Vol. 56.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxXV 



top of the formation is absent: so that we have probably a 

 difference at the rate of 2 to 1. It is shown, too, from several 

 borings that the least thickness is in the middle part of the Basin 

 and in our own neighbourhood here, while the greatest is in the 

 north-east. 



As regards the divisions of the Chalk not much has been learnt, 

 but enough to indicate that all three, Upper, Middle, and Lower, 

 extend underground, as might be expected. 



The Upper Greensand. 



Coming to the formations beneath the Chalk, except at sites 

 near their outcrop there is no evidence of underground extension 

 westward of Winkfleld, near Windsor ; and so, for the present, our 

 view must be limited to that part of the London Basin which lies east 

 of a north-and-south line from Wendover to Aldershot. There can 

 be little doubt, however, that Upper Greensand and Gault occur 

 throughout the tract to the west. 



Eastward from Winkfield, where it is 31 feet thick, the Upper 

 Greensand occurs in all the deep borings along the broad Valley of 

 the Thames, at Richmond, and in London to Crossness, never 

 exceeding the thickness at Winkfield, but irregularly decreasing to 

 12 feet at Crossness and to 10 at Shoreham, in the Valley of the 

 Darent. 



Beyond this eastward, in Kent, it has not again been found in any 

 of the twelve or more borings that have been carried deep enough 

 to prove it. This agrees with the thinning at the outcrop, no 

 Upper Greensand that could be mapped having been found beyond a 

 few miles eastward of the Darent, except for a short distance near 

 Folkestone. 



North of the Valley of the Thames it is absent at Bushey, 

 near Watford, which is practically on the line between the thinning- 

 out on the southern outcrop, at Kemsing, and the point beyond 

 which Upper Greensand cannot be mapped on the northerly outcrop 

 near Tring, though it is again seen a little near Dunstable. 



So far, therefore, the underground evidence agrees remarkably 

 well with that obtained aboveground. But north of London the 

 conditions are different, the Upper Greensand extending underground 

 in renewed force up the valley of the Lea, being 44 feet thick at 

 Cheshunt and 40 at Ware, quite a respectable thickness for this 



