466 Proceedings of Philosophical Societies. [Dec. 



The following is the table of equivalent beds above alluded to : 



Nov. 19. — A paper was read, " On the Purbeck and Port- 

 land Beds;" by T. Webster, Esq. Sec. G. S. 



The author observed, that the great general features of the 

 geology of the Isle of Purbeck had been already traced out by 

 iiim in his letters to Sir Henry Engiefield. He now confined 

 himself to some details respecting the series of limestone beds 

 in the Isle of Purbeck, and to those in the Isle of Portland. 



He then proceeded to give a description of the strata from 

 which the well known Purbeck stone used in London, for side 

 pavements, &c. is derived. This stone is composed almost 

 entirely of fragments of shells. The Purbeck marble contains 

 chiefly" univalves in a compact limestone, and these in general 

 are smaller than the univalves in the Petworth marble, both hav- 

 ing been supposed to belong to freshwater shells ; but the 

 author possessing specimens that contain a mixture of marine 

 with freshwater shells, he cannot consider this as a decided 

 freshwater formation, a term that, in his opinion, ought to be 

 restricted to those beds supposed to have been formed in lakes 

 only. The common Purbeck stone appears to consist of frag- 

 ments of small bivalves, of which the origin is doubtful. 



Mr. Webster then gave a detailed account of the quarries in 

 the Isle of Portland, which furnish the Portland stone much 





