Mr. Webster oh the Strata lying over the Chalk. 217 



the north side of the Isle of Wight, and may be seen at Cowes, 

 Ride, and Bembridge. 



In the enumeration of the various strata in the basin of Paris, our 

 attention is particularly called to a thin bed of bivalve shells cover- 

 ing the upper beds of gypsum and gypseous marls of the lower 

 freshwater formation, the shells of which are placed close, and as 

 it were locked into each other. This bed, though apparently of 

 little importance, is remarkable by its great extent ; having been 

 observed over a space of more than 10 leagues in length, and more 

 than 4 in breadth, and always retaining the same situation and 

 thickness. It is still farther distinguished as marking out the be- 

 ginning of the new series of strata deposited by the sea. 



Over this thin bed of bivalves there is one of greenish marl with- 

 out fossils ; and then several alternating beds of argillaceous and 

 other marls and sands, containing marine fossils, shells, and bones 

 of fish. Two beds of oysters are particularly noticed ; and it is 

 observed that these must have lived upon the spot where they now 

 are, because they are locked into each other as in their natural beds, 

 with many of their hinges entire. The uppermost beds of marls 

 are however not constant ; sometimes, as at Chellis, there is only 

 a thin bed of sand between the green marl and the upper fresh- 

 water formation. It is in these marls that the silex menilite is 

 found. 



The changes which the surface of the earth has experienced have 

 no doubt destroyed so much of the last depositions in the London 

 as well as in the Isle of Wight basins, that it is impossible to say 

 what the complete series was originally. 



Of the existence of this upper marine formation therefore in 

 the London basin, I must speak with diffidence, more particularly 

 since an extensive freshwater formation might never have existed 



Vol. II. 2 e 



