﻿Vol. 66. 1 , THE WESTERN END OF THE WEALD. 649 



Beacon Hill, and the high plain (now nameless) which connects them together. 

 Chert is abundant in some Drift near Clare Park, to which more attention will 

 be directed later ; and, lastly, I have found a few pebbles on the summit of the 

 Doaviis above Bentley. 



These facts accord very well with the hypothesis of marine 

 planation, whether we regard the sea or the rivers as the trans- 

 porting agent of the chert ; but how they affect the theories of 

 Prestwich and Prof. Davis must now be separately considered. 



(B) Hypothesis of a Subaerial Peneplain. 



The general features of Prof. Davis's hypothesis have already 

 been noticed, and it is unnecessary to repeat them here. What we 

 have to examine is whether it affords any satisfactory explanation 

 of the distribution of chert along the North Downs. 



In the maturity of the first cycle there might be much trans- 

 ference of this material across the Chalk by consequent rivers, 

 more numerous than those surviving at the present day ; but a 

 prolonged period of old age followed, during which the consequent 

 streams were reduced in number, and the hills were gradually 

 degraded to mere elongated mounds. Having regard to our 

 experience elsewhere as to the disappearance of drift materials, it 

 seems to me impossible that any appreciable remnant of the earlier 

 river-gravels could have survived such extensive denudation as this. 

 When upheaval followed, and the second cycle began, no fresh 

 transference of pebbles from the Lower Greensand to the Chalk 

 could occur, except along the lines of the old consequent rivers. 

 Prof. Davis lays it down as an essential difference between a cycle of 

 denudation on an uplifted marine plain, and a second cycle following 

 a subaerial plain, that the drainage of the former would for some 

 time be solely by consequent streams, whereas that of the latter 

 would from the first consist largely of revived subsequent rivers ; 

 and these subsequent rivers would evidently form an effectual barrier 

 to such a direct transference of material as we are considering. 

 Nor do I see how this distribution of chert could be accounted for 

 by oscillations of existing consequent rivers on the peneplain : for, 

 in the first place, the Southern Drift is often close to the escarpment, 

 the straightness of which is attributed to planation ; and secondly, 

 rivers meandering in such wide curves could scarcely have the 

 necessary transporting power. 



It might perhaps be thought possible, without altogether 

 abandoning Prof. Davis's hypothesis, to assume the marine origin of 

 the drifts containing Lower Greensand pebbles, and treat them 

 as belonging to a coastal plain continuous (after upheaval) with 

 the subaerial peneplain : but, apart from objections which will be 

 raised in connection with Prestwich's views, it must be remembered 

 that Prof. Davis's conclusions rest, not on two separate lines of 

 argument, but upon the combination of a bevelled escarpment 

 with the mature drainage-system. With the admission of marine 

 action along the North Downs we destroy all evidence of subaerial 

 bevelling, and thereby undermine the whole argument. 



