CHAP. XI.] HA.NTONIAN PERIOD. 197 



The Oligocene series may also be simply divided into : 



Uroer<! HempsteadB6ds ' 

 1 Bembridge Beds. 



Lower Headon Beds. 



The Hantonian strata of north-western Europe occur in 

 several broad basins or trough- shaped areas separated by 

 parallel anticlines or regions of elevation. The most 

 northern of these is that which we know as the " London 

 basin," but which is really only part of a large trough- 

 shaped area extending from Belgium across the North Sea, 

 and terminating in Wiltshire ; the second is known as the 

 Hampshire basin, and a third as the Paris basin. 



A study of all these areas is necessary for a proper com- 

 prehension of the geographical changes which took place 

 during the period, and the stratigraphy of the Paris basin 

 is especially important from the fact of that area lying 

 nearer to the southern shore of the Hantonian sea. 



1. Stratigraphical Evidence. 



Eocene. The gap between the Chalk and the Tertiaries 

 is not bridged over by beds of passage in any part of 

 England, but in Belgium and Denmark there are deposits 

 which show that the Cretaceous period was brought to a 

 close by a general upheaval of western Europe a change 

 which led to the expulsion of the Cretaceous fauna from 

 the European area, and the introduction of a very different 

 shallow-water assemblage, which we call the Eocene fauna 

 because it contains the ancestors of our modern shallow- 

 water European species. 



In England, therefore, there is a considerable break 

 between the Cretaceous and Hantonian systems, but tl.e 

 upheaval which caused this break seems to have been such 

 a gradual and uniform movement that it did not lead to 



