66 H. A. Baker — Quarizite Pebbles of the Oldliaven Beds. 



similar to those from the Woolwicli and Eeading Beds. The same 

 minerals occurred, in the same relative order of abundance and 

 exhibiting the same individual features in each case. No mineral 

 occurred, which had not been noted in the Eocene residues, and, of 

 the specially characteristic minerals of the latter, there was no 

 absentee in the residues from the quartzites. In view of this 

 evidence it would seem that these pebbles are not foreign rocks 

 derived from a distance and incor]3orated in the pebble-beds, but 

 that they are closely related to the Lower Eocene beds. 



In pursuance of this idea it becomes necessary to inquire into the 

 evidence for the occurrence, in the Lower London Tertiary beds, 

 of material similar in character to these pebbles, and at once the 

 much debated " grey wethers " or " sarsen-stones " are recalled. 

 Prestwich, as long ago as 1854,^ dealt in a full and admirable manner 

 wdth the question of the source of the " Druid Sandstones ", and the 

 arguments he brought forward are just as cogent to-day as they 

 were when he wrote. He concluded first, from the then available 

 evidence, that the distribution of these " Druid Sandstones " was 

 more in accordance with the range of the Lower London Tertiaries 

 than with that of the Bagshot Sands. This conclusion has been 

 endorsed by later workers. Prestwich then proceeded to consider 

 the three members of the Lower London Tertiaries separately as 

 possible sources of these sandstone blocks. He discarded the 

 Thanet Sands on the ground that although they often present 

 favourable elements and are occasionally semi-indurated, they 

 are rarely consolidated. In the case of the section at the Reculvers, 

 where the Thanet Beds contain a bed of concretionary sandstone, 

 he pointed out that the sandstone has a calcareous cement, contains 

 no pebbles, and frequently exhibits the impressions of shells ; 

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

 neither calcareous nor fossiliferous, and are not uncommonly sub- 

 conglomeratic. He pointed out, further, that the Thanet Sands 

 do not range more than 6 to 10 miles westward of London. He 

 next considered his "Basement-bed of the London Clay", with 

 which at that time (1854) were included the present Oldhaven or 

 Blackheath Beds. He alluded to the facts that the " Basement- 

 bed " presents a very small development westward of London, and 

 that although it often contains concretionary blocks they are 

 almost invariably small, calcareo-argillaceous, and fossiliferous. 

 He then stated his belief that the greater portion of the blocks 

 known as Druid Sandstones, greywethers, sarsen-stones, and 

 puddingstones were derived from the middle division of the Lower 

 London Tertiaries. He admitted that it was very rarely that 

 solidified jDortions of the strata were found in situ, but stated that 

 the same difficulty occurred in as great a degree with the sand- 

 stones of the Bagshot Sands. He was able, nevertheless, to put 



' Prestwich. " The Woolwich and Reading Series " : Quart. Journ. GeoL 

 Soo., vol. X, 1854, pp. 123-30. 



