260 Rev. W. D. Conybeare on the Phenomena of Geology 



in abruptly escarped basset edges, but that their planes were 

 prolonged so as to expire insensibly against the more elevated 

 portions of the substrata, which, on emerging, usually rise to 

 higher levels ; so that the whole surface at first presented one 

 uniform declivity, nearly uninterrupted. The Fluvialists must 

 then suppose that the drainage across this declivity excavated 

 the transverse valleys as its main channels, while the lateral 

 drainage into these main channels excavated the longitudinal 

 valleys. But in order to constitute the supposed original uniform 

 declivity, the mass of materials formerly upfilling the whole 

 space, and which we must imagine to have been subsequently 

 removed, is stupendously great ; for these longitudinal valleys 

 usually present very extensive plains at the foot of the basset 

 escarpments, whereas the transverse valleys are comparatively 

 narrow defiles. If then we attribute the latter to the main 

 course of drainage, and the former to its lateral action, we 

 attribute an inferior effect to what must surely be considered 

 as the most favourable line of action, and a vastly superior 

 effect to that least favourable. I would refer to the analysis 

 of my paper on the Thames, in your Magazine for July 1829, 

 for the particulars of my argument, as far as that district af- 

 fords any grounds of illustration. 



With regard to the evidence then adduced, it was remarked 

 by a writer, anxious at the same time to point out the neces- 

 sity of confining the inferences so as to leave untouched all 

 the districts in which facts of an opposite tendency might be 

 observed, " similar facts are supplied by nearly all the greater 

 valleys of England ; and on the whole they point to one con- 

 clusion, that fluviatile erosion, as a mere solitary agent, has 

 produced but small effects in modifying the prominent features 

 of our island." ( President Sedgwick's Address to the Geological 

 Society, 1830.) Mr. Lyell, on the contrary, in a passage ap- 

 parently designed as an allusion to this paper, has objected 

 to reasoning from the actual form of the surface in any given 

 district as to what may have taken place (as to drainage, &c.) 

 under the original configuration of the district, which he con- 

 ceives may have been entirely different. I can only reply, that 

 the actual configuration must in some manner have resulted 

 from that original one. He supposes the agent employed in 

 the transformation to have been fluviatile erosion. Now the 

 scope of my argument was intended to prove that no original 

 form of surface could be assigned from which fluviatile erosion 

 could have educed the actual form. This, 1 repeat, was the 

 aim of my argument; whether or not I succeeded in that aim 

 is another question. On this, as on other controverted subjects, 

 temperate discussion can alone elicit truth ; and I shall feel gra- 

 tified 



