^0 Professor Forbes on the Geology ofAuvergne. 



observed phenomena. Many circumstances lead me to conclude 

 that the changes undergone by this district have been vastly 

 more complicated, more numerous, and more prolonged ; and 

 that it would be unreasonable to expect that such phenomena 

 should always be of so simple and so elementary a description. 

 Some facts inferring many successive changes in the materials 

 constituting the Mont Dor have been already adverted to. 

 Others it would be easy to cite. The numerous dykes of tra- 

 chyte and basalt are especially remarkable, and if accompanying 

 or in any part posterior to the general elevation, must necessa- 

 rily have altered most materially the configuration of the ground, 

 and the relations of its constituents. That the three points or 

 centres of elevation indicated by MM. Elie de Beaumont and 

 Dufrenoy, viz. the Puy de Sancy, the Puy de la Tache, and the 

 Roche Sanadoire, were really points of disturbance and eleva- 

 tion, seems scarcely to admit of doubt. The last, indeed, speaks 

 for itself; the spectator, placed on one of the magnificent pho- 

 nolitic masses of La Thuilliere or La Roche Sanadoire, finds 

 himself nearly surrounded by mural escarpments. But I con- 

 tend that there must probably have been many more centres of 

 effervescence, whose successive outbreaks may have materially 

 changed the original configuration of the district. The termina- 

 tions of many of the valleys differ from what the elevation theory 

 would point out, neither expanding into craters, nor vanishing 

 in mere rents, but having frequently rounded mural termina- 

 tions, which may be compared (as I rather think M. Elie de 

 Beaumont himself has done) to the oules or theatre-like termi- 

 nations of many of the Pyrenean valleys. One of these partial 

 disturbances may, I conceive, be seen in a very remarkable 

 crater-like cavity close to the Roc Crusau ; nor does it appear 

 that the disturbing energy was exhausted until it had spent it 

 self by the comparatively modern orifices of the Puy de Tar- 

 taret, the lakes Pavin, Servieres, and others. 



Edikburgh, bth December 1 835. 



