292 NEOZOIC TIME. [CHAP. XIII. 



the present rivers ; (3) the occurrence of mammalian 

 remains on the bed of the North Sea, and especially on the 

 Dogger Bank. 



In the first place, it is certain that when Palaeolithic 

 man and the animals which are associated with his re- 

 mains invaded Britain, the rigour of the Ice Age must 

 have passed away, and the climate of the southern portion 

 of the country must have been mild enough for the growth 

 of an abundant vegetation ; the country must, in fact, have 

 been in a condition to support large herds of herbivorous 

 animals. 



To enable terrestrial animals to cross freely from France 

 and Belgium to England, the elevation required would not 

 be more than 25 fathoms (150 feet) ; but the central part 

 of the Irish Sea and St. George's Channel is everywhere 

 more than 50 fathoms deep, except over a small area 

 opposite Cardigan Bay, where the deepest parts are between 

 40 and 50 fathoms. It would therefore require an eleva- 

 tion of at least 50 fathoms (about 300 feet) to unite Ireland 

 with Wales. Even then long and deep lakes would remain 

 in the bed of the Irish Sea, for there is a tract extending 

 from off the west coast of Carnarvon to the Sound of Jura, 

 where the soundings are continuously over 50 fathoms, 

 and generally over 60, so that if the elevation reached the 

 latter figure these lakes and their excurrent river would 

 form a definite line of separation between Great Britain 

 and Ireland. 1 



It might be supposed that the formation of the Straits 

 of Dover and of St. George's Channel was effected by 

 marine erosion rather than by subsidence, and that the 

 present depth of these channels could not be taken as 

 affording any measure of the elevation of the land previous 

 to their formation ; so that in order to restore the condi- 



1 J. Geikie in " The Great Ice Age," second edition, p. 294, and map. 

 See also " Prehistoric Europe," by the same author. 



