76 EARLY MAN IN BRITAIN. [CHAP. iv. 



From this table it is clear that the southern shells 

 were being driven away from their habitation by the 

 depression of the temperature of the water in which 

 they had lived, and that they were being slowly replaced 

 by those of a northern habit ; the increase in the number 

 of the latter, as Sir Charles Lyell acutely points out, being 

 from 5*0 per cent in the Coralline Crag to 107 per cent in 

 the Bed, and 14*6 in the Norwich Crags. This was due to 

 a more intimate connection with the Arctic Ocean, and to 

 the consequent invasion of the British area by currents 

 of cold water. But we have other evidence that this 

 was the case. Professor Prestwich calls attention to a 

 large block of porphyry in the Coralline Crag at Button, 

 which is undoubtedly ice-borne, and Sir Charles Lyell 

 mentions unworn and angular chalk flints in the Eed 

 Crag which have been transported by the same agency. 

 From these facts we may infer the presence of floating 

 ice in the North Sea in the Pleiocene age, and it is very 

 probable that this was brought about, not merely by a 

 general lowering of the temperature in the northern 

 regions, such as Scandinavia, but also by the sub- 

 mergence of the tract of land uniting Iceland with the 

 continent of Meiocene Europe, by which currents of cold 

 water from the Polar regions obtained free access to the 

 North Sea of the Pleiocene age, 1 from which they had 

 before been shut out by a barrier of land. 



1 For a full statement of the arguments see Lyell, Antiquity of Man, 

 1873, p. 248 et seq. ; Prestwich, Quart. Journ. GeoL Soc. Lond. xxvii.; 

 Searles Wood, Crag Mollusca, Palceont. Soc. 1873, and Quart. Journ. Geol. 

 Soc. Lond,. xxii. p. 541, 



