Mr. Webster on the Strata lying over the Chalk. 213 



The whole of the north shore of the Isle of Wight has been 

 for ages in a state of constant ruin by the action of the sea and the 

 sliding down of the soil. It is difficult therefore to find any part 

 of the strata in their original situation ; on this account freshwater 

 and marine shells are frequently found together in confusion. I 

 have however observed some places where they occur in alternate 

 layers.* 



The cerithia, cyclades, cythersea, oysters, and other fossil shells, 

 which are so numerous on the shore near Cowes, are derived from 

 the blue clay of the upper marine formation, which is situated 

 above that which we are now considering, and of which the 

 sloping banks chiefly consist. 



That occasional alternations and mixture of marine and fresh- 

 water shells should occur, may, a priori, be expected. They 

 would denote either the gradual nature of the change that has 

 taken place in an arm of the sea before it became completely a lake 

 of freshwater, or the occasional irruptions of the ocean at a sub- 

 sequent period. 



These beds may be traced a considerable way eastward of Ride, 

 and I believe as far as Nettlestone, but I have not had an opportu- 

 nity of ascertaining their precise boundaries. Neither have I been 

 able to learn with certainty that any part of this formation is to be 

 found on the coasts of Hampshire on the opposite side of the 

 Solent ; and I do not find it eastward on the Sussex side. 



Woodward, however, in his valuable catalogue of fossils, fre- 

 quently makes mention of freshwater shells in a marly stone from 



* The only shells which I uoticed thus alternaling with those of freshwater, appeared 

 to belong to the genus Cerithium, but not having specimens of them it has since occurred 

 to me, and it will deserve future observation, whether these were not the Potamides 

 Lamarkii. They arc abundant at Gurnet Poiat. 



