IJ4 THE STORY OF THE EARTH. 



The strata and their fossils show that the 

 level of the land fluctuated. The lakes became 

 sometimes occupied with brackish water, so that 

 marine life divides up the fresh-water deposits. 

 After the lower fresh-water beds were formed, 

 the land was submerged, so as to give rise to the 

 Middle Headon beds, which are essentially ma- 

 rine. There are great banks of oysters with nu- 

 merous marine shells, most of them similar to the 

 types which had previously been known in the 

 Barton clay. These marine beds became better 

 developed at Brockenhurst in the New Forest, 

 where SO me corals are found, together with the 

 vertebral column of Zeuglodon, a marine mammal 

 of the whale type, with teeth like seals, already 

 known from the Barton clay. These marine beds 

 are widely spread in Germany. After they v. 

 deposited the land was raised once more, and the 

 Upper Headon beds formed, which reach a con- 

 siderable thickness. They are fresh-water de- 

 posits, consisting of marls frequently green, full 

 of the large Paludina lenta y the Cyrena obovata and 

 the extinct Potomamya pUuia, which alternate with 

 thick limestones, commonly full of fresh-water 

 shells of the genera Planorbis and Limnaa. Tl 

 limestoi almost entirely the product of the 



growth and decay of the fresh-water plant Clnira 

 which precipitates carbonate of lime upon its 



by absorbing carbonic acid gas from the 



water charged with carbonate of lime. In tl 



remains are found of terrestrial mam- 



mals of the types present in the Gypseous beds 

 at Paris, although they are not so nun as in 



the Bembridge beds. 



wix i [eadon beds are followed to the 



coast of Hampshire, the limestones disappi 



