THE NEOZOIC AGES. 273 



material not explicable by glaciers, and evidences of 

 marine erosion, occur at still higher levels, and it is 

 natural that on high and exposed points fewer remains 

 of f ossiliferous beds should be left than in plains and 

 valleys. 



At the present day the coasts of Britain and other 

 parts of Western Europe enjoy an exceptionally warm 

 temperature, owing to the warm currents of the 

 Atlantic being thrown on them, and the warm and 

 moist Atlantic air flowing over them, under the influ- 

 ence of the prevailing westerly winds. These advan- 

 tages are not possessed by the eastern coast of North 

 America, nor by some deep channels in the sea, along 

 which the cold northern currents flow under the 

 warmer water. Hence these last-mentioned localities 

 are inhabited by boreal shells much farther south than 

 such species extend on the coasts and banks of Great 

 Britain. In the Glacial period this exceptional advan- 

 tage was lost, and while the American seas, as judged 

 by their marine animals, were somewhat colder than 

 at present, the British seas were proportionally much 

 more cooled down. No doubt, however, there were 

 warmer and colder areas, determined by depth and 

 prevailing currents, and as these changed their 

 position in elevation and subsidence of the land, 

 alternations and even mixtures of the inhabitants of 

 cold and warm water resulted, which have often been 

 very puzzling to geologists. 



I have taken the series of drift deposits seen in 

 Britain and in Canada as typical, and the previous 



