﻿688 ME. H. BURY ON THE DENUDATION OF [Nov. I 9 10, 



(Gr) General Remarks. 



The general tendency of the foregoing pages has heen to re- 

 establish, though on a very different foundation, Itamsay's hypo- 

 thesis of marine planation. Perhaps only in the case of the 

 Blackwater will the proofs be deemed sufficient ; the other rivers, 

 indeed, offer a good deal of evidence, derived mainly from their 

 relation to the folds, and the distribution of chert, which appears 

 to me to point in the same direction and to have a cumulative 

 strength ; but it may be thought wiser to reserve our final judgment 

 on them until the whole river-system of the Wealden area lias been 

 critically examined. 



There is a feature in the Chalk Plateau of the North Downs 

 which demands further attention. In each interspace between 

 rivers we notice one point which reaches the summit-line (figs. 1 

 & 2, p. 642), and from this the Downs generally slope away on 

 either side, gradually at first, down to about 600 or 700 feet O.D., 

 and then abruptly into the river-valley. It thus often happens 

 that we find large areas of plateau, which appear to be parts of 

 the plain, and yet are theoretically too low. Such curvature of 

 the summit-line is normal where it is accompanied by horizontal 

 curvature of the escarpment (3, p. 134) ; but in this region the latter 

 is far too straight, and the inclination of the plateau too small, to 

 produce the observed effect. Transverse synclines would be capable 

 of giving rise to such curves, but so far their existence has not been 

 satisfactorily demonstrated. In addition to this, a correspondence 

 occurs in many cases, both in the North and in the South Downs, 

 between the highest points on the summit-line and the watersheds 

 of the subsequent rivers in the Lower Greensand. There is no need 

 to give full details of this interesting fact, since it can easily be 

 verified on any good map : it will be found especially conspicuous 

 between the Wey and the Mole, on both sides of the Darent, at 

 Lenham (between. the Medway and the Stour); and in the South 

 Downs at Ditchling Beacon (Adur and Ouse). 



At first sight it appears as if, in the combination of these two 

 features, we had evidence of the great age of existing consequent 

 rivers, and of something approaching Prof. Davis's subaerial pene- 

 plain. We might suppose, if they stood alone, that, in the course of 

 long ages, transverse watersheds were established in the Chalk area, 

 on each side of which gentle slopes led down to the consequent rivers 

 themselves ; and that, on the retreat of the escarpment, the same 

 watersheds had been preserved in the Lower Greensand. But 

 further study reveals obstacles which appear to me fatal to this 

 hypothesis. In the first place, it seems to require that the same 

 watersheds should still extend over the Chalk itself, and it is seldom 

 that any trace of such extension is found ; whatever may be their 

 ultimate connections over the Tertiary strata, nearly all the valleys 

 which drain the Chalk-slopes start at right angles to the escarp- 

 ment, and exhibit no relation whatever to the neighbouring con- 

 sequent rivers. Secondly, there is the distribution of chert, so often 



