CHAP, iv.] EVIDENCE OF ICEBERGS IN THE NORTH SEA. 75 



deserted by the expiring energy ; and the volcanoes 1 of 

 the Western Islands, inert and cold, became a prey to 

 the ravages of the elements which have reduced them to 

 their present condition. How vast this destruction was 

 has already been placed before the reader in treating of 

 the Meiocene mountains, p. 44. 



The Pleioceiie rivers in Britain were in their present 

 positions, though they flowed at a higher level, and 

 those on the west opened upon the Atlantic coast-line, 

 then far away from their present mouths, as is shown in 

 the map (Fig. 10). 



Evidence of Icebergs in the North Sea, off the 

 Coast of Britain. 



The researches of Professor Prestwich 2 and Mr. 

 Searles V. Wood into the mollusca of the British 

 Pleiocene strata show that the temperature of the sea 

 gradually became lowered in passing from the period of 

 the Coralline Crag towards the upper deposits. This 

 lowering of the temperature caused the area of the North 

 Sea to be invaded by northern forms, and compelled the 

 retreat of the southern species now found in the warm 

 waters of the Mediterranean, as may be seen from the 

 following table, constructed by Professor Prestwich, 

 from which all the species common to the crags and 

 the British seas of the present time are omitted. 



Species now restricted to 



Northern Seas. Southern Seas 



Norwich Crag . . 130 19 11 



Red Crag . . 216 23 32 



Coralline Crag . . 264 14 65 



1 Judd, Quart. Geol. Journ. Lond. xxx. p. 274. 

 2 Quart. Geol. Journ. Lond. xxvii. p. 474. Crag Mollusca, Palceonl. Soc. 



