VOLCANO OF MOULEYRES—ITS LAVAS. 19 
and within it, that we are to look for the source of the great eruption which has 
at one time levelled the whole valley. A small ravine and bridge may be ob- 
served between the volcano and the village. This is called the Gueule d’ Enfer by 
Favsas, and is figured by him ; the Pont du Diable is concealed by the foreground 
of the drawing. The Gueule d’Enfer now affords passage to the stream belong- 
ing to the lateral valley. It is almost entirely formed of granite, and the con- 
trast of the granite and basalt may be traced on the left of the bridge by which 
the ascending road from Aubenas reaches Thuez. 
Now, in the first place, as to the origin of the lava there can be no doubt. 
The volcano of Mouleyres or Thuez has evidently produced at one prodigious throe 
all this mass, and may be said to have expired in the effort ; for there is no appear- 
ance of any repetition of the action, and the crater is burst completely open on 
the side of Thuez, so as to represent, like many volcanic cones near Clermont, the 
figure of an arm-chair. A portion of the lava seems, however, to have escaped 
from the crater towards the south, and to have formed one of the basaltic plateaux 
in the lower part of the valley, and at a much lower level. The highest part of 
the crater of Mouleyres or Thuez is 2026 feet above the sea, or 500 feet above 
the general level of the lava-bed on which the village is placed.* 
We shall call the Jateral valley of Thuez that which is seen in the pano- 
ramic view extending towards the Gravenne of Montpezat. I have repeatedly 
examined the whole of this valley with the greatest care, in order to decide 
whether the last-named volcano had any share in producing the lava which fills 
it (as some authors have supposed). There is certainly an obscure appearance of 
a slaggy lava-stream having descended the Gravenne on the side of Thuez, but it 
is everywhere covered with loose cinders, whose boundary with the granite may 
be traced right and left. These cinders are in contact with the slag of the volcano 
of Mouleyres to the east, and they are lost in the bottom of the valley amongst 
the multitude of granite blocks by which it is entirely choked, and through which 
the rivulet (the same which passes through the Gueule d’Enfer) makes its way 
without leaving a trace of a section which should decide whether or not there is 
a lava-stream beneath. Even the undoubted lava of Mouleyres can be traced but 
a very little way above Thuez, in the bed of the stream. It is important to men- 
tion that amidst the granite blocks many are found evidently altered by heat ; 
they are heavy, red, and friable, and have no doubt been ejected from the Gravenne. 
Similar blocks are likewise common at Jaujac and elsewhere. 
If we would now trace more accurately the composition and dimension of 
the lava-stream, as shewn in the valley of the Ardéche, the best way is to descend 
* The elevations in this part of the valley were deduced from barometrical observations chiefly 
made in 1841, and referred to Thuez as a standard height. This latter has been estimated at 1545 
feet above the sea at Marseilles from six observations in 1839 and 1841, compared with those of M. 
Valz at Marseilles, and kindly communicated to me by that excellent observer. 
