.1821.] of Headeu Hill, Isle of Wight. 217 



•short examination of Headen Hill, in the Isle of Wight, which, { 

 •reoret, it has been impossible for me to complete owing to the 

 very contracted period it was in my power to allot to the pur- 

 pose. This spot, it is well known, has been repeatedly examined ; 

 a description of it forms the principal part of a memoir published 

 in the second volume of the Transactions of the Geological 

 Societv. The particular object of my visit to it was to collect 

 the fossil freshwater shells that abound there, for the illustration 

 of that part of the subject in the admirable work on Land and 

 Freshwater Mollusca now publishing by De Ferussac. I wished 

 also to obtain a regular series of the strata above the chalk, in the 

 uppermost of which exist such multitudes of freshwater fossils. 

 Of course, I took Mr. Webster's memoir, abovementioned, with 

 me; I was to be guided by it. I, therefore, trust I shall be 

 acquitted of having gone there with an intention to criticize its 

 respectable and learned author; if, entertaining difference of 

 opinion, though founded upon the examination of the same spot 

 of ground, and the same kinds of organized fossils, 1 should find 

 myself compelled to express my dissent from what he has 

 advanced ; following the strata in the same order ; but as regards 

 the nature of one particular stratum, guided by a more intimate 

 conchological knowledge. 



One or two general observations shall suffice for the " lowest 

 marine formation above the chalk, including the plastic clay and 

 sand, together with the London clay." The present state of the 

 vertical cliffs is such as renders it quite impossible to trace the 

 sand and clay in the order in which Mr. W. has described them. 

 Indeed these cliffs appear to consist rather of vertical beds than of 

 continuous strata; and must, therefore, be as constantly varying 

 as the weather and other natural causes operate to produce 

 changes in the form of the cliffs. Every thing is in favour of the 

 opinion, that from the chalk to the lowest part of the freshwater 

 stratum, the whole is but one formation, consisting of various 

 beds of sand and clay. Indeed Mr. W. says, " upon reviewing 

 the whole of this lower marine series of strata in Alum Bay, and 

 comparing it with other sections of the strata immediately over 

 the chalk, we shall find it useful, for the present at least, to 

 separate it into t%vo great divisions: 1. Sand and plastic clay. 

 2. London clay. From the irregularities in the beds in the few 

 places where there are good sections, these divisions, however, 

 can as yet scarcely be considered as distinctly determined ; " 

 but he gees on to say, " thus much is certain, that the plastic 

 clay and sand are always beloic, and never above, the London 

 clay." But it appears to me that he has himself described a bed 

 of London clay, which he marks d, in Plate XI. of vol. ii. Geol. 

 Trans. very near to the chalk, and placed below the greater part 

 of the beds of plastic clay and sand, which are again surmounted 

 by the bed of London clay marked B in the same plate. It is 

 true he advances an opinion that the bed marked d is not conti- 



