﻿Vol. 64.] ME. H. BUET ON THE EIVEE WET. 331 



recent ongin — without any narrowing of its valley or steepening 

 of its sides — is well-nigh incredible. Yet that section of the valley 

 which lies between Alton and Cuckoo's Corner (the junction of 

 Chalk and Upper Greensand) shows a bottom as wide and almost 

 as flat as at Farnham itself; and indeed from one watershed to 

 another it is actually much wider than at that town, some 8 miles 

 lower down. There is plenty of evidence in the neighbourhood to 

 show that the Lower Chalk is comparatively non-resistant, and 

 is readily levelled down when exposed ; but, if the valley is an 

 outgrowth from the Weald, how did such a width of these strata 

 become exposed ? And, even if we assume (without any justi- 

 fication) that all the Chalk is soft in this region, we are still in 

 difficulties with the Upper Greensand, which is, beyond all doubt, 

 a highly-resistant rock. The only explanation that I can find 

 which will meet all these facts is, that a broad valley must 

 have been already formed here in the Chalk (and perhaps Eocene 

 Beds) and have \ieen already connected with the Blackwater near 

 Parnham, before the Wealden strata of this region were uncovered. 

 An examination, however imperfect, of the history of the denuda- 

 tion of the Weald will show us that this conclusion is not so 

 improbable as it may at first sight appear. 



It is generally agreed that the Weald, after passing through 

 a first cycle of denudation was reduced almost to a plain. This 

 used to be referred to as a plain of marine denudation, but perhaps 

 at the present time Prof. W. M. Davis's view would receive more 

 support — that it was a peneplain due to fluviatile erosion. But, 

 although this belief in a plain has long existed, no satisfactory 

 attempt has, so far as I know, been made to show at what horizons 

 we may seek at the present day for remnants of it. I am far from 

 possessing enough knowledge to attempt such a reconstruction, and 

 in what follows I am merely seeking to ascertain the lowest points 

 that may have been reached hj parts of that plain, so as to get 

 some idea of the extension of the Chalk at the close of the first cycle 

 of erosion. In the western portion of the Weald, with which alone 

 I am concerned at present, the highest point is Blackdown Hill 

 (918 feet) immediately south of Hindhead ; but nothing as high as 

 this is found in either the North or the South Downs. Therefore we 

 may probably infer that the upheaval which ushered in the present 

 cycle of denudation was of a differential character, affecting the 

 central axis more than the sides, and perhaps also raising the 

 southern slopes more than the northern, since we find several 

 points in the South Downs rising above 800 feet (including Butser 

 Hill, 889 feet), while in the North Downs we find only two hiUs 

 west of Dorking which rise above the 700-foot level — White Down 

 (712 feet), near Dorking, and Holybourne Down (728 feet), close to 

 Alton. If, then, we wish to ascribe any lower points to the plain, 

 we must, on the marine theory, postulate some longitudinal 

 irregularities in the upheaval, as well as the above-mentioned 

 transverse one. But it is less necessary to do this if we adopt the 

 fluviatile hypothesis, since a peneplain allows of the existence of 

 hills of moderate height above the general base-level of denudation. 



