1866.] -VTHITAEIER LOWEE LONDOX TERTIAEIES. 419 



the junction, is owing to the formation of pipes after, and not to 

 the wearing away of the Chalk before, the deposition of the beds. 

 Moreover, if Mr. Hughes be right in accounting for the formation of 

 the bed of green- coated flints*, it follows that the destruction of Chalk 

 needful for its production also took place after the deposition of the 

 Thanet Beds. Of course the process which would effect this result 

 would alike efface the signs of conformity ; and therefore the evenness 

 of junction may be owing to this, and not to conformable deposition, 

 as said before (pp. 406, 407). 



2. The Woohuich and Heading Beds, though far more constant in 

 their presence, are less so in their structure. In the far east they 

 pass into the sands below, whilst on the west the two are sharply 

 divided, and the pebbly greensand of the bottom-bed sometimes rests 

 irregularly on the light- coloured Thanet sandf. West and north of 

 London the Reading Eeds lie immediately on the Chalk, as is also the 

 case in the Hampshire basin. I think, therefore, that we should be pre- 

 pared to find this series make a like overlap southwards from the 

 mass of the Kentish Tertiaries, and should not be surprised if outliers 

 of it should be found resting on the Chalk of the North Downs. 

 Now on the hills just north-east of Otford there do occur (besides 

 the loamy mottled brick-earth) masses of bright- coloured mottled 

 plastic clay, just like that of Berkshire, &c., associated with the 

 sands which Mr. Prestwich has classed as CragJ. 



3. The Oldhaven Beds are the most local, the most uniform in 

 structure, and the most irregular in occurrence of the w^hole. Mr. 

 Prestwich long since noticed the way in which the pebble -beds 

 rested on a worn surface of the Woolwich Series §, sometimes indeed 

 cutting off almost the whole of the latter (see Sections 6, 7, 8, 

 PI. XXII.); and near Chiselhurst I have lately seen the former 

 cutting through the clay with shells (4), the pebble-bed (2 a), and 

 the bottom-bed (1), until it rested on the Thanet sand (c). If, then, 

 we ought not to be surprised at the Woolwich Beds overlapping the 

 underlying series and resting on the Chalk of the North Downs, still 

 less so should we be at fi.nding the Oldhaven Beds in that position. 

 Here, again, Mr. Prestwich is before me ; for, from having found some 

 of the peculiar green-coated flints of the <' base-bed" in the Old- 

 haven sand near Heme Bay, he infers that " it is probable that the 

 denuding action acted not only on the upper part of the un- 

 derlying (Woolwich) series, hut that it in places extended to the chalJc 

 itself \\. 



There seems to be another reason why the Oldhaven Beds must 

 have somewhere reached to the Chalk ; for else how is one to account 

 for the great mass of flint-pebbles which they contain ? — a mass too 

 great to have been derived from any beds between the two, none of 

 which, indeed, contain anything like that amount of flints. 



* See above, p. 402. 



t See Prestwich, Quart. Journ. G-eol. Soc. vol. x. p. 101. 



+ Ibid. vol. xvi. pp. 323-324. 



§ Ibid. vol. X. pp. 106-107. 



II Ibid. vol. vi. p. 277. 



