PRESTWICH WOOLWICH AND READING SERIES. 125 



:his bears to the mass, in which these concretions are of a merely 

 local and rare occurrence. Nevertheless on the slopes of this ridge, 

 especially along its south-western flank, the nmnber of blocks of 

 Druid sandstone scattered over the surface of the ground just below 

 the outcrop of the sands is very considerable ; as they become more 

 numerous they also become larger. These hills, if prolonged, would 

 pass by Larabourue six miles further to the N.W., and it is on the 

 downs about that village that the Druid sandstones are particularly 

 numerous and large. That the tertiary strata ranged in that direc- 

 tion is proved by a few traces of them yet remaining *. 



Again, in Buckinghamshire, Oxfordshire, and Hertfordshire, grey- 

 weather sandstones and pudding-stones occur in districts throughout 

 which are scattered numerous outliers of the Lower Tertiaries, once 

 continuous over the whole area, and in the wreck of which, at their 

 denudation, these blocks seem, as in Wiltshire, to have been left 

 behind. So also with the large accumulations of sandstone blocks 

 on the north downs above Maidstone. 



If therefore we admit the distribution of the Druid sandstones to 

 be in accordance with the range of the Lower London Tertiaries 

 rather than with that of the Bagshot Sands, the next question is to 

 ascertain what evidence there is of the occurrence in situ of similar 

 masses of stone in the different groups of the former series. 



1 . With regard to the Thanet Sands : — Although often presenting 

 favourable elements, and occasionally semi-indurated, they are rarely 

 consolidated. At the Reculvers, however, they contain a bed of 

 concretionary sandstone ; but it has a calcareous cement, contains 

 no pebbles, and exhibits frequently the impressions of shells ; 

 whereas the erratic sandstones of Kent, Bucks, and Wiltshire are 

 neither calcareous nor fossiliferous, and are not uncommonly sub- 

 conglomerate. Further, the Thanet Sands do not range more than 

 six to ten miles westward of London. It is therefore not probable 

 that the Druid sandstones belong to beds of this age. 



2. The Basement-bed of the London Clay presents a very small 

 development westward of London, and although concretionary blocks 

 are often found in it, they are all comparatively of so small a size, 

 besides being generally calcareo-argillaceous, and almost invariably 

 fossiliferous, that it is not in this direction we must look for the 

 origin of these sandstones. But to the eastward of London this bed 

 becomes more important, and at Boughton it contains a subordinate 

 bed of a siliceous sandstone, often extremely hard, very local and 

 very variable, and of a character which would harmonise perfectly 

 well with some of the blocks on the slopes of the East Kent Chalk 

 ranges. 



3. My belief, however, is that the greater portion of the blocks 

 known as Druid Sandstones, Greyweathers, Sarsen Stones, and 

 Pudding-stones are derived from the middle division of the Lower 

 London Tertiaries. It is very rarely that solidified portions of the 

 strata are found in situ, but the same difficulty occurs in as great 



* As, for instance, at Liquid Farm, five miles W.N.W. from Wickham. 



