Vol. 56.] ANNIVEESARY ADDRESS OE THE PEES [DENT. lxxvii 



made a little too narrow here on the Geological Survey Map, the 

 uppermost part of the Gault having apparently been classed with 

 the Upper Greensand, into which, indeed, it passes up. 



At Winkfield, on the west, is the next greatest thickness, 

 264 feet, decreasing thence eastward, along the valley of the 

 Thames, to 201 at Richmond and to between 130 and 188 in and 

 near London. There must be a rapid thinning northward from 

 Caterham. 



Easterly thinning from Caterham is also fairly marked, there 

 being something more than 237 feet at Sundridge, where the very 

 top has been removed, and at Shoreham 226. 



The many borings in the valley of the Medway show a thickness 

 decreasing from 234 feet on the south, where the river enters the 

 Chalk tract, to 192 at and below Rochester. 



Pursuing an easterly course there is no certain evidence for some 

 distance ; but the Gault ultimately thins in that direction, for at 

 Ottinge, in the parish of Elham, it is 127 feet thick, at Ropersole 

 (? in Barham) between Canterbury and Dover 119, and on the 

 coast at Folkestone about 100. Although north-east of the last- 

 mentioned place, in the two borings near Dover, it reaches a higher 

 thickness (121 and then 144 feet), thence northward it decreases to 

 only 63 at Margate. 



Along or near the northern outcrop the thickness is over 215 feet 

 at Long Marston, near Tring, where the top beds have been removed, 

 and 214 at Hitchin ; but this decreases underground towards London, 

 the figure at "Ware being 163. 



The multitude of borings in the neighbourhood of Cambridge 

 show a decided north-easterly thinning, the thickness varying 

 generally from 175 to nearly 110 feet, with considerably more in a 

 single case and considerably less in another. Farther northward 

 the thinning continues to 90 feet at Soham, 56 at Stoke Ferry, and 

 20 at Narborough. 



Leaving out of account the boring at Saffron Walden, of which 

 there is no trustworthy record, but which, judging by its depth, 

 must pass through the Gault, there are four deep borings in 

 northern Essex and in Suffolk that certainly do so (at Weeley, 

 Harwich, Stutton, and Culford), and in these the thickness varies 

 from 49 to 76 feet, while in the one boring in northern Norfolk, 

 at Holkham, it is but 10. This last shows, however, that underground 

 the Gault clay extends northward to the coast, though at the 

 outcrop it thins out several miles southward thereof, where its place 



