1862.] WHITAKEK— LONDON BASIN. 269 



of this paper, that they are not more than 15 feet thick. They do 

 not increase further westward in Marlborough Forest, the last point 

 where they occur in the London Basin. 



There is one other fact that seems to point to a thinning of the 

 Beading Beds, though in what direction is not clear. In the western 

 part of the London Basiu, the basement-bed of the London Clay is 

 remarkable for the common occurrence of large rounded flintsin it 

 (generally in a liae at its lowest part), often 6 or 8 inches in their 

 longest diameter, and sometimes as much as 14 inches*, besides the 

 ordinary flint-pebbles. Now, where any pebbles are found in the 

 Eeading Beds in the same district, they are not of large size. The 

 most westerly placet where I have seen rounded flints of any great 

 size in that formation is at Chorley Wood Kiln, about two miles 

 W.N.W. of Rickmansworth (iu an outlier) ; and these were in the 

 " bottom-bed," which there consists of 10 or 12 feet of green sand 

 full of pebbles. It would seem likely, therefore, that the large 

 rounded flints of the " hasement-hed " of the London Clay were derived 

 at once from the Chalk, or that, if they came from the Reading Beds, 

 it was from the lower ^art of that formation ; or, in other words, that 

 the London Clay sea stretched over the Chalk where the latter was 

 either wholly uncovered, or hut slightly covered, hy any older Tertiary 

 formation. In confirmation of this, I may quote Mr. Prestwich's 

 words : "■ It is probable that the denuding action (which accompanied 

 the formation of the basement-bed of the London Clay) acted not 

 only on the mottled clays and the pebble-beds forming the upper part 

 of the underlying series, but that it in places extended to the Chalk 

 itself J." Mr. Prestwich, however, thinks that the rounded flints 

 were all derived from older Tertiary beds, and not directly from the 

 Chalk. 



The Basement-hed of the London Clay. — This bed§ seems to reach 

 its greatest thickness near Eeading, where the light-brown loam, with 

 green sand, shells, flint-pebbles, and masses of Hmestone and of iron- 

 stone, of which it there consists, is 5 to 12 feet thick, whilst at 

 Northcot (to the west) and at JN"ettlebed (to the north) it is 9 feetl|. 



In well-sections in and near London it has been found to be from 

 2 to 5 feet thick. IS'ear New Cross it is only about a foot (in one of 

 the sections at Loam-pit Hill, near Lewisham, it is, however, only 

 three inches), and near Bromley from a foot to 3 feet. Further 



* This great size is noted by Mr. Prestwich in Quart. Journ. Geol. Soc. vol. vi 

 p. 259 (explanation of fig. 4). 



t I speak of the northern outcrop of the Reading Beds. According to the 

 sections given by Mr. Prestwich (in Quart. Journ. Greol. Soc. vol. x.) and by 

 Mr. Bristow (in the Geological Survey Memoir on Sheet 12), clay chiefly prevails 

 along the southern outcrop at the western part of the London Basin, and the sands 

 do not contain pebbles. 



I Quart. Journ. Geol. Soc. vol. vi. p. 277. 



§ Not including therein the pebble-bed of Blackheath, &c., nor the sands just 

 beneath the London Clay near Heme Bay. (See above, p. 267.) 



II See Memoir illustrating Sheet 13 of the Maps of the Geological Survey, 

 pp. 49, 40, 52. Mr. Prestwich is mistaken in saying that " westward of London 

 in no case does the basement-bed of the London Clay present a thickness of 

 more than 5 feet " (Quart. Journ. Geol. Soc. vol. vi. p. 280). 



