6 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
lets of melted matter were far more widely spread than this idea of a central vol- 
cano would permit. Perhaps it may added that the close proximity of .the 
granite to the surface, wherever the volcanic materials have been wasted suffi- 
ciently to expose it, and ¢iat particularly in the valley of the Salliouse, as shewn 
in M. Bertrann’s Geological Map, almost close to the Cirque de Clusels, seems at 
variance with the supposition that Mont Mezenc is to be regarded as the sole vol- 
canic centre which gave rise to such widely-spread phenomena. 
I must still more emphatically dissent from the theory that the hills of 
phonolite, forming the north-east barrier of the basin of Le Puy, are the relics of 
a felspathic eruption proceeding from the Mezenc.* Not to multiply arguments 
against so very bold, and, in itself, I must think, so improbable a supposition, I 
will only observe, jist, the rarity of any appearances, in trachytic mountains, in- 
dicating that the matter of which they are composed has been sufficiently liquid 
to flow, after the manner of a current, over any extent of country. Secondly, the 
absence of any appearance of a current in the chain of detached sugar-loaf 
shaped eminences here referred to. Thirdly, that if it were a current, it would 
not have occupied the axis of a granitic elevation, constituting one of the oro- 
graphic features of the country; and, dastly, that the direction of the chain, coin- 
cident with that of other important chains, and especially of the chain separating 
the Allier and the Loire, and the chain of La Margeride, beyond the former, evi- 
dently points out an axis of elevation which, in other instances, in this singular 
country, is the fertile source of local explosions and eruptions, to which these in- 
sulated phonolitic peaks may, in my opinion, be, with far more probability, 
ascribed. 
The route, southwards from the Mezenc, presents some singular features. 
We follow a ridge, sometimes composed of trachyte or phonolite, sometimes of 
basalt, which separates the gentle slopes towards the basin of the Loire and the 
more precipitous ones towards that of the Rhone. The views, in the latter direc- 
tion, are eminently singular, and even romantic; a country intersected with deep 
ravines, and divided by deep hilly ranges, often capped by fantastic summits, 
stretches away for many miles in the direction of Chalangon and La Voulte. 
Some idea of the scenery may be formed from the sketch in Plate V., fig. 5, 
taken from a spot commanding a view towards the Mezenc, across the country 
now referred to. The form of the phonolitic summits (those whose names 
are given on the drawing) marks their composition in a manner scarcely to 
be mistaken; but the basis of the whole is granite or gneiss, as pointed out 
in M. Berrranp’s map, and in the new Geological Map of France. It is pro- 
* «The uniformly progressive declination of this series of phonolitic summits from the Mezene 
to the bed of the river where they terminate, proves them, in my opinion, to be the remains of a 
single enormous lava current prior in date to the excavation of the actual channel of the Loire, 
and far the most considerable in bulk and extent of any which I haye had occasion to observe in the 
phlegrean fields of France.””—Scrore’s Geology of Central France, p. 129. 
