22 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
this point was undoubtedly the ancient issue of the stream of the lateral valley 
which now escapes at the Gueule d’Enfer. I have ascertained by barometrical 
observations that this is really the lowest point of contact of the lava with the 
ancient surface of the granite valley; and was therefore the thaliveg or water- 
way until it was choked by the lava of Mouleyres. If, then, the Gravenne gave 
birth to any stream of lava, however small, it might be expected to have flowed in 
this direction: If this change of the water-way be admitted (and it appears to be 
unquestionable), there is another proof, amounting almost to demonstration, that 
the Gueule d’Enfer has been excavated in the granite or gneiss since the lava of 
Mouleyres flowed ; for before that time no water could have run through it. 
The following are the results of my barometrical observations :— 
Eng. Ft. above Sea. 

Summit of Lava Cliff at Echelle du Roi, . , : . 1476 
Foot of i ae be me cS H 5 ; 1283 
Summit of Lava Cliff at Gueule d’Enfer, : ; 2 1514 
Foot of sae pee nee ae : F 3 1343 
Height of Cliff at Echelle du Roi (A), - : ; : 193 
oe Gueule d’Enfer (B), . : ; ; 171 
Surface of Lava at A below Surface at B, : F ; 38 
Contact of Lava with Ground at A below contact at B, 60 
The lava of Thuez gradually thins out as we ascend the valley ; continuing, 
however, to present a basaltic cliff towards the stream. Where the lava ceases, 
the valley expands ; and the river has a wide bed formed of detritus, preserving a 
uniform level, evidently occasioned by a lake which once existed, due to the ob- 
struction caused by the lava to the waters of the Ardéche.* 
Montpezat. 
A short walk from Thuez leads to the top of the volcano called La Gravenne 
de Montpezat ; but the ascent is rapid, the strata of pozzuolana constituting the 
southern face of the Gravenne dip at an angle of about 30°, the superficial debris 
25°. The principal discharge of lava being towards the north, filling the valley of 
Montpezat, we shall next in order consider the phenomena which that district, 
watered by the Fontaulier, presents. The Gravenne itself rises to a height of 2727 
feet. The view from it is extensive and striking. It has an exceedingly well- 
formed crater, which, however, is seen from but few points. Plate IIL. fig. 4, 
shews its appearance, as seen from a hill to the north, on a ridge between Mont- 
Fi About 5 hours’ walk across the hills to the south-west of Mayres, or 7 hours’ from Thuez 
(passing le Chambon and Bornes), are the remote mineral-springs of St Laurent les Bains, having 
a temperature of 125° Fahr. They rise from mica slate, in the neighbourhood of granite, and 
traversed by granite veins. They contain salts of soda almost exclusively, and particularly the 
carbonate. 
