ANCIENT GLACIERS IN EASTERN HEMISPHERE. 189 



deposits of the Grlacial period, both in western Europe and 

 in America. 



A species of loess — differing, however, somewhat in 

 color from that on the Ehine — covers the plains of north- 

 eastern France up to an elevation of 700 feet above the 

 the sea, where, as we have already said, it overlies the high- 

 level gravels of the Seine and the Somme. Above this 

 height the superficial soil in France is evidently merely 

 the decomposed upper surface of the native rock. 



The probable explanation of all these deposits, included 

 under the term " loess," is the same as that already given 

 by Prestwich of the loamy deposits of northern France. 

 But in case of rivers, which, like the Ehine, encountered 

 the ice-front in their northward flow, a flooded condition 

 favouring the accumulation of loess was doubtless promoted 

 by the continental ice-barrier. In the case of the Danube 

 and the Rhone, however, where there was a free outlet 

 away from the glaciated region, the loess in the upper part' 

 of the valleys must have accumulated in connection with 

 glacial floods quite similar to those which we have de- 

 scribed as spreading over the imperfectly formed water- 

 courses of the Mississippi basin during the close of the Ice 

 age. That the typical loess is of glacial origin is pretty 

 certainly shown, both by its distribution in front of gla- 

 ciers and by its evident mechanical origin when studied 

 under the microscope. It is, in short, the fine sediment 

 which gives the milky whiteness to glacial rivers. 



In central Eussia there is a considerable area in which 

 the glacial conditions were, in oue respect, similar to those 

 in the northern part of the Mississippi Valley in the United 

 States. In both regions the continental ice-sheet sur- 

 mounted the river partings, and spread over the upper 

 portion of an extensive plain whose drainage was to the 

 south. The Dnieper, the Don, and the western branch of 

 the Volga, like the Ohio and the Mississippi, have their 

 head-waters in the glaciated region. In some other respects, 



