part 1] sectiox at woems heath. 15 



.area where the Heading Beds rest on the Chalk, this is rarely the 

 ■case, the flints being like those at the base of the Thanet Beds. 

 It is along the northern outcrop of the London Basin, from the 

 edge of Buckinghamshire through Hertfordshire, that pebbles are 

 found in the bottom-bed, where it rests on the Chalk. 1 



In Hertfordshire there are also pebbles higher up in the Beading 

 Beds ; but generally in mere layers, rarely in quantity enough to 

 warrant the name ' pebble-bed,' 2 unless we include the beds that 

 have yielded the Hertfordshire puddingstone, which have been 

 seen in place near Aldenham. 3 



Along the southern outcrop in the London Basin the only 

 marked pebble-bed recorded is in Kent, next above the bottom-bed, 

 at St. Mary's Cray, Lewisham, and Chislehurst, the greatest 

 thickness being 12 feet, while elsewhere there is not half that 

 amount.* 



In the Hampshire Basin, at the far west, in Dorset, the 

 Reading Beds take on a gravelly condition, but not altogether a 

 pebbly one. Beds of coarse gravel, flint conglomerate, and breccia, 

 sometimes with Greensand chert and quartz-pebbles have been 

 grouped by C. Reid with this division. 5 These gravels, which 

 are described as 'more like river-gravels than like beach -deposits,' 

 seem hardly to have any connexion with the question of the origin 

 and formation of the pebbles of the Blackheath Beds. 



Now, the whole of these occurrences of pebbles below the Black- 

 heath Beds, spread over so wide an area, show 7 an amount of pebbles 

 that is small, compared with the great concentrated mass of Black- 

 heath, Bexley, Bromley, etc., so that unless (and without any 

 justification) we suppose that there were pebble-beds beyond the 

 present Tertiary tract, we cannot look to the Woolwich and 

 Beading Beds for the supply of pebbles to the Blackheath Beds. 



These latter, on the other hand, with their former large extension 

 over the Chalk tract (shown by the many outliers still left), 

 are bulky enough to have yielded the material for later pebble-beds 

 in the Tertiary formations ; but the comparatively small areas in 

 which the Blackheath Beds now occur lead one to think that much 

 of the later pebble-beds may have been directly derived from the 

 Chalk. 



(b) Above the Blackheath Beds. 



Pebbles are of common occurrence in the basement-bed of the 

 London Ckiy, and layers are occasionally found in the clay itself. 



1 ' The Geology of London. &c.' Mem. Geol. Surv. vol. i (1889) pp. 192, 

 193, 196-200, 203, 205. 



2 Ibid. pp. 194, 201, 202, 207 ; and 'The Geology of the London Basin' 

 Mem. Geol. Surv. vol. iv (1872) p. 227. 



3 ' The Geology of London. &c.' vol. i (1889) p. 200. 



4 Ibid. pp. 116, 127, 137, 138. 



5 ' The Geology of the Isle of Purbeck' Mem. Geol. Surv. 1898, pp 192-94, 

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

 pp. 17-22. 



