CRETACEOUS BOCKS. 7 



Only in the south-west has it been proved. At Ottershaw Park. 

 3 miles south-west of Chertsey, 12 feet of coarse brown quartz 

 sand was encountered, and yielded a plentiful supply of water. 1 

 The bed is absent at Southall, but has been found in two borings 

 near Slough, 2J miles west of the boundary of our map, so that 

 its eastern limit presumably passes out of our area to the west 

 near the middle of the western margin. In the newer Slough 

 boring, at the Motor Depot, a thickness of 130 ft. has been 

 proved, consisting mainly of fine brown sands with a band of 

 clay 12 ft. thick, 83 ft. from the top. 



SELBOBNIAN. 



Under this title are included the beds known as Upper 

 Greensand and Gault. The Gault, which consists of dark grey 

 clays with pyrites, surmounted by grey sandy marl, has been 

 met with in every deep boring that has been carried beneath 

 the Chalk in the London District. It is usually fossiliferous 

 and has at its base a pebbly layer with coprolites. The Upper 

 Greensand on the northern side of the London Basin comprises, 

 in the lower part, beds of siliceo-calcareous rock or malmstone 

 which owes its texture to the silica derived from sponge -spicules, 

 and in the upper part dark green sand which becomes marly 

 in places and merges upward into the Chalk Marl. The forma- 

 tion is not persistent in this district, as it is replaced in some 

 tracts, as at Bushey in Hertfordshire, by the upper clayey beds 

 of the Gault. On account of the variable lithology, the two 

 formations have been included under the name ' Selbornian,' 

 as suggested by Jukes-Browne. 



Along the southern side of the London Basin the Upper 

 Greensand consists of beds of malmstone, a glauconitic sandstone, 

 more or less micaceous and calcareous. These are represented 

 by the ' firestone ' and k hearthstone ' that outcrop north of God- 

 stone and Reigate, at the foot of the North Downs, a little south 

 of the limits of our map. 



In deep borings, beds of greensand, sandy marl and clay, 

 like those of the northern outcrop, were proved at Kentish 

 Town and Crossness ; and beds of sandstone resembling the 

 southern type were encountered at Meux's Brewery, Richmond 

 and Streatham. In a boring at Purley, below the depth of 

 467 ft., a thickness of 35 to 47 ft. was assigned to Upper 

 Greensand, the Gault Clay being entered only for 8 feet; at 

 Shoreham, in Kent, the thickness of Greensand was only 

 10 ft., succeeded by 226 ft. of Gault. At Ottershaw the section 

 showed 25 ft. of marl followed by 15 ft. of hearthstone and fire- 

 stone passing down gradually into typical Gault. From the 

 table it will be seen that under London itself the thickness of 

 the Selbornian is from 150 to 200 ft. In the south-west it 



1 ' Geology of Windsor and Chertsey ' (Mem. Geol. Surv.), 1915, pp. 5 

 and 112, 113'. 



