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



to oppose an uniform resistance to it ; when any circumstance 

 occurs which creates a variation in the resistance, such as the 

 change from softer to harder strata, faults traversing them, 

 and the like, a corresponding deflexion in the course of the 

 current seems a necessary consequence. 



The origin of the valleys of excavation, then, I am inclined 

 to refer in part to the currents of the ocean in which they 

 were first deposited, in part to those accompanying the gradual 

 retreat of that ocean. But since we have also sufficient evidence 

 that subsequent convulsions, such as the elevation of the Isle 

 of Wight for instance, must have disturbed the oceanic level 

 sufficiently to have occasioned renewed diluvial waves sweep- 

 ing over tracts which had previously emerged, we have hence 

 a third class of currents, which must undoubtedly have tended 

 greatly to modify the results of the two former. 



Do I then deny that fluvial erosion has ever produced a 

 single valley? and if so, how do I dispose of the evidence 

 which has been brought forward in favour of this view ? I will 

 avow the tendency of my arguments openly and frankly. I deny 

 that all valleys of excavation have been so produced: I deny 

 that many have been so produced : I deny that any have been 

 so produced, except under extraordinary circumstances. And 

 to the evidence I reply, that it relates to districts in which these 

 extraordinary circumstances undoubtedly exist, volcanic di- 

 stricts for instance, such as Auvergne and the neighbourhood 

 of lEtna. Now I cannot admit the action of torrents occa- 

 sioned by, and cooperating with, volcanic convulsions, as an 

 example of the ordinary action of common streams; but that 

 under these extraordinary circumstances, and even under such 

 more common but still comparatively rare incidents as the 

 late floods in Scotland, fluvial action may occasionally produce 

 considerable effects, / do not deny. 



Art. III. The phenomena of cataracts are inconsistent 

 with the fluvial hypothesis. 



The fluvial hypothesis requires me to believe, that since the 

 emergence of our continents the atmospheric drainage has 

 commonly furrowed them into valleys hundreds of miles in 

 length, and hundreds of feet in depth ; that the streamlets 

 forming the Thames, for instance, have done this. But if such 

 have been the effects of these comparatively tranquil streams, 

 what must the effects have been of a volume of water like that 

 of Niagara precipitated in thundering fury ! 1 here indeed 

 take it for granted that Thames and Niagara have been 

 acting on the surface for the same period. If this be denied, 

 I shall certainly require a reason for that denial, and shall 

 wish to be informed what is the exact seniority of old Father 



Thames 



