Mr. Webster on the Strata lying over the Chalk. 215 



in the gypsum being thicker, and in tlie occurrence of a bed of 

 very indurated clay containing fossil fish. The third or uppermost 

 mass is the most important, the beds of gypsum being very thick ; 

 it is here alone that the bones of unknown birds and quadrupeds 

 are met with. Freshwater shells are also found in it, though rarely. 



Over the gypsum arc very thick beds of calcareous and argil- 

 laceous marls ; one of the latter, which is white and friable, contains 

 silicified trunks of palm trees, and in the former are found many 

 freshwater shells. In this freshwater formation are found neither 

 the meulieres, nor any other flints, except the silex menilites, and 

 the hornstone of the upper gypsum beds. 



The total absence of beds of gypsum and of the remains of qua- 

 drupeds, in this formation in the Isle of Wight, (at least as far as 

 my observations extend) exhibits a striking proof, that although 

 probably of contemporaneous origin, yet the circumstances ac- 

 companying its formation were very different from those under 

 which that of the basin of Paris was produced. The gypsums of 

 Montm.artre are well knov;n ; and they are considerably different 

 in their appearance from those of England, which belong only to 

 strata far below the chalk. It has been already observed, that 

 selenite is very abundant in our blue clay ; but this arises from the 

 decomposition of the pyrites contained in it. 



§ 4. Upper Marine Formation » 



Over the lower freshwater formation in the Isle of Wight, a 

 stratum occurs, consisting of clay and marl, which contains a vast 

 number of fossil shells wholly marine. Few of these shells agree 

 with the species that have been found in the London clay, and they 

 are also considerably different from them in their state of preserva- 



