clvi Report of the Mineralogical Survey [No. 126*. 



333. In the separation of these two facts,* continuity of elevated 

 ground and identity of geological structure, and which is everywhere 

 so strongly marked in these mountains, we may perceive proof in- 

 controvertible, that the present hydrographical arrangement of the sur- 

 face has been posterior to the original formation of these mountains ; in 

 other words, that their vallies or hollows are effects of denudation, 

 and not of original structure. This conclusion could not be more 

 firmly established, even if we saw the corresponding but disjoined ends 

 of the strata on opposite sides of the valley ; such an appearance, 

 however,'is by no means rare, as may be seen in the details given in the 

 preceding section. But the proofs need not stop here. For if there be 

 a truth more firmly established than another, by many and various 

 circumstances, it is this of the extensive waste which this surface has 

 undergone, and evidently from causes far exceeding in power any that 

 are now in operation. 



334. It is sufficiently obvious, however, that though the system of 

 vallies or drainage, generally speaking, be the effect of denudation, yet 

 we are not to attribute all the irregularities of the surface to this 

 cause. Some it is certain originally existed ; some may have been caused 

 by a sinking in of the strata. In the first way, we may account for 

 much of the great depth assigned to vallies in Art. 41. The excava- 

 tion of a valley of 15,000 feet in depth, and having a slope of nearly 

 30°, would indeed be incredible whatever force of water we employ, or 

 however long the period we have at our disposal. Even with this 

 abatement, enough remains to stagger our belief. Our incredulity 

 may, however, be softened by recollecting the continually recurring 

 difficulty of accounting for so many openings in the line of the strata, 

 without any marks of displacement or dislocation. 



335. There are some facts which, though they throw no light on the 



manner in which this great change of the surface has been effected, yet 



are sufficient to shew, that such a conclusion is not to be rejected, even 



though there may occur a difficulty in explaining all the details. The 



beds of some of the rivers are, for a part of their course, in the solid 



* One of the first impressions made on the mind in examining the Tortworth 

 district is, that the existing form of the surface appears to a certain extent to be 

 unconnected with the nature of the rocky formations that compose its base, an obser- 

 vation indeed that may admit of almost universal application, and be deemed a 

 maxim in Geology. — Mr. Weaver Geol. Trans, vol. \,p. 319. 



