1854.] PRESTWICH LONDON CLAY. 401 



That at some former period the ferruginous sand of Sheppey, though 

 now limited in extent, was coextensive with the London Clay beneath, 

 seems evident from the existence of the kilns at Scrap Gate, and of 

 others which appeared to me to be on the Warden end of the cliffs. 

 These clearly show, that although the arenaceous deposit does not 

 exist in such localities as a distinct mass, yet there is, in the necessary 

 loamy state of the brick clay, evidence of a transition state between 

 the London Clay below and the commencement of some sandy bed 

 above. 



I do not attempt to prove the identity of the Sheppey sand vnth 

 that of Bagshot. My wish is to bring those facts which I observed 

 under the notice of others, who may have an opportunity of minutely 

 investigating the subject. The discovery of fossils, or of lithological 

 characteristics, or actual sequence, must be added to the present 

 order of superposition before we can really prove that the high lands 

 of Sheppey bear an outlier of the Bagshot Sand formation, in common 

 with the other elevated portions of the London Clay. 



The accompanying woodcut gives an approximate section of the 

 cliffs on the N.E. coast of the Island. 



Above the bed of ferruginous sand, and over the surface of the 

 highest parts of the Isle of Sheppey, rounded pebbles are found, not 

 deposited en masse, but detached and scantily scattered over the sur- 

 face. Hence the boggy nature of the Island in wet weather, the London 

 Clay coming up to the surface almost uncovered. These pebbles, 

 both in their shape and mode of deposition, are precisely similar to 

 those which are found in some localities N. of London, as at Muswell 

 Hill and Mill Hill ; and they appear to be quite distinct from the 

 beds of gravel found in various parts of the valley of the Thames. 



3. On the Thickness of the London Clay; on the Relative 

 Position of the Fossiliferous Beds q/" Sheppey, Highgate, 

 Harwich, Newnham, Bognor, ^c. ; and on the Probable 

 Occurrence of the Bagshot Sands hi the Isle of Sheppey. 

 By Joseph Prestwich, Jun., F.R.S., F.G.S. &c. 



§ 1 . Thickness of the Clay. 



Having frequently had my attention directed by Mr. Searles 

 Wood to the question of the exact position in the London Clay of 

 the peculiar and remarkable fossiliferous beds of Sheppey, and of 

 the thickness of the London Clay itself at that place, I have, with 

 that special object in view, made a further examination of the Island, 

 and now beg to lay before the Society the result of these observ^a- 

 tions. I have availed myself of this opportmiity to determine also 

 more correctly the importance of the London Clay in other parts of 

 its range, and to settle approximately the position of the strata of some 

 other localities, well known for their fossils — separate lists of which 

 I give at the end of this paper. 



This question of the thickness of the London Clay remains nearly 



