THE ARCTIC FAUNA. 173 



that coast have generally been traced to the action of 

 land-ice, but the supposition that they have been 

 carried by icebergs the older theory appears to 

 me the more probable one. Such boulders begin 

 to make their first appearance in the Red Crag, a 

 deposit which is now looked upon as belonging to 

 the newer pliocene series. But whether we call it 

 pliocene or pleistocene really matters little. The 

 important fact is, that glacial phenomena, consisting 

 of the appearance of boulders foreign to the country 

 together with an invasion of Arctic shells, are now 

 ushered in upon a coast which shortly before teemed 

 with the southern life of a Mediterranean character. 

 Among the new arrivals in these English crags there 

 are no less than eighteen species of North American 

 marine mollusca. Since the German Ocean had then 

 no direct communication with the Atlantic, these 

 mollusca could only have come from the White 

 Sea, and Forbcs's Arctic current would offer an 

 explanation of the manner in which they were 

 enabled to migrate there from their original 

 home. 



It might be urged that we have no grounds 

 for the supposition that the German Ocean was 

 practically a closed basin; and that these American 

 species probably inhabited at that time the whole of 

 the North Atlantic Ocean. But if such had been the 

 case, we ought to have evidence of the occurrence of 

 some of these species in the newer Tertiary deposits 

 along the west coasts of the British Islands. Such 



