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XIV. — On the London and Plastic Clay Formations of the Isle of Wight. 



By Mr. BOWERBANK, F.G.S. 



[Received March 12th, 1839.] 



XiARLY in June 1838, in company with my friend Mr. White, I visited White 

 CUfF and Alum Bays, for the purpose of examining the London and Plastic Clay 

 Formations, and of determining, if possible, by a careful examination of their 

 fossils, how far the great mass of the London clay and the beds of coloured clays 

 and sands beneath it are entitled to be considered as separate formations*. We 

 were induced to visit them with this view, from having some years since, in 

 company with Mr. Morris, carefully examined the sandy beds between the London 

 clay and the chalk at Heme Bay ; and having been forcibly struck by the great 

 similarity which the fossils procured from these sands bore to several of the species 



* When this paper was read before the Geological Society, I was not aware that Mr. G. B. Sowerby 

 had examined the strata which occur between the chalk and lower freshwater formation of Mr. Webster 

 at Alum Bay, and that he had published the result of this examination in the ' Annals of Philosophy' for 

 Sept. 1821. In that paper he questions the propriety of Mr. Webster separating the upper or London 

 clay from the lower or plastic clays and sands ; and he says, " Everything is in favour of the opinion, 

 that from the chalk to the lowest part of the freshwater stratum the whole is but one formation, con- 

 sisting of various beds of sand and clay," p. 217. The merit of this idea is therefore undoubtedly due to 

 Mr. G. B. Sowerby ; neither was I aware of the existence of the paper published by Professor Sedgwick 

 in the same work in May 1822, in which he expresses a difference in opinion on the subject, and advo- 

 cates a return to the arrangement into two formations, as proposed by Mr. Webster. Professor Sedgwick 

 prefaces his observations on Mr. Sowerby 's views by stating, that " The separation of the two formations 

 above mentioned is not marked by any extraordinary natural epoch, but is merely assumed as a conve- 

 nient classification, founded on constant geological relations ; on a decided difference in the composition 

 of the constituent beds ; and in a still more decided difference in their zoological phaenomena." p. 339. 

 Now the fourteen well-known characteristic London clay fossils, found by myself in the clay and sand 

 near the chalk, marked a in Webster's section, decidedly negatives the assertion that that part of the 

 formation differs in zoological phaenomena from the upper portion of the London clay, marked B in 

 the same section. Mr. Lyell also, in the ' Principles of Geology,' 1st edit., vol. iii. p. 278, states, that in 

 the coloured sands and clays of Alum Bay there is a " distinct alternation of the two groups," that is, 

 of the London and plastic clay beds. Why, therefore, they should be designated as separate forma- 

 tions, even in the latest geological publications, I cannot conceive, unless it be that it is more difficult 

 to eradicate a confirmed error than it is to establish a new truth. — February 184'1. 

 VOL. VI. SECOND SERIES. Z 



