16 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
the whole exterior of the volcanic cone, and found the sandstone everywhere, ex- 
cept in the narrow space occupied by the issue of the lava-stream from the crater 
(see Plate II., fig. 5), which abuts against a hillock of sandstone, marked C on the 
plan. It does not, however, appear to reach the banks of the Alignon, and ter- 
minates abruptly against the two hills of gneiss, A and B, which form part of 
the boundary of the valley, which is everywhere of primitive rock. Near A, the 
junction of the coal-formation and the gneiss is well seen. The strata of sand- 
stone and gneiss resemble each other so much, that, being nearly conformable, it 
was some time before I could assure myself that the fine granite veins which occur 
there were not in the coal-formation. 
The crater is probably the largest in the Vivarais. It is low, strong, and of 
an elliptical form, and has burst at one end of the longer axis, being that to- 
wards the village of Jaujac. From the firmness and dimension of the lava walls, 
I presume that it must have been lofty, as is indeed probable, if it contained any 
large part of the prodigious flow of lava which proceeds from it, and which is 
evidently the result of one eruption, and probably the only one which this vol- 
cano has experienced. The interior of the crater is filled with clay and ashes, 
which sustain, however, a magnificent growth of chestnut trees. The open lip is 
narrow, and just gives vent to a stream of fine compact lava with little slag, 
which then fills the valley; the town of Jaujac stands on the extreme upper 
part of it. Near the mouth of the crater, and 1531 feet above the sea, is a spring 
plentifully charged with carbonic acid, whose temperature was 53°2. A spring, 
issuing from below the basalt, at the junction of the Alignon and Rioclat, had a 
temperature of 54°7 (23d June 1841). These temperatures are both above the 
mean of the place. The extreme height of the Coupe (by the mean of barome- 
trical observations in 1839 and 1841) is 1923 English féet above the sea. The 
level of the surface of the basaltic formation at Jaujac is about 1400 feet; and the 
top of the coulée of Neyrac, at the junction of the Ardéche and Alignon, is 1117 feet. 
Valley of the Ardéche—Thuez. 
If, instead of pursuing the course of the Alignon to Jaujac, we follow the 
principal stream of the Ardéche to Thuez, we first of all seem to leave all traces 
of basalts in the valley. Very soon, however, patches of volcanic formation 
appear upon the right of the road; whilst, on our left beyond the river, we have 
the volcano of Neyrac or Souillols, formerly mentioned as occupying the ridge 
between the two valleys, and as having unquestionably thrown out its principal 
stream into that of the Alignon. The summit of the volcano (which is at a 
height of 2178 feet above the sea, barometrically determined in 1839) commands 
a very fine view of the upper district of the Vivarais, and of the volcanoes which 
