Ml. 55.] FOUNDATIONS OF ST PAUI/S. 205 



I remember some years ago somebody showing me a section 

 which was dug in our time at St Paul's, and my notion is that 

 the strata belonged to the Plastic Clay and sands below the Lon- 

 don Clay. Can you tell me whether this is the case, and whether 

 anything has been printed on the subject ? I suppose Wren 

 would call any fossil marine univalves, periwinkles. 



Milman also asks me what the pot-earth is. I suppose, as he 

 says that the Eomans made pottery of it, that it may be an 

 argillaceous bed of the Plastic Clay series ? Wren says that, 

 viewed by a microscope when dissolved in water, this pot-earth 

 was impalpable fine sand which would vitrify with fire. 



As Milman leaves town in a few days, I should be very glad of 

 an early reply. Excuse so much trouble, and believe me, ever 

 truly yours, CHAS. LYELL. 



J. Prestwich to Sir Charles Lyell. 



10 KENT TEREACE, H*A June 1867. 



DEAR SIR CHARLES, I know of no account of the strata 

 beneath St Paul's besides that given by Wren. Some time 

 ago I went carefully into the matter, and the conclusion I 

 came to was that the beds he described were all Drift beds. 



In the 1st place, the London Clay under St Paul's must be 

 about 140 to 150 feet thick, and in it no bed of sand occurs 

 about the level of low-water mark. Secondly, if the L. C. had 

 been traversed, the sand-bed beneath it would at that time (what- 

 ever may now be the case) have been found full of water. If by 

 " hard beach " had been meant any of the conglomerate beds of 

 the Woolwich Series, " sand " and not " natural hard clay " would 

 most probably have been found under it. It is true that the 

 words "periwinkles and other marine shells" might naturally 

 enough have been applied to the fossil Paludina lenta, the 

 Pectunculus, &c., of those beds, or might apply to some fossils 

 of the London Clay, especially the masses of univalves occasion- 

 ally found in the blocks of Septaria ; but the other reasons are, 

 I think, too strong against the description referring to the Ter- 

 tiary strata. 



On the other hand, " pot-earth," 6 feet thick, and thinning off 

 to 4 feet, and containing fine sand, applies very well to the top 

 brick-earth ; under this comes dry sand (the upper part of the 



