70 LONDON CLAY. [1849. 



this Drift period. At the same time, I hazard this opinion in its 

 extension to the Wealden with considerable doubt, although I 

 have reason to hope that with regard to the other districts it 

 will be found fairly grounded. I do not know whether in this 

 short explanation I have made myself clear, or whether I have 

 entered at sufficient length on the points you wish. In any case, 

 I shall be most happy to communicate to you any other facts I 

 may be in possession of, or to enter more fully into any of my 

 views requiring explanation ; and believe me to remain, my dear 

 sir, very sincerely yours, J. PRESTWICH. 



To the Same. LONDON, 20th August 1849. 



MY DEAR SIR, I had written the enclosed letter on the night 

 of the 31st July, when on the 1st August I received your second 

 letter with further inquiries respecting the Drift period. I post- 

 poned, therefore, sending it until I had again considered the 

 subject, and seen more of the district in question. 



Since the publication of my papers on the London Clay and 

 Bagshot Beds, I have only communicated to the Society short 

 papers on isolated facts, and have not therefore gone again into 

 the general views of subsidence and elevation affecting large 

 areas and requiring lengthened observation. This subject I am 

 about to resume in a paper on the beds between the London 

 Clay and the Chalk, as well as in a paper on the Diluvial period. 

 "With regard to ground where the many hundred feet of London 

 Clay and overlying beds were derived, I yet feel at a loss to form 

 an opinion. I have thought much about it, and have sought in 

 vain for any transported rock specimens in the body of the beds 

 to show their origin. I have only one such specimen from the 

 London Clay, and that is not very distinct. It is, at all events, 

 some old and distant rock. The clays of Sheppey indicate the 

 proximity of land on some point, I think, southward of that island. 



In the beds below the London Clay the evidence is, however, 

 stronger and clearer. The fluviatile beds of Upnor, Woolwich, 

 and Lewisham, and of Guildford appear evidently to have been 

 local things small rivers, and flowing apparently from a land 

 on the south, as the deposits do not seem to have extended 

 themselves far from the then existing shore, and they are lost 



