By Professor T. Rupert Jones, F.R.S., F.G.S., 8fc. 131 



and that the region was at a much lower level than now, and was 

 covered by the sea. The water, charged with silex, filtering among 

 the sand-grains, cemented them here and there, making these great 

 blocks of concreted stone, which accordingly formed irregular sand- 

 stone beds, interrupted by spaces, where no concretionary action 

 went on. 



The sands of the Woolwich -and-Reading Beds are some of those 

 that underwent this concretionary change at places ; also those of 

 the Bagshot-Sand series. The relative successional order (from 

 above downward) of these formations is as follows : — 



Upper. 300ft. thick in Berks ; with a few pebbles, 

 and with a pebble-bed at the base (Q.J.G.S., 

 vol, xlii., p. 414). 

 Middle. 20ft- in Berks ; with a few small pebble-beds. 



Barton Clay and Bracklesham Beds ; 400ft. 

 in Isle of Wight, with a pebble-bed. 

 [_ Lower. 150ft. in Berks ; with a few small pebble-beds. 

 i f London Clay and Bognor Beds. 500ft. in Essex ; 60ft. in 

 f & I Western Berks ; 15ft. near Great Bedwyn, Wilts ; 



^ -j 0 in Marlborough Forest. 



I Basement-bed of the London Clay. With pebble-beds. Near 

 L London 2 to 5ft; west of London, 9ft. to 0. 



"Old-Haven Beds (Whitaker). 30ft. in Kent : 0 to the 

 west. 



Woolwich-and-Reading Beds. 50ft. near London and 



p Bagshot Sands. 

 A 



* § 



d 

 o 



in Berks ; in Wilts, 15ft. (Great Bedwyn) ; j g 

 0 further west. I t> 



I 0 further west. J 



£ (JThanet Sands. 60ft. in West Kent ; 0 in Berks. 

 CRETACEOUS.— Chalk. 



The above table indicates that the " London Tertiaries " thin out 

 westward ; consequently in Wiltshire the Bagshot Beds get gradually 

 to lie nearer to the Chalk, which persists throughout the south-east 

 of England. Not far west of Marlborough Forest the Woolwich- 

 and-Reading Beds, as well as the London Clay and its Basement- 

 bed, all disappear, and consequently the Bagshot Beds must have 

 lain directly on that rock, just where the " Grey wethers " occur in the 

 greatest number. Hence these blocks near about Clatford are most 

 probably the concretionary sandstones of the Bagshot Sands. See 

 Tig. 1. In Berks the Sarsens for the most part were derived also 

 from the Bagshot Beds, though some may have come from the out- 

 cropping edges of denuded Beading Beds. In Surrey the Bagshot 

 Sands were surely their source, for they lie on surfaces 200ft. and 



