﻿Vol. 66.'] THE WESTERN END OE THE WEALD. 671 



Lower Greensand for a considerable distance on either side, and so 

 rob neighbouring streams of their water-supply before their valleys 

 were actually breached by the subsequent river. Although, of course, 

 this principle is well understood, it is easily overlooked ; and, in 

 order to emphasize its importance, I may perhaps be permitted to call 

 attention to an actual example of its action in this district. A little 

 to the east of Farnham there were, till recently, two rivers flowing 

 over the Gault and Chalk, at the same level, into the Blackwater ; 

 of these the Farnham River has been captured by the Wey, and 

 now flows in a Lower Greensand valley 50 feet lower than its 

 former companion — the Seale stream — and only a mile away from it. 

 The consequence is that, at all ordinary times, the water which 

 falls on the Seale basin is carried away underground into the Wey, 

 and only when rain has been unusually heavy does any of it appear 

 on the surface and flow northwards into the Blackwater. The 

 Seale stream is therefore in an interesting transitional stage, in 

 which it is robbed of most of its water-supply, although its valley 

 has not yet been invaded. 



But it is not only upon general considerations such as these that 

 I found my opinion. Apart from arguments which might be derived 

 from other districts (for instance, the Vales of Pewsey, Kings- 

 clere, etc.), the drainage-area under consideration supplies direct 

 proof of the rapidity with which rivers, both subsequent and obse- 

 quent, can be developed. The presence of chert all over the Alice 

 Holt Plateau (including Farnham Common) affords strong grounds 

 for the belief that, when the plateau was formed, the Tilford River 

 was not yet in existence, at any rate between Headley Park and 

 Tilford ; and it is practically certain that the Waverley Valley was 

 at that time occupied by the consequent head of the Blackwater. 

 The present level of the river at Tilford is about 170 feet ; that of the 

 Alice Holt Plateau (the date of which is fixed by the palaeoliths of 

 late type in its gravels) 360 feet ; and that of the Farnham Plateau, 

 610 feet. It is, therefore, clear that the formation of the Tilford 

 Eiver and the reversal of drainage in the Waverley Valley belong 

 to the second half (and perhaps the smaller half) of the cycle of 

 which the Farnham Plateau marks the initial stage. If that cycle 

 has evolved the whole drainage-system from a marine plain, these 

 facts are intelligible ; but if, on the other hand, it is only part of a 

 vastly longer cycle of subaerial denudation, the delay in the forma- 

 tion of a Lower Greensand subsequent appears to me very difficult 

 to explain, in view of the rapidity with which it attained maturity 

 when once it started. 



So far I have only dealt with the hypothesis of complete marine 

 planation on the one hand, and of prolonged subaerial denudation 

 on the other ; there remains Prestwich's alternative of a combina- 

 tion of the two. This is so elastic a theory that it is obviously 

 impossible for me to meet, in this paper, all the various aspects 

 which it may be made to present ; but I think it will be found 

 that, unless we practically identify it with the marine hypothesis, 



