Mr. Webster on the Strata lying over the Chalk, 229 



described it agrees most nearly with its corresponding formation in 

 the Paris basin, with this difference however, that none of ours is 

 siliceous ; I shall therefore, in the succeeding comparison, confine 

 myself to the calcareous part. 



The external characters of this in both countries are sufficiently 

 different from every other known rock to render them distinguish- 

 able even without the shells. That of France is described as white, 

 or yellowish ; sometimes as tender and friable as chalk or marl, 

 and sometimes very hard, compact and solid, with a fine grain and 

 conchoidal fracture. In the latter case it breaks into shaip fragments 

 like flint, and cannot be worked as stone ; sometimes it will even 

 admit of being polished as marble. It is also frequently filled with 

 infiltrations of calcareous spar. 



This description corresponds very nearly with the freshwater rock 

 of the Isle of Wight, and an examination of the specimens from both 

 places leaves no room to doubt of the similarity of the strata. 



The fossil shells which I found in the upper freshwater formation,, 

 and which have been described by Mr. Parkinson, are the following. 



Names given by Lamarck. Linnean names, 



Planorbis, much resembling that which 



Brongniart says approaches to 

 P. cornu 

 Planorbis, two other species 

 Planorbis, much resembling P. 



prevostinus J 



Ampullaria --------- Helix 



Cyclostoma --------- Helix 



Limneus longiscatus ------- Helix stagnalis 



acuminatus 



) Helix planorbis 



