﻿Vol. 6 1.] SARSEN -STONES IN A CLAYPIT. 41 



then, seemed clearly that we are dealing here with swallow-holes 

 formed by underground solution in the Upper Chalk. The flints 

 sank downward by superincumbent pressure against the sides and 

 bottom of each, and the inflowing ooze of wet clay gave way 

 beneath the heavy sarsen-blocks, which sank deeply into it. For 

 the sarsens in clay show this remarkable arrangement : — They 

 slope on all sides towards the centre of the pits, and at the bottom 

 they are horizontal, and the quarrymen work the pits with this 

 observed fact to aid them. The original containing-walls of the 

 swallow-holes were Chalk-with-Flints, that by process of dissolution 

 became Clay-with-Elints, the flints (large and unworn, having been 

 subjected to no detrition by movements over any but a very small 

 area) remaining in situ, like the flints found at the base of the 

 Thanet Sands, which were clearly never deposited by marine action, 

 but simply ' remain ' in their old position while the chalk that 

 contained them has disappeared. 



In conclusion, I wish to express my thanks to Mr. Bristow, under 

 whose direction the pits are worked, for the help which he courte- 

 ously afforded me in the pursuit of my investigations. 



Discussion. 



Mr. Barrow drew attention to the deep cuttings on the new 

 railway between Uxbridge and High Wycombe. Near Gerard's 

 Cross the lowest Tertiary deposits (Woolwich and Reading Series) 

 were exposed, and one here saw that a bed of white sand, when 

 followed to a sufficient depth, passed first into isolated and rounded 

 blocks of nearly-white sandstone, and finally into a solid bed. This 

 material was almost certainly the same as that exhibited by the 

 Author. It was clear that this bed, when near the surface, 

 decomposed into loose sand more rapidly than ordinary denudation 

 could remove that sand. Thus the solid bed could never be found in 

 a natural outcrop. But under special conditions, when these beds 

 had once formed a continuous thin sheet above the Chalk, powerful 

 floods or ice in motion had swept away the loose sand and picked 

 up the remaining hard cores, involving them in the general mass 

 of Drift of the country. The truth of the Author's explanation of 

 the inverted-cone shape of the hollows in which the sarsens were 

 found, namely, that they were essentially swallow-holes in the 

 Chalk, was firmly established by the cutting a little farther west 

 of Gerard's Cross. There a continuous series of these inverted 

 cones (swallow-holes) had been cut open, and the basement-beds of 

 the Tertiary rocks passed alternately over the edges and down 

 to the apices of these cone-shaped hollows. Seen from a distance, 

 the beds overlying the Chalk seemed as if they had been affected 

 by some powerful earth-movements. The sections were so striking 

 that they ought to be photographed. 



