274 THE STORY OP THE EARTH AND MAN. 



discussion has had reference to them. But it would 

 be unfair not to inform the reader that this succession 

 of deposits after all belongs to the margins of our con- 

 tinents rather than to their great central areas. This 

 is the case at least in North America, where in the 

 region of the great lakes the oldest glaciated surfaces 

 are overlaid by thick beds of stratified clay, without 

 marine fossils, and often without either stones or 

 boulders, though these sometimes occur, especially 

 toward the north. The clay, however, contains 

 drifted fragments of coniferous trees. Above this 

 clay are sand and gravel, and the principal deposit 

 of travelled stones and boulders rests on these. I 

 cannot affirm that a similar succession occurs on the 

 great inland plains of Europe and Asia; but I think 

 it probable that to some extent it does. The ex- 

 planation of this inland drift by the advocates of a 

 great continental glacier is as follows : (1) In the 

 Pliocene period the continents were higher than 

 at present, and many deep valleys, since filled up, 

 were cut in them. (2) In the Post-pliocene these 

 elevated continents became covered with ice, by the 

 movement of which the valleys were deepened and 

 the surfaces striated. (3) This ice-period was followed 

 by a depression and submergence, in which the clays 

 were deposited, filling up old channels, and much 

 changing the levels of the land. Lastly, as the land 

 rose again from this submergence, sand and gravel 

 were deposited, and boulders scattered over the .surface 

 by floating ice. 



