16 ME. W. WHITAKEK ON THE [vol. lxXV r 



Where the pebbly basement-bed rests directly on Blackheath Beds 

 there can hardly be a question as to the pebbles of the former 

 being derived from the latter. 



It was said long ago by Prestwich that ' it is probable that the 

 denuding action (which accompanied the formation of the base- 

 ment-bed of the London Clay [in which he included great part of 

 the Blackheath Beds]) ... in places extended to the Chalk 

 itself, 1 1 though he thought that the pebbles were derived from 

 older Tertiary Beds. A dozen years later, however, it was sug- 

 gested that the large flint-pebbles, so characteristic of that bed in 

 the western end of the London Basin, ' were derived at once from 

 the Chalk ' in a tract in which ' the London Clay sea stretched 

 over the Chalk, where the latter was either wholly uncovered, or 

 but slightly covered, by any older Tertiary formation.' 2 



Clement lleid spoke of ' the mass of flint-shingle which forms 

 the basement-bed of the London Clay,' between Moreton and 

 Turner's Puddle, at the western end of the Hampshire Basin, 3 but 

 perhaps this deposit may be claimed as belonging to the Blackheath 

 Beds. 



The chief deposit of pebbles in the Eocene Tertiaries, above the 

 Blackheath Beds, is in the Bagshot Series, and not altogether at 

 one horizon. Layers of pebbles are found in various places ; but 

 masses are less frequent and of no great extent. It is these only 

 that need now be noticed. 



In the London Basin the most westerly occurrence that has been 

 noted is in Western Berkshire, near Kintbury, at a spot called 

 Pebble Hill on the old map (Sheet 12), a fit name that does not 

 appear on the new one (Sheet 267). This pebble-bed (several feet 

 thick) is far from any Blackheath Beds, and in a tract where the 

 London Clay is thin, so that there is far less between Bagshot Beds 

 and Chalk than is usual. Indeed, it has been inferred, from the 

 thinning of the Beading Beds ar.d the London Clay, that some 

 miles farther west, at the western end of the Tertiary Beds, Bagshot 

 Sand may rest direct on the Chalk. 4 



Another occurrence in Berkshire is in the eastern part of the 

 county, westward of Wokingham, where the term ' Pebble Beds ' 

 is engraved on the old map (Sheet 8). In speaking of this part, in 

 the Memoir of the new map (Sheet 268), Mr. H. W. Monckton 

 notices the presence of ' a few minute pebbles of quartz.' 5 



A few miles farther eastward is a thin pebble-bed at East- 

 hampstead, which has been described by Mr. H. Dewey. G 



1 Quart. Journ. Geol. Soc. vol. vi (1850) p. 277. 



2 Ibid. vol. xviii (1862) p. 269. 



3 'The Geology of the Country around Dorchester' Mem. Geol. Surv. 1899, 

 p. 27. 



4 Quart. Journ. Geol. Soc. vol. xviii (1862) pp. 259, 261 65, 270, 271. 



5 'The Geology of the Country around Reading' Mem. Geol. Surv. 1903, 

 pp. 57, 58. 



G ' The Geology of the Country around Windsor & Chertsey ' Mem. Geol. 

 Surv. 1915, p. 34. The publication of the map (269) has been delayed by 

 ■fche War. 



