388 



EEV. A. LEVIN G OF THE PHYSICAL HISTOET OF 



the probable Upper Bagshot, 5 ft. thick in -the cutting 550 yards east 

 of the station, and 5 ft. below the surface, said to be 6 ft. thick in 

 a well 70 yards north of the cutting. Space compels me to omit here 

 full details which I possess of other well-sections in this outlier ; 

 but it must be stated that in two of them, at the foot of Bill Hill 

 (330 ft. high), " green sand yielding putrid water " is recorded, and 

 beds of pebbles in these and in two others (6 in. to 24 in. thick) 

 were met with, at about 265 (O.D.). There is a very good section 

 of Upper Bagshot at the south end of Bill Hill, at about 300 (O.D.). 

 The dip discovered at Bracknell, and the trend somewhat to the north 

 of the general strike of the London Clay mav account partly for the 

 altitude (250-118=) 132 (O.D.) of the surface of the London Clay 

 in the Ascot well. The specimens which I have from that well show 

 (1) real "blue clay" at that depth, (2) a striking correspondence of the 

 sandy beds for some distance higher with nos. 11 to 13 of the Welling- 

 ton well-section. A nearly perfect shell of a CeritMum is in my 

 possession, probably from still higher beds, very likely equivalents of 

 nos. 9, 10, and of the laminated clay-and-sand bed at Bracknell. 

 The last is certainly not the bed which occurs next above the 

 London Clay in the Ascot well. 



I believe that beds of about the horizons of nos. 9 and 10 run 

 through this side of the country, and rest, in places, on the eroded 

 London Clay surface at Wokingham, Buckhurst, and Bracknell* ; that 

 is to say, there is a certain (not very great) double unconformity, beds 

 not the lowest of the Bagshots resting upon beds not the highest of 

 the London Clay. The evidence can only be really estimated on the 

 ground, and will have to be taken into account in a resurve} 7- of the 

 district. If such marginal conditions exist, we must be prepared 

 for some things "unusual" (Quart. Journ. Geol. Soc. vol. xlii. p. 406), 

 such as the occurrence of a pebble-bed in the Upper Bagshot, the 

 objection to which is, moreover, neutralized by the record of pebbles 

 at Pirbright (Joe. cit. p. 414). 



GrENEEAL CONCLUSIONS. 



1. My recent work, of which a mere outline is here given, tends 

 to bear out the general conclusions arrived at in my previous paper 

 (Quart. Journ. Geol. Soc. vol. xli. pp. 506-508), except on one or two 

 points. 



£. The Bagshot Beds of the London Basin are, upon the whole, an 

 estuarine series, which admits of differentiation, both on physical 

 and pakeontological grounds, into : — 



(ci) An upper marine-estuarine group ( = " Upper Bagshot 

 Sands ? '). 



(h) A lower freshwater group ( =" Middle and Lower Bag- 

 shots"). 



* Local erosion on a minor scale is a matter of direct observation at all those 

 three places. The occurrence usually of a few feet of fine quartz-sand beneath 

 those beds shows the absence of a " passage " between the two formations 

 {cf. Mem. Geol. Surv. vol. iv. pp. 316, 317). 



