THE SIBERIAN MIGRATION. 235 



Such I assume to have been the geographical con- 

 dition of Northern Europe during the deposition of 

 the Red Crag. Arctic mollusca were then brought 

 to the east coast of England, and boulders were 

 scattered through the beds laid down on that coast 

 by icebergs which had been cast off by Scandinavian 

 glaciers on reaching the sea. Bedded clays which 

 have yielded arctic shells He beneath the lower con- 

 tinental boulder-clay on the Baltic coast-lands and on 

 the coast of the White Sea. According to Professor 

 Geikie, marine clays on the same geological horizon 

 reach an elevation of some 230 feet. " It would seem, 

 then," he says, "that before the deposition of the 

 lower boulder-clay of those regions the Baltic Sea 

 had open communication with the German Ocean " 

 (p. 442). All these clays are evidently deposits of the 

 same sea. But apart from the fact that the Red Crag 

 and these Baltic deposits are the oldest of the upper 

 Tertiary beds containing arctic shells, there is no 

 evidence that they are contemporaneous. 



Overlying the same Baltic deposits comes the lower 

 boulder-clay, reaching a thickness of several hundred 

 feet in some parts of Germany. It presents, like the 

 upper clay, frequent interstratification with well- 

 bedded deposits of sand and gravel. The scarcity of 

 marine mollusca, the occurrence of striated surfaces, 

 and the occasional presence of so-called giants' 

 kettles, appear to favour the view, which at present 

 is generally adopted by both British and Continental 

 geologists, that the boulder-clay owes its origin to 



