266 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 16, 



"Woolwich and Eeading Series*," and "On the Thickness of the 

 London Clay f." It will be better to begin with the lowest formation, 

 and to work upwards. 



The Thanet Sand. — Mr. Prestwich has fully noted the westerly 

 thinning out of this bed of fine soft light- coloured sand ; and I can- 

 not do better therefore than quote his words on the matter, from 

 the first of the above-mentioned papers (p. 241). " In some parts 

 of the neighbourhood of Canterbury they cannot be much less than 

 80 to 90 feet thick. They then apparently maintain a tolerably 

 uniform thickness of from 60 to 70 feet, as far as Chatham, Upnor, 

 and Gravesend. At Bexley Heath they have been ascertained to 

 vary in thickness from 45 to 55 feet, and at Woolwich I find that 

 they are 60 feet thick. Beneath London their thickness averages 

 from 30 to 40 feet. They then become more rapidly thinner as they 

 trend underground further westward, being only 20 feet thick at 

 Wandsworth, 17 feet at Isleworth, 7 feet at Twickenham, and 3 feet 

 at Chobham, beyond which they thin out, although I believe that 

 originally they probably had a range westward coextensive in some 

 measure with the green-coated flints overlying the Chalk ij:." 



Along its line of outcrop in Surrey, the Thanet Sand thins west- 

 ward from Croydon and Beddington (where it is full 30 feet thick), 

 until at Ashstead it is but a few feet in thickness. Further to the 

 west I know of no section in it. 



Its thickness beneath London and the country to the west is known 

 by means of well-sections : thus near Westboume Grove it was found 

 to be 18 feet thick §. Mr. Prestwich says, " At WiUesden there are 

 several deep wells, but I have not been able to obtain an exact sec- 

 tion of any of them. From a good supply of water, however, being 

 obtained before reaching the Chalk, it is probable that the Thanet 

 Sands have here commenced j| ." At the Hyde, 2 J miles north of the 

 village, the following beds were found : — 



1. London Clay, and its '' basement-bed " 66 feet. 



2. Woolwich and Heading Beds. — Sands, clays, and pebbles 34 ft. 8 in. 



3. Chalk. 



* Quart. Journ. G-eol. Soc. vol. x. p. 75. t Ibid. p. 401. 



\ I hardly think that such is the case ; for the bed of green-coated flints above 

 the Chalk in Berkshire, &c., is a part of the "bottom-bed" of the Eeading 

 Beds, which lies on the top of the Thanet Sand when that formation is present, 

 and is therefore not to be confounded with the bed of flints at its base. It is 

 possible, however, that (as Mr. Prestwich believes) the two beds may join together 

 to the west of London where the Thanet Sand has thinned out, and thus that 

 the roughly laminated grey clay and the clayey green sand, with oyster-shells 

 and green-coated flints, that overKe the Chalk at Eeading and Newbury (see 

 Memoir illustrating Sheet 13 of the Map of the Geological Survey, p. 23, and 

 also the Memoir on Sheet 12, p. 27) may represent the bottom-bed not only 

 of the Woolwich and Eeading Beds, but also of the Thanet Sand. Speaking 

 generally, where the Thanet Sand is present the bottom-bed of the Woolwich 

 and Eeading Beds does not contain the green-coated angular flints so common 

 in Berkshire, &c., but the flints are in the state of pebbles : this need cause no 

 surprise, however, a^ where the latter formation was not deposited directly on the 

 Chalk, it is not likely that it should contain unworn flints that must be derived 

 directly from that rock. 



§ Quart. Journ. Gbol. Soc. vol. x. p. 96. || Ibid. p. 95. 



