OF THE BAGSHOT BEDS OF THE LONDON BASIN. 177 



mately associated with the green earths. On these grounds the beds 

 e to ^ of this section may be assigned with some probability to the 

 horizon of Nos. 7 and 8. That they are not part of the London 

 Clay is well shown by the position of the bed r/, and the caution 

 with which I first described them is fully justified. This leaves it 

 an open question whether the pebble-bed c of this Barkham section 

 is on the horizon of No. 3 or No. 6 of the College-Well section. 

 Compare also Sections K, L, 0. 



Puller details of the structure of these Bearwood Hills are given 

 in a previous paper (Geol. Mag. loc. cit.). I see no reason to depart 

 from the view therein expressed, that these hills are capped by an 

 outlier of the Upper Sands, although the horizon of their base is at 

 a rather lower level than the corresponding horizon in the Finch- 

 ampstead Church Hill, the elevations of the beds in that hill having 

 been, I venture to think, sufficiently accounted for. A section along 

 the eastern flank of Finchampstead Ridges would show the corre- 

 lation of the beds in the Barkham Hill to be in reality a very simple 

 matter. 



Farley Hill, to the west, cut off from the Finchampstead Hills by 

 a valley along which London Clay is exposed, has, I believe, from 

 exposures on its cap and flanks, a similar capping of Upper Sands, the 

 Middle Group being mainly represented by strongly developed clays 

 and loams of horizons Nos. 9 and 10, which appear to overlap the 

 quartz-sand series found on the south side, and to come directly on 

 the London Clay further north. Further exploration is needed 

 here *. 



Results. 



It will be seen that there is a general but very irregular attenu- 

 ation of the quartz-sand series along this line of country f. While 

 they thin out entirely in the Bracknell Hill at Wokingham and 

 Barkham, and admit of the overlap of beds Nos. 9 and 10, they 

 thicken to 80 feet or more in the Ascot Well +, thinning again to 

 from 10 to 15 feet on the north side of Englefield Green. The 

 most probable explanation of this is that about Ascot we are on 

 the site of the influx of an important Eocene river, which was an 

 affluent to the main river-mouth in which these sands were deposited. 

 We have similar evidence of an affluent of less magnitude from the 

 north-west in the fluviatile sands found on the north side of the 

 tongue of London Clay on which St. Paul's Church, Wokingham, 



* I haye estimated 30 ft. for the quartz-sands at Woodbury, the residence 

 of W. Simonda, Esq. (March 27). 



t Prof. Prestwich (Quart. Journ. Geol. Soc. vol. xliv. p. 107) suggests Eocene 

 denudation as the cause of this attenuation ; but I venture to think that the 

 evidence taken altogether shows rather that it is due to conditions of original 

 deposition. 



X The rectified reading of this section given in my last paper was based on the 

 following data : — (1) specimens which came into my possession after the first 

 account of it was published ; (2) the discovery of the clip of the London Clay at 

 Warfield ; (3) some notes by Mr. Whitaker ; (4) some calculations made by 

 Mr. B,. S. Herries. 



