226 HISTORY OF THE EUROPEAN FAUNA. 



moraine of the great northern ice-sheet, we might 

 expect to find that the Caspian deposits were not 

 contemporaneous with it. Curiously enough, it has 

 been shown by Mr. Sjogren that all observations 

 have pointed to the fact that these two deposits do 

 not overlie one another, but occur side by side, 

 and are therefore contemporaneous. This seems to 

 warrant our belief, that while the boulder-clay was 

 being laid down in Northern Europe, the Aralo- 

 Caspian Sea had some communication with the 

 White Sea. 



The boulder-clay of Northern Continental Europe, 

 as already stated, is now generally recognised to 

 be the product of a huge ice-sheet which invaded 

 the lowlands of Continental Europe from the Scan- 

 dinavian mountains. Though Alpine glaciers at the 

 present day produce little or no ground moraine, 

 these ancient larger ice-sheets, or "mers-de-glace," are 

 believed to have deposited immense layers of mud 

 containing scratched and polished stones. Many of 

 the latter have been carried great distances from 

 their source of origin. The Scandinavian ice-sheet 

 is supposed to have advanced as far south as the 

 line indicated on the map, after which it gradually 

 retreated. On this point, however, as in almost every 

 detail connected with the Glacial period, geologists 

 are at variance. Professor James Geikie maintains, 

 that there were no less than four Glacial periods, 

 separated from one another by milder inter-glacial 

 phases. On the Continent the view of two Glacial 



