THE HUMAN OR PRESENT PERIOD. 519 



imply a westward elevation. These and collateral phenomena, taken 

 with the remarkable movement of the Keewatin ice-sheet from what 

 is now the lower to what is now the higher side of the plains, seem 

 best satisfied by the view that until about the close of the Glacial period 

 the western side of the Great Plains was lower than now, or the eastern 

 side higher than now, relative to the common surface-level. The 

 composite view, that the area under the great ice-sheet was relatively 

 depressed and the area on its western border relatively elevated, is 

 perhaps the best special interpretation. On the western side of the con- 

 tinent there is much evidence of recent movement, some of which 

 appears to have taken place since the close of the Glacial period, as 

 usually defined. Similar phenomena affect other continents. 



It is not therefore wholly clear whether the present is to be regarded 

 as a part of that time of deformation which had its climax in the Plio- 

 cene, or whether it belongs rather to the initial stage of a period of 

 quiescence that is yet to develop characteristically. It may perhaps 

 best be regarded as a transition from the one to the other. 



In any case, two movements are probably peculiar to it : (1) the 

 elevation of the glaciated surface due to the removal of the weight 

 of the ice-sheet and its return to a normal temperature, and (2) the 

 restoration of the water temporarily locked up in the ice-sheets to 

 the ocean, which tended to raise the sea-level, while the removal of 

 the attraction of the ice-mass and the accompanying change in the 

 position of the earth's center of gravity tended to cause the waters to 

 recede from the glaciated region. 1 When the special effects of these 

 exceptional agencies are deducted, the amount of the post-glacial 

 movement is appreciably reduced, which is favorable to the view 

 that the earth is now passing slowly into a period of quiescence. 



The suggestions of existing physiography. — This view is further 

 strengthened by the present physiographic features of the earth's sur- 

 face. These are a direct inheritance from the Tertiary deformations 

 superposed upon pre-existing configurations. They have been modi- 

 fied by the gradational agencies that have been working since, includ- 

 ing the recent glaciation. They should tell us whether the face of 



1 The attraction of the ice-mass is discussed mathematically by R. S. Woodward, 

 Bulletin U. S. G. S., No. 48, 1888; also, 6th Annual U. S. G. S., 1884-85, pp. 291-97. 

 The effects of the accumulation and the melting of ice are discussed by Croll, 

 Climate and Time, 1890, p. 388. 



