ICE ATTRACTION AND EPEIROGENIC MOVEMENTS. 229 



The departure of the sheets of laud ice which spread drift formatious 

 over the uortheru part of North America, uorthwesteru Europe, and Pata- 

 gonia, was iu each of these great and widely separated areas attended by 

 a depi'ession of the land. While each of these ice-sheets was melting 

 away, the land on which it had lain was somewhat lower than now, and its 

 coasts were partially submerged by the sea. These are the only extensive 

 regions of the earth which have lately borne ice-sheets that have now 

 melted, and it seems* to be a most reasonable- inference that the vast weight 

 of their burdens of ice was an important element in the causes of their 

 subsidence. Since the disappearance of their ice-sheets, each of these con- 

 tinental areas has been uplifted, probably in large measure because of the 

 withdrawal of the ice-load. In Europe these ejjeirogenic movements of 

 depression and reelevation seem to have been more nearly proportionate to 

 the volume and extent of the ice-sheet than on our continent. Both in 

 North and South America, other great epeirogenic movements, affecting 

 large areas which Avere never glaciated, have been iu progress, apparently 

 during the same time and in close association with the oscillations of the 

 glaciated regions. In another chapter, treating more fully of the causes of 

 the changes in level of the beaches of Lake Agassiz, these complex move- 

 ments of our continent and other parts of the world during the Quaternary 

 era will be reviewed for the })urpose of learning, if possible, how and why 

 such sfibsidences and uplifts of great areas take place. 



At present we need only to inquire what were the amounts of depres- 

 sion of the basin of Lake Agassiz and of contiguous parts of this continent, 

 since these would affect the history of tliis lake iu its reduction from its 

 highest level to its lower shore-lines and to Lake Winnipeg; and what was 

 the manner of the reelevation, whether by regular and continued move- 

 ment, or by intermittent uplifting and stages of repose, and whether the 

 basin was uplifted«differentially as a whole or in successive portions. 



Depression of the continent shoivn by coastal submergence. — Answering the 

 first jjart of these inquiries so far as we may by the known extent of oceanic 

 submergence of the land when it became uncovered from the ice, we have 

 the testimony of marine fossils in beds overlying the glacial drift, which 

 show that the country southwest of Hudson and James bays then stood 



