18 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
the greater part both of the lateral and principal valley, damming back the river 
Ardéche so as to form a lake, which was only drained when the unceasing and all- 
powerful action of water had excavated a channel in the hard basalts, and even 
in the granites beneath them, leaving a cliff, which has been already described, 
towering above the walls of primitive rock, between which the stream struggles, 
forming a scene far more varied and picturesque, though less regular, than the 
columnade of Jaujac which we have lately quitted. 
Dire must have been the confusion which the element of fire wrought in 
this quiet valley. Scarcely less appalling the confusion and organic change pro- 
duced by the subsequent action of the water upon the intruded masses which had 
for a while staid its course. But now all is again tranquil, and the progress of 
events is marked by almost imperceptible gradation ; for now, the lava being no- 
where in contact with the stream, the action of the river on the granite may be 
considered as inappreciable. The platform raised by the lava, and terminated by a 
tremendous chasm on the side next the valley, is covered by a most luxuriant 
vegetation. The village, surrounded by its vines and maize, is exactly opposite 
to the spot whence the volcano of Mouleyres or Thuez must have evacuated the 
most abundant part of the fiery flood which charged it; the crater itself, which 
rises to 2026 feet above the sea, is filled (as usual) with splendid chestnut trees, 
and its porous cone is planted, like the sides of Vesuvius, with productive vine- 
yards; the site of the lake once formed by the waters of the Ardéche is now a 
fertile meadow ; and the chasm once so ruined and bare, in which the river flows 
past Thuez, is now ornamented with luxuriant wood of walnut, chestnut, and beech, 
which give a great charm to the contrasted outlines of the jagged granites, and 
the alternately level and perpendicular basalts. In many places the primitive 
soil of the valley, the granite surface, cleared of the prodigious load of black rock 
which for ages covered it, is again brought into cultivation and yields abundant 
crops. 
[have attempted to give (in Plate VI., fig. 5) a panoramic sketch of the 
position of Thuez, which may serve to illustrate this description. It is taken from 
the granitic heights on the opposite side of the Ardéche. In the centre of the 
view, in the middle distance, is the village of Thuez, resting, as has been explained, 
on the almost horizontal plateau of basalt, whose front or section is presented to 
the spectator, and which stretches for a great way to the left, or wp the valley of 
the Ardéche, with gradually diminishing elevation. In the nearer part of the draw- 
ing, on the left, is seen the deep chasm 7n the granite rocks through which the 
river passes. Immediately behind Thuez, at the closed end of the little lateral 
valley which we have mentioned, rises the Giravenne of Montpezat, a volcano 
which has thrown by far the greater part, if not all, of its lava in a contrary direc- 
tion, and which, therefore, has not produced at least any sensible portion of the 
basalts of Thuez. It is to the open and degraded crater on the right, whose vivid 
red colour contrasts splendidly in nature with the bright green of the trees around 
