12 Professor Forbes on the Geology of Auvergne. 



of the scenery (although it is not on a very great scale) in the 

 Gorge d'Enfer in the Mont Dor. I think we are bound to con- 

 clude that the rocks of this country have undergone great 

 changes in the constitution, structure, and mutual relation, by 

 causes which have acted subsequently to the convulsions which 

 brought them into juxtaposition. Such changes are what 

 theory can take little account of, though they may be most nu- 

 merous and most important ; and this is one of the cases in 

 which the processes of nature seem more involved than we can 

 ever venture to render our theories, which, from our limited 

 sources of induction, are perhaps universally ioo simple, though 

 we dare not render them more complicated without deserting 

 altogether the track of logical analogies, and running the risk of 

 shipwreck amidst a chaos of possible contingencies. 



The relations of the igneous to the stratified rocks are also 

 very obscure ; we have mentioned the instance of Gcrgovia, and 

 we might extend our remarks to the fresh -water formations of 

 the Cantal. A sketch of these has been given by Messrs 

 Lyell and Murchison,* and also by M. Dufrenoy-|- and others. 

 I examined the curious sections occurring between Aurillac and 

 Tiesac, particularly that at the latter place, where I spent two 

 days. But I own that the dynamical conclusions from these 

 formations seem extremely^difficult. We are forced to consider 

 the limestone as anterior in point of date to the tufas and other 

 volcanic rocks, yet we frequently find masses of the latter con- 

 tained in the former. 



Perhaps the origin of these tufas is the most difficult, and one 

 of the most important problems presented by the Cantal and 

 Mont Dor. I shall hazard a few remarks respecting them, in 

 concluding with a notice of the theory of elevation. 



II. Theory of Elevation-Craters. — The hypothesis proposed 

 long ago by Von Buch, to account for certain appearances 

 in volcanic countries, (as in the Isle of Palma, one of the Ca- 

 nary Islands), amounts to this ; that, in some cases, the conical 

 form of volcanic mountains, instead of being assumed in virtue 



• Annales des Sciences Naturelles, torn, xviii. 

 f Annales des Mines, 2de Serie, torn, vii- 



