16 Professor Forbes on the Geology of Auvergne. 



surface of the mountain groups : he has hkewise observed that, 

 as proper lava streams are always narrovv, such an arrangement 

 could not occur in all directions. But there is almost a physical 

 impossibility to such an arrangement, namely, that, unless the 

 crater had an edge or lip mathematically level all round, a uni- 

 versal effusion of the kind which I have alluded to could not 

 take place; some lateral deficiency must have drawn off the ris- 

 ing fluid, unless we conceive a supply so boundless as to resem- 

 ble that of a very copious spring rising up under hydrostatic 

 pressure, — a phenomenon far less reconcilable with those now ob- 

 served than the theory of Von Buch. 



3. The enormous disproportion of the valleys to the drainage 

 of a single conical mountain next strikes us ; and this is equally 

 applicable to the Mont Dor. It is hardly credible that such 

 and so numerous valleys could be the mere work of erosion.* 



4. This and similar difficulties seem to have been so strong- 

 ly felt, that many or most writers have taken refuge in the 

 theory of earthquakes having produced these rents ; — this, how- 

 ever, M. Elie de Beaumont justly observes, is a begging of the 

 question, since separation by fracture in this case implies eleva- 

 tion above the horizontal plane. [The contraction from cooling 

 might have been mentioned, but that is wholly inadequate.] 

 But there seems even a stronger objection, which brings those 

 who entertain such views directly under the ranks of the eleva- 

 tion theory. Such earthquakes could not have accompanied or 

 occurred between the deposition of these successive layers ; for, 

 had this been so, we should have wanted the perfect coincidence 

 between five, six, or seven successive beds exhibited on each side 

 of these valleys, and which seem perfectly to correspond. Indeed 

 no overflow could have taken place after the formation, of a sin- 

 gle such valley, as the lava must have followed the direction 

 of the valley, as we see constantly amidst the Monts Dome. 

 Consequently the earthquakes could not have taken place until 

 the effusions had completely ceased, and hence correspond in 



• In a memoir published since this paper was read, M. Elie de Beaumont 

 puts this difficulty in a forcible, and, it seems to me, not an exaggerated 

 light : — " On ne pourrait concevoir la reunion de Tune et I'autre circonstances, 

 la divergence des vallees jointe a I'existence des barrages, qu'en imaginant 

 poetiquementi des courans diluviens descendant du ciel en ligne perpendicu- 

 laire !"— Memoires pour servir, &c. tom. iii. p. 293. 1836. 



