ANCIENT GLACIERS. 



125 



successive glacial and interglacial epochs, and that when 

 there was a period of elevation west of the Kocky Mount- 

 ains there was a period of subsidence to the east, and vice 

 versa. In short, he supposes that the east and west for a 

 long time played a game of seesaw, with the Eocky 

 Mountains as the fulcrum. We give his scheme in tabu- 

 lated form. 



Scheme of Correlation of the Phenomena of the Glacial 

 Period in the Cordilleran Region and in the Region 

 of the Great Plains. 



CORDILLERAN REGION. 



Cordilleran zone at a high 

 elevation. Period of most severe 

 glaciation and maximum devel- 

 opment of the great Cordilleran 

 Glacier. 



Gradual subsidence of the 

 Cordilleran region and decay of 

 the great glacier, with deposition 

 of the boulder-clay of the interi- 

 or plateau and the Yukon basin, 

 of the lower boulder-clay of the 

 littoral and probably also, at a 

 later stage (and with greater sub- 

 mergence), of the interglacial 

 silts of the same region. 



Re-elevation of the Cordil- 

 leran region to a level probably 

 as high as or somewhat higher 

 than the present. Maximum of 

 second period of glaciation. 



REGION OF THE GREAT PLAINS. 



Correlative subsidence and 

 submergence of the great plains, 

 with possible contemporaneous 

 increased elevation of the Lau- 

 rentian axis and maximum de- 

 velopment of ice upon it. De- 

 position of the lower boulder- 

 clay of the plains. 



Correlative elevation of the 

 western part, at least, of the great 

 plains, which was probably more 

 or less irregular and led to the 

 production of extensive lakes in 

 which interglacial deposits, in- 

 cluding peat, were formed. 



Correlative subsidence of the 

 plains, which (at least in the 

 western part of the region) ex- 

 ceeded the first subsidence and 

 extended submergence to the base 

 of the Rocky Mountains near the 

 forty-ninth parallel. Formation 

 of second boulder-clay, and (at a 

 later stage) dispersion of large 

 erratics. 



