THE SIBERIAN MIGRATION. 237 



position of Scandinavia at this time an isthmus of 

 land with a high mountain range lying between the 

 warm Atlantic and the cold Arctic Sea the snowfall 

 must have been excessive, and large glaciers were 

 evidently forming. These produced icebergs as soon 

 as the lower parts had advanced to the Baltic coast- 

 land and deposited their detritus in the sea. Immense 

 masses of mud and stones were thus cast to the 

 bottom of the sea, and under these circumstances no 

 delicate mollusca or other marine life probably could 

 have developed within a considerable distance from 

 the shore. To judge from the direction pursued by 

 the majority of the boulders from their source of 

 origin, the prevailing current during the deposition of 

 the lower boulder-clay was from north-west to south- 

 east. It is possible that little marine life, except 

 free-swimming forms, would have been able to live 

 within the Russian area of this sea. But the free- 

 swimming larvae of molluscs and other surface 

 species were not prevented from passing from the 

 White Sea south-westward, and in sheltered localities 

 where little or no mud deposition was going on, 

 these no doubt might have developed into adults 

 on the sea-floor. It is quite conceivable, therefore, 

 that in one portion of the North European Sea, which 

 was fully exposed to the destructive influences of the 

 iceberg action, the fauna was scanty or totally absent, 

 while in another part there lived a fairly abundant 

 one. The unfossiliferous state of the lower continental 

 boulder-clay does not, therefore, offer any serious 



