14 



to the northern hemisphere. It is remarkable while 

 the eocene flora of Europe was largely Australian in 

 character the miocene has an American facies. The 

 retreat of the great miocene sea and the elevation of 

 the Alps and Pyrenees were the two great events of 

 the pliocene age. The presence of mountain-ranges 

 covered with snow would materially lower the tempera- 

 ture, and had doubtless a considerable influence ; but 

 the glacial state of Europe cannot be accounted for by 

 this phenomenon alone ; it may have been aided, 

 according to Count Saporta, by the diffused sun-light 

 and a densely-clouded atmosphere reducing the contrast 

 between the polar summer and winter, or, according to 

 Professor Geikie, to an alteration of the position of the 

 poles and the winter of our hemisphere happening in 

 aphelion. That a gradual depression has taken place 

 is clearly shown by the norwich red and coralline crags, 

 the latter, which is the older, differs less in the character 

 of its fauna than the other two, as it contains twenty- 

 seven molluscs now living in the Mediteranean and one 

 West Indian species ; thirteen only occur in the red- 

 crag associated with three fresh southern species, while 

 the whole disappear from the Norwich beds, and are 

 replaced by others of a boreal type, sixty-nine of which, 

 out of eighty-one, are still living, and among them are 

 no species of southern latitudes, we may infer, therefore, 

 that the temperature of the sea must have gone on 

 gradually diminishing. In the immediate overlying 

 forest-bed of Cromer which extends along the 

 Norfolk coast for about forty miles may still be 



