at length, in 

 the weight of the ice which accumulated over the elevated 

 region, a new force to hasten and increase the subsidence, 

 which would have taken place in due time in the natural 

 progress of the orographic oscillations already begun. 

 Professor Le Conte illustrates the subject by the following- 

 diagram, which, for simplicity's sake, treats the Glacial 



A _--^ 



pliocene 



GLACIAL 



CHAM PLAIN 

 Fig. 102. 



PRESENT 



epoch as one ; the horizontal line, A B, represents time 

 from the later Pliocene until now ; but it also represents 

 the present condition of things both as to land-level and 

 as to ice-accumulation. The full line, c d e, represents 

 the oscillations of land (and presumably of temperature) 

 above and below the present condition. The broken line 

 represents the rise, culmination, and decline of ice-accu- 

 mulation. The dotted line represents the crust-move- 

 ment as it would have been if there had been no ice-accu- 

 mulation. 



It is seen from the diagram that the ice-accumulation 

 culminated at a time when the land, under the pressure 

 of the ice-load, had already commenced to subside ; and 



