18 Professor Forbes on the Geology of Auvergne. 



afterwards in the Mont Dor. The lowest tufaceous bed in the 

 section at the cascade of Mont Dor already given, which con- 

 tains recent wood, indicates, I think, a fluviatile or lacustrine 

 origin. The latter is rendered more probable (and it is the 

 most conformable to the views just stated) from the very re- 

 markable tufaceous deposit in a neighbouring valley, above the 

 spot called La Verniere, where a pumiceous conglomerate occurs 

 inter stratified with the most delicately deposited beds of fine 

 white clay, which must evidently have had their origin in still 

 water, l^his does not accord with the idea of Mr Scrope in his 

 beautifully illustrated work on this district, that such beds were 

 formed from the debris accumulated by torrents. 



6. I must, however, observe, that the origin of these tuface- 

 ous beds seems to be distinct from that of the great mass of 

 conglomerate which occupies the base of almost all these valleys 

 to an immense depth. It is a proper rock, and is, as MM. Elie 

 de Beaumont and Dufrenoy observe, to be distinguished care- 

 fully from the mud eruptions by which Herculaneum was co- 

 vered. It has a trachytic basis, and the inclosed fragments are 

 generally trachyte ; so that it is probably the result of some in- 

 ternal interrupted process, not a recomposed rock in the proper 

 sense of the words. This is entirely confirmed by the undoubt- 

 ed mechanical energy it has exerted, as, for example, in dis- 

 turbing, elevating, and imbedding the tertiary stratified deposits 

 with which it came in contact, as described by Messrs Lyell and 

 Murchison*, and others. I own that the strong impression 

 made upon my mind (notwithstanding its prima facie improba- 

 bility) by the Cantal was, that this conglomerate had been the 

 agent of elevation of the whole group. The evidence seems to 

 strengthen upon reflection, nor can I easily account otherwise 

 for the entire occupation of the bottom of the valleys by this 

 conglomerate, which must have been itself fissured had it been 

 merely passive. At all events, the disturbance of the stratified 

 rocks by this material must be considered as entirely conforma- 

 ble to the elevation theory, and as indicating beyond any doubt 

 a certain amount of convulsion. 



7. Upon the whole, it seems to me that the evidence of earth- 

 quakes subsequent to the deposition (in whatever way) of the 



• Annales des Science Nat. xviii. 172. 



