BEDS OF THE LONDON BASIN. 353 



11 in Prestwich's White cliff-B ay section leads me to the conclusion 

 that it should be placed in that portion of the Bracklesham termed 

 Group C by the Eev. Osmond Fisher (Quart. Journ. Geol. Soc. 

 vol. xviii. p. 65). 



The Bagshot bed resembles Prestwich's bed 11 at Whiteciiff Bay 

 in colour and texture, in the abundance of Cardita jolanicosta and 

 large Turritellce, and in the occurrence of Nummulites Icevigatus. 



Passing now to the upper fossil-bed at Bagshot, marked A, and 

 best seen at Pirbright, I venture to submit that the shells clearly 

 prove it to be of Barton age. 



The occurrence of such shells as Volvaria acutiuscula, Littorina 

 sulcata, Dentalium striatum, Lucina divaricata (or Rigaultiana), Tel- 

 Una scalaroides, and Clavagella coronata, all of which are well- 

 marked types and not easily mistaken, taken together with the ab- 

 sence of Cardita planicosta, Pecten corneus, &c, which are so 

 abundant in the underlying green sand, is, I think, quite sufficient to 

 prove that we are here in Barton beds. On the other hand, if we 

 compare the list of shells from Pirbright Common with that from 

 Long Mead End, Hordwell, published by Mr. Tawney (Proc. 

 Cambr. Phil. Soc. iv, p. 150), we can, I. think, feel no doubt that the 

 Long-Mead-End Upper Bagshot Sand, with Cerithiumpleurotomoides, 

 cannot be correlated with the Upper Bagshot Sand of Bagshot 

 Heath. The upper beds of the Bagshot Sand of Bagshot Heath 

 must therefore be correlated with the Barton beds of Hampshire 

 and the Isle of Wight. 



It might be objected to this correlation that it does not ac- 

 count for the absence of the Upper-Bracklesham beds in the 

 London basin ; but I think that, in the first place, the distance be- 

 tween the two basins is enough to account for almost any amount 

 of thinning-out or change in the nature of the strata ; and in the 

 second place, the clear evidence of a break in the London-basin 

 series, which is furnished by the remarkable pebble-bed marked C 

 in the table of strata, is sufficient to account for the absence of the 

 Upper Bracklesham in that basin. 



The only question remaining to be considered is the point at which 

 the division between the Barton and Bracklesham should be placed 

 in the Bagshot area. On the whole, it appears to me to be most 

 convenient to place it at the pebble-bed marked C in the table of 

 strata (this was suggested by the Eev. A.Irving [Proc. Geol. Assoc. 

 iv. pp. 334, 335], and is in accordance with Prof. Prestwich's Golds- 

 worthy section [Q. J. G. S. iii. p. 382]), giving the Barton beds a thick- 

 ness of 226 feet at least, and the Bracklesham an average thickness 

 of about 45 or 50 feet. 



In the Geological Survey Map some of the overlying sands are 

 included in the Bracklesham, on the ground that they contain green 

 grains. This, however, does not appear to me altogether satisfactory, 

 a few green grains being no proof that a bed is not of Barton age ; 

 and I think it better to take the pebbles as a division than the very 

 uncertain line proposed by the Survey. 



