264 PEOCEEDINOS OF THE GEOLOGICAL SOCIETY. [Apr. 16, 



whether the "bottom-bed" occurs here or not, I do not know. 

 Higher up, the sands and clays of the Heading Beds are overlain by 

 a continuous bed, a few inches thick, of black flint-pebbles of various 

 sizes, many large ; and above this pebble-bed there are white and 

 light- coloured sands, with thin seams of pipe-clay, about 12 or 15 

 feet thick. Now in the sections of the Reading Beds in the western 

 part of the London Basin pebbles are very rarely seen, except in the 

 green sand Q' bottom-bed ") lying directly on the Chalk ; in that bed 

 they are not always found, and when they do occur it is not in great 

 numbers nor of large size : I have never seen, in that district, any 

 regular pebble-bed in the Heading Series *. The basement-bed of 

 the London Clay, however, usually contains pebbles, and generally 

 many of large size, as was found to be the case at Stoke Farm, in 

 the mass of London Clay nearest to the section now under notice ; 

 and very often there is a layer of them at the lowest part of this 

 bed. I have therefore no doubt whatever that the pehhle-bed of this 

 section belongs to the basement-bed of the London Clay ; and therefore 

 that the overlying sands are part of the Bagshot Beds, and that the 

 London Clay proper has here thinned out. The section would then 

 stand thus : — 



1. Lower Bagshot Beds. — Light- coloured sands, with seams of pipe- 



clay about 12 or 15 feet. 



2. Basement-bed of the London Clay. — A pebble-bed . . a few inches. 



3. Beading Beds. — Sands and plastic clays about 15 feet, 



I will now try to show that the above conclusion is borne out by 

 other facts. We have seen that at Stoke Farm the Reading Beds 

 and the London Clay are neither more than 15 feet thick. That 

 the latter thins westward from London has been shown, by Mr. Prest- 

 wicht, although he seems to have underrated the extent of the 

 thinning ; and as from Beading to Great Bedwin, a distance of about 

 28 miles, it has dwindled from 350 feet to 15 (or at the rate of about 

 12 feet in a mile), one can have no difficulty in inferring that two 

 miles further westward it has thinned out altogether (with the ex- 

 ception of part of its basement-bed). Moreover the upper sands of 

 the section in question, with their seams of pipe-clay, are lithologi- 

 cally more like Lower Bagshot Beds than anything else. 



At the brickyard on the western side of the outher (close to the 

 yard just noticed) the section was not very clear when I saw it (in 

 May 1859). Chiefly sands were shovm; and at one part there was, 

 at the top, a small irregular patch of green sand : could this be a part 



* The statement of Mr. Prestwich (ia Quart. Journ. Gaol. Soc. vol. x. p. 79) 

 with regard to Marlborough Forest, that "the greater part of these fine woods 

 are planted on a thin and irregular capping of the clays and pebble-beds [of the 

 Reading Beds] on the Chalk," is likely to mislead, although, in a foot-note, Mr. 

 Prestwich includes also "a clay and gravel drift." Only a small part of the 

 Forest, not more than a square mile indeed, is on Lower Tertiary beds ; but a 

 very large part is on the drift-clay, brick-earth, and clayey pebble-gravel so 

 abundant in this neighbourhood. 



t Quart. Journ. Geol. Soc. vol. x. p. 401. 



