26 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
culties. It must be a stream from the Crater of Pal, already adverted to in de- 
scribing the route by the Bauzon from Le Puy to Montpezat. This well-formed, 
nearly circular crater occurs alittle behind the granite rocks and cliffs which com- 
pose the back-ground of the ravine of Chambon ; and so far it would appear to 
be a very natural origin of the lava. But the singular circumstance is, that the 
crater in question belongs to a different system of valleys ; inasmuch as the water 
issuing from it, forms the source of the Fontaulier, and reaches the vicinity of 
Montpezat after a long circuit (see the General Map, Plate I.). It is also pretty 
open to the north ; but it seems completely cut off from the ravine of Chambon by 
a considerable granite ridge running NW. and SE., forming the Col or passage o. 
Le Pal, and rising to a height of 4537 feet, exactly between the crater of Pal and 
lava of Chambon, being 644 feet higher than the former, and 403 feet above the 
extreme point of the latter. There are therefore two difficulties to be accounted 
for; jivst, the passage of a great mass of lava and other volcanic matters over 
this barrier ; secondly, the excavation of the ravine of Chambon, and the removal 
of all the upper part of the lava stream. 
With regard to the first difficulty, it is important to remark that most of the 
granite heights between Le Pal and Chambon are covered more or less thickly 
with a volcanic conglomerate forming horizontal beds (one granite top which has 
escaped, is uncoloured on the map). This conglomerate descends to the crater of 
Pal, and stops short abruptly on the face nearest Le Chambon (see section, 
Plate IV., fig. 2). Consequently, the hills about C (fig. 2, Plan) did not probably 
always form the highest part of the edge of the crater; for the conglomerate is of 
too compact a character to have been formed by a dejection of volcanic materials 
merely; and the same cause which permitted the deposition of these conglo- 
merates (such as the heightening of the crater by the formation of a cone which 
has since disappeared), would permit equally the passage of a part of the fused 
materials across the ridge into the neighbouring valley. And the fact of the oc- 
currence of abundant conglomerates in both positions, confirms, if it does not 
render absolutely necessary, this supposition ; although I am aware that there is 
some mineralogical difference in their composition, —that of Chambon being more 
friable, and more generally composed of volcanic ingredients, containing, however, 
granitic masses,—that of Pal being chiefly of granite boulders, cemented by a very 
hard volcanic basis. The posteriority of the conglomerate eruption may account 
(together with the great declivity) for the absence of any trace of the lava flow 
on the granite heights of Le Pal. 
The second difficulty, that of the excavation of the ravine of Chambon since 
the lava flowed, must remain, I fear, unanswered, upon any theory. Of the fact 
there appears to be no reasonable doubt. This (geologically) recent excavation 
of a perfect mountain of hard granite, at the head of a ravine which possesses no 
drainage sufficient to procure a powerful current of water, and which is near the 
culminating point of three pretty extensive mountain-ridges, is merely one of a 
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