By Professor 7. Rupert Jones, F.RS., F.GS., Fe. 181 
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). 
& Bagshot Sands. 4 Middle. 20ft. in Berks; with a few small pebble-beds. 
B Barton Clay and Bracklesham Beds ; 400ft. 
_|a in Isle of Wight. with a pebble-bed. 
- Lower. 150ft. in Berks; with a few small pebble-beds. 
a Ea ondon Clay and Bognor Beds. 500ft. in Essex; 60ft. in 
oO se ole Western Berks; 15ft. near. Great Bedwyn, Wilts ; 
2 2 (Pb 0 in Marlborough Forest. 
3 Basement-bed of the London Clay. With pebble-beds. Near 
aS London 2 to 5ft; west of London, 9ft. to 0. 
ES ( Old-Haven Beds (Whitaker). 30ft. in Kent: O tothe) 4 , 
Sy | west. ag 
rg Woolwich-and-Reading Beds. 50ft. near London and + 2 
ee in Berks ; in Wilts, 15ft. (Great Bedwyn) ; | Ea 
= wigs = 
E 0 further west. 
43 Thanet Sands, 60ft. in West Kent; 0 in Berks. 
CRETACEOUS.—CuHatx. 
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 “ Greywethers ” occur in the 
greatest number. Hence these blocks near about Clatford are most 
probably the concretionary sandstones of the Bagshot Sands. See 
Fig. 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 Reading Beds. In Surrey the Bagshot 
Sands were surely their source, for they lie on surfaces 200ft. and 
K 2 
