44 Mr. Webster on the Purheck and Portland Beds. 



and the beds in the Weald clay, called, in the Isle of Wight, Flatten, and in 

 Sussex, Petworth Marble : that the shells in the latter are fresh-water shells, is 

 an opinion which has been advanced by several geologists. The most abun- 

 dant of the small univalves and bivalves in the Purbeck beds are the same as 

 those in the Hastings rock, which contains scarcely any other shell. Beds of 

 the latter are quarried near Battle Abbey in Sussex, having a very close re- 

 semblance to some of those in Purbeck, and like them are separated from each 

 other by shale. These and other circumstances may lead us to unite the Pur- 

 beck beds, the Hastings beds, and the Weald clay into one group ; while the 

 same reasons will tend to remove the Purbeck beds from the Portland rock, 

 which contains a suite of marine shells only, as theTurritella, Trigonia, &c. ; 

 added to this, I have no where seen in the Purbeck or Hastings beds any of 

 those oolitic grains of which the Portland almost entirely consists. 



The chief question is, perhaps, where the line should be drawn between this 

 group and the proper oolite formation ; and here, I own, I do not see my way 

 clearly. The beds called the Top Cap have no shells to enable us to decide ; 

 but there is something in their general aspect that strongly reminds me of the 

 fresh-water rocks in the basin of Paris, particularly that of Chateau Landon. 

 Should this resemblance be at a future time found to be owing to similar 

 agencies, the division may be made at the top of the chert, which is evidently 

 connected with the oolite by its containing the same fossils. 



The Cap of Portland has much more analogy with the lower part of the 

 Purbeck beds than with the ooHtic beds ; indeed I have seen among the beds 

 of Purbeck some not to be distinguished from the Cap of Portland. 



