PHYSICAL AND CLIMATAL CONDITIONS. 123 



" Mr. Belt, in an interesting paper (Quart. Journ. Geol. 

 Soc., Nov., 1874), deals with similar difficulties in explain- 

 ing the glaciation of Siberia. The northern part of Asia 

 appears in many ways to resemble that of America ; 

 surrounded by mountain-chains on all sides save the 

 north, it is a sort of interior continental basin covered 

 with 'vast level sheets of sand and loam.' As in the 

 interior regions of America, marine shells are absent, or 

 are only found along the low ground of the northern 

 coast. To account for these facts, Mr. Belt resorts to a 

 theory first suggested by him eight years ago, by which 

 he supposes the existence of a polar ice-sheet capable of 

 blocking up the entire northern front of the country, and 

 damming^ back its waters to form an immense fresh-water 

 lake. The outfall of this lake, during its highest stage, 

 he supposes to have been through the depression between 

 the southern termination of the Ourals and the western 

 end of the Altai to the Aral and Caspian seas." 



The main difficulty in the way of this masterly ex- 

 planation is the great height above the sea of the western 

 part of the plains ; but this is now met by the probability 

 of the depression of the plains contemporaneously with 

 the elevation of the Cordillera, since suggested by the 

 author of the extract. To the absence of marine shells 

 from the deposits of the plains no importance need be 

 attached. The water may have been cold and brackish, 

 and in all geological periods gravels, sands and conglom- 

 erates usually have few marine fossils. 



In 1883 I had an opportunity of going over the same 

 ground, and my notes respecting it are as follows : * 



The Great Missouri coteau to which Dr. Cr. M. 

 Dawson first directed prominent attention as a glacial 



* See Journal Geol. Society of London, 1883. 



