38 w. whitae:ee on some boeings in kent. 



the base of the Thanet Beds, it shows that the whole of the Chalk is 

 present. 



The Chalk therefore being passed through from top to bottom, "we 

 have, for the first time, a measurement of its total thickness in the 

 district. This is from 682 to 689 feet, according to the various 

 accounts, a less amount than one might have expected, but which is 

 confirmed by the section of the neighbouring boring at Prindsbury 

 (Cement-works), showing 618 feet of Chalk at a point some way 

 below where the Thanet Beds come in. The higher of the above 

 figures (689) exceeds only by 18 feet the thickness of the Chalk in 

 the Eichmond boring*, and by 44 feet that in the Kentish Town 

 section, which gives the least thickness of all the borings in the 

 London Basin that pierce the Chalk from top to bottom f. In 

 marking the probable position of the base of the Chalk in Sheets 78 

 and 84 of the so-called Horizontal Sections of the Geological Survey, 

 which cross the Chalk and Tertiary district just eastward of Chatham, 

 and in which that line was kept low for safety, 1 have erred by 

 about 180 feet, having estimated the thickness of the Chalk as pos- 

 sibly reaching to about 870 feet. 



The absence of the Upper Greensand is no more than we should 

 expect ; for there is none of this formation at the base of the Chalk 

 hills to the south, although a narrow strip has been coloured on the 

 Geological Survey Map (an error corrected in the Drift Edition now 

 being engraved). The 18 inches of greenish-grey marly sand de- 

 scribed by me as Upper Greensand (in the Memoir on the Weald, 

 p. 153) is, of course, far too thin to be mappable ; and, moreover, 

 I should now class it as the base-bed of the Chalk Marl, the same 

 as the Cambridge nodule-bed J. 



With regard to the Gault there is a diflPerence of 6 inches in the 

 two Chatham borings, and the Prindsbury well agrees remarkably 

 with them, being but a foot less ; so that the figures 192 and 193 

 may fairly be taken as the thickness of this bed here, where the 

 slight dip would have but a trifling effect in exaggerating it. This 

 tends to show that, as has been the case further west, too small a 

 thickness may have been assigned to the Gault at the outcrop, which 

 has been given as low as 100 feet, and that the supposed abnormal 

 thickness at TrottesclifiPe, where the Gault was not bottomed after 

 183 feet had been passed through, is not so exceptional as has been 

 thought §. Here, again, there is a fair agreement with the Eichmond 



* Prof. Judd, Quart. Journ. Geol. Soc. vol. xl. p. 724 (1884). 



t 'Guide to the Geology of London,' ed. 4, pp. 20, 21 (1884). 



J In Ed. 4 of the ' Guide to the Geology of London,' p. 16, first paragraph, 

 the statement with regard to the deep wells of the London basin, that " in eadh 

 case the Upper Greensand and the Gault succeeded in due order," should have 

 been "in most cases the Upper Greensand and in all the Gault . . . ." The 

 mistake is a case of survival from a former edition, in which the statement was 

 correct. 



§ ' The Geology of the Weald,' Mem. Geol. Survey, p. 148 (1875). For the 

 thicknesses of the various divisions of the Lower Greensand and of the Wealden 

 beds at their outcrop I am indebted to this work. I have also to thank the 

 author, Mr. Topley, for the trouble he has taken in getting together informa- 

 tion on the Chatham borings. 



