1850.] PRESTWICH ON THE LOWER TERTIARY STRATA. Vll 



the various sorts of sand associated with those beds, were also Ukely 

 to have been thus derived. I hold this opinion because this base- 

 ment bed does not present these diiferent substances in separate and 

 sedimentary order — the bed is composite, and its materials derived, 

 not by a river action bringing down sediment into the sea, but, if 

 I may so term it, by apparently a scouring and general sea action on 

 the pre-existing and underlying beds. That the pebbles in this bed 

 are not in the position in which they received their present form, is, I 

 think, evident from the excessive and lengthened attrition which the 

 flints must have undergone to have formed pebbles of such uniform 

 roundness and finish — a state of things incompatible with the co- 

 existence and preservation of the remains of a delicate and abundant 

 fauna in the same stratum. But we have other and independent 

 proofs of this bed having originated in the destruction of part of the 

 underlying beds. Thus in Section, fig. 1, at White Cliff Bay, it has 

 been shown that rough pebbles of red clay, derived from the harder 

 parts of the underlying mottled clays, occur in this bed ; and in Sec- 

 tion, fig. 11, at Heme Bay cliff, I have found in this bed specimens 

 of the peculiar uncouth green-coated flints, which form the charac- 

 teristic bed, reposing almost everywhere in this country and in the 

 North of France immediately on the chalk. These half-rolled green- 

 coated flints have an appearance so perfectly distinct and constant, 

 that their origin cannot for a moment be doubted. Therefore it is 

 probable that the denuding action acted not only on the mottled 

 clays and the pebble beds forming the upper part of the underlying 

 series, but that it in places extended to the chalk itself. It is doubt- 

 ful however whether the latter suffered much denudation at this time. 

 There are but few traces of its direct debris in this bed. The pebbles 

 of flint I suppose to have been derived from a long-continued wear- 

 ing away of the chalk at a previous period. The disturbance at the 

 period of the basement bed of the London clay does not appear to 

 have acted with sufficient power on the chalk, nor during sufficient 

 time on the flints, to have produced a large destruction of the former, 

 or to have reduced the latter to the state of such well-rounded flint 

 pebbles. 



The irregular and worn upper surface of the lower beds of sands 

 and mottled clays, upon which the basement bed of the London clay 

 reposes, is a corroborative proof of their partial denudation prior to 

 the deposition of the latter. The erosion is certainly not very great, 

 yet it forms an extremely well-marked phsenomenon, as at Sonning 

 Hill and Guildford (see Sections 6 and 1 2), at which latter place it 

 has removed all but a few patches of the Woolwich beds. In the 

 western parts of the London tertiary district it has worn to some 

 extent into the mottled clays. 



That the setting-in of this denuding action was sudden, is evident 

 from the abruptness of the change ; of its force, the size of the 

 transported pebbles, and the amount of erosion, lead us to judge 

 that it was moderate ; and that it was not of long duration is evi- 

 denced by the thinness of the deposit. 



The erosion is more apparent, and the exhibition of transporting 

 force greater, on the southern flank of the tertiary beds than on the 



