7 
GEOLOGY OF MONT MEZENC. 5 
the fragments of trachyte found, as already mentioned, completely imbedded in 
the basalt, near the church at Fay-le-froid. 
That the Mountain of Mezenc is situated near what was once a centre of vol- 
eanic action is highly probable, not only from being the most elevated summit of 
the whole country, and itself composed of volcanic rocks, but from the man- 
ner in which the basaltic plateaux which have since been broken up, and in 
great part removed by denuding causes, may be traced from the very environs 
of Le Puy up to this chain, and also from the peculiar evidences of local fire 
which the torrefied materials exhibit in the section just presented. I am, how- 
ever, disposed to agree with those geologists who consider that since the produc- 
tion (perhaps in a good measure by elevation) of the Mont Mezene, the contour of 
the ground has been so completely altered and disfigured as to leave no ground 
for inferring that we are to trace in its lineaments the actual point of ejection, 
still less that the Cirgue de Clusels or Boutieres is really a “* crater of elevation,” 
for it appears to want the essential characters of a crater at all. It would be dif- 
ficult to prove that the strata (ill-defined even where they exist) dip away uni- 
versally from a common centre ; for only the northern and western portions of the 
supposed circus can be found; and the argument which has been drawn* from the 
precipitousness of the rocks in this place compared to their gentle slope in other 
directions is worth little, since the same reasoning would apply to the whole eastern 
flank of the range, between Mont Mezenc and the Gerbier des Joncs, compared to 
the western, which depends upon some general, but probably posterior cause of 
denudation of which I can give no account. That the expansive action of the 
basalt, whilst fluid, elevated the previously existing beds of trachyte, and thus 
contributed to give to the Mezenc its present height, I think there can be no 
reasonable doubt ; and the dikes of basalt occurring here and elsewhere appear to 
confirm this opinion. It is, however, worthy of note, that if this be cited as an 
argument in favour of “elevation craters,” and as confirming the usual chronology 
of volcanic rocks of different mineral characters, in the Cantal at least, we find a 
mountain of phonolite, the Puy Griou, subjacent to the basalts which it is sup- 
posed to have elevated. 
Whilst I admit, with M. Berrranp, that immense lava streams proceeded 
from some point or fissure near the Mont Mezene, and flooded the fresh-water for- 
mations of a great part of the basin of Le Puy, I should hesitate before ascribing, 
with Mr Scrorr, the immense basaltic plateaux of the Coyrons in the lower Vi- 
varais, with the least degree of certainty, or even of probability, to an eruption 
of the Mezenc.} Amongst other arguments, the numerous dikes of basalt, tra- 
versing granite, and other formations in this country, seem to shew that the out- 
* Burat Terrains Voleaniques de la France Centrale, p. 230. 
Mr Scropr’s General Map gives an altogether erroneous idea of the proximity and mutual 
* dependence of the basaltic formations of the Mezene and those of the Coyrons. 
VOL. XX. PART I. B 
