386 



HEY. A. IRVING ON THE PHYSICAL HISTORY OF 



the clays worked in the California pits are the beds nos. 9 and 10 of 

 the well-section ; (2) that the section published last year (Quart. 

 Journ. Geol. Soc. vol. xlii. p. 409, fig. 2) is erroneous in several impor- 

 tant particulars, (a) in the altitudes (error of about 15 ft.), (b) in 

 the omission of the middle beds of the section, (c) in the assign- 

 ment of too small a thickness to the clayey beds of the brick-yard. 

 The evidence, and that furnished by a comparison of numerous other 

 sections in the neighbourhood, including sections on both sides of 

 Finchampstead Ridges, is altogether against the assignment of the 

 California clays to the Lower Bagshot. The outcrop of the London 

 Clay in the pit at the west end of Mne-mile Ride and at Barkham 

 demonstrates therefore, when altitudes are taken into account, the 

 attenuation of beds nos. 11 and 12 of the College well-section also in 

 this direction. On the strength of the stratigraphical evidence, of 

 which only the salient points are here briefly indicated, I re-asserfc 

 my entire agreement with the judgment of the Officers of the Survey 

 in mappiug the California clays as the basement-beds of the Middle 

 Bagshot. The train of reasoning which is based on the assignment 

 of these beds to a Lower Bagshot horizon (Quart. Journ. Geol. Soc. 

 ,vol. xlii. p. 408) seems to me, therefore, to fall to the ground. The 

 base of no. 10 is 215 (O.D.), about the level of the base of the same 

 bed in section E (p. 384), 1^ miles due east of this point. 



Fig. 2. — General Section. 



For details of the Bearwood outlier, see Geol. Mag., March 1887; 

 for the Wokingham outlier, see supra (p. 383) and my former paper 

 (Quart. Journ. Geol. Soc. vol. xli.). The Buckhurst outlier is probably 

 for the most part Upper Bagshot *. As to the Bracknell outlier the 

 following sections should be considered : — 



Section F. Warfield Brick-yard {North end of outlier). 



ft. 



a. Coarse brown irony sand with irony concretions and 1 

 irregular layers of strong white unctuous clay (? a Bag- I 8 

 shot bed) J 



c. Pebble-bed in loam with included lumps of pink and) -, f about 



white clay (about) J \ 260 (O.D.) 



d. Laminated clay-and-sand beds, with ferruginous con- j 



creticns (a "mellow loam" when well mixed), hori- I 5 

 zontal in sections at right angles J 



e. Dark-coloured sandy clay passing down into ordinary j 



London Clay with a true dip of 10° in a direction 20° I 31 

 S. of E. (exposed to) ... J 



Vertical exposure 45 



This, it is seen, furnishes direct evidence of unconforrnability. The 

 dip is shown by even layers of pebbles and a calcareous rocky fossili- 



* Judging from road-sections and from the loose materials brought up from 

 a well 52 feet through the yellow sands. In the railway-cutting beds nos. 10 

 and 1 1 rest on a highly eroded surface of London Clay. 



