and on the On^in of the Vulkijs of Auvetgne. 



mark, that we are much indebted originally to Mr Scrope, and 

 afterwards to Messrs Murchison and Lyell, for shewing, in a 

 more decisive manner than had before been done, the degree to 

 which the accumulated waters of even a small stream are capably 

 of eating into the substance of the most compact rockstp jntjijgiFi 



As the exclusive consideration of one set of causes rendera u» 

 less alive to indications of an opposite nature, it was the natural 

 tendency of the diluvial theory to lead us to depreciate the ope- 

 ration of existing causes; and it cannot be doubted but that the 

 collision of opinion has in this instance brought to light facts» 

 which the influence of one exclusive system might have caused 

 us for some time to overlook. ;,....,,,. .., 



. Important, however, as their observations may be with refe-* 

 fence to the general question at issue, they have evidently no pe- 

 culiar bearing on that respecting Auvergne, the analogy be- 

 tween the ravines in that district which Mr Scrope represents as 

 formed by the present rivers, and the valleys contiguous to them» 

 being much less close than that existing between the latter in 

 this ami in other countries. 



I might therefore concede the whole of this part of the argu* 

 pepty without abandoning the position I have taken up in this 

 memoir ; but, as I chanced this summer to visit the neighbour^ 

 liood of l*ont Gibaud, whence Mr Scrope has derived some of 

 his most striking proofs of the action of rivers, it may not be 

 irrelevant to point out certain circumstances relative to the phe* 

 nomena there exhibited, that seem to throw doubt upon his 

 conclusions in certain cases, and limit the effects there attributa* 

 ble to causes now in action within a narrower circle than he is 

 inclined to suppose. 



;f I shall begin by observing, that, in order to shew that a lava 

 current has been cut through by a river since the occurrence of 

 a supposed deluge, it must be ascertained, 1^^, That the coulee 

 itself exists on both sides of the river ; *ildly^ That it has de- 

 scended from a crater ; and, 3^%, That the crater itself is com- 

 posed of such loose materials, that it would have been washed 

 away by any great body of water which might have overflowed 

 the country. 



Now, Mr Scrope has presented us with two examples near the 

 town of Pont Gibaud, one of which fulfils all the required con- 

 ditions, and establishes the fact that the River Sioule has worn 



