THE ARCTIC FAUNA. 175 



to the Gulf of Bothnia, and to the four-horned sting- 

 fish (Cottus quadricornis, Fig. 14, p. 178), neither of 

 which occur on the west coast of Scandinavia. But 

 there are others which point in an equally unmistakable 

 manner to the former existence of a marine connection 

 between the Baltic and the southward prolongation of 

 the Arctic Ocean known as the White Sea. It is 

 generally admitted now that such a union between 

 these two seas, viz., the Baltic and the White Sea, 

 occurred in recent geological times, but opinions 

 differ as to the duration of this connection. I 

 adhere to the view expressed by Murchison and 

 others, that the boulder-clay is a marine deposit. I 

 am also convinced that the Arctic Ocean, as I have 

 already mentioned, transgressed over the lowlands 

 of Northern Russia at about the time when the 

 newer crags were being deposited on the east coast 

 of England; that the same large sea also covered 

 Northern Germany, Denmark, Holland, and the low- 

 lands of Sweden, and laid down the lower continental 

 boulder-clay which is spread over such vast tracts of 

 land in those countries. I shall have occasion to refer 

 to this again more fully in the next chapter; mean- 

 while, it should be remembered that this stage was 

 followed by a partial retreat of the northern sea, 

 though Scandinavia did not become joined to the 

 Continent. The date of this retreat of the sea, repre- 

 sented in Fig. 13, corresponds probably to what is 

 know as the inter-glacial phase of the Glacial period, 

 and I think it must have been during this time that 



