VALLEY OF THE ARDECHE—NEYRAC—THUEZ. 17 
occur there, as well as of the valley of Thuez. It has a distinct, though degraded 
crater, which has given way on the south-eastern side. The sides of the ridge 
are wooded; but I traced two distinct streams, though of no great dimensions, 
into the valley of the Ardéche, which evidently came from the crater of Neyrac. 
They include between them the hamlet which bears that name; and one at 
least of them (the most westerly) may be traced down the bed of an insignificant 
watercourse in the granite, down to the valley, where it has formed columnar 
basalt. for a short distance. The other patches of basalt, between this point and 
the junction of the Alignon, are probably due to this stream. 
Opposite to the village of Neyrac, the following section occurs (Plate IIL, fig. 1), 
‘which attracted my attention from the complete state of aggregation of the sand 
and gravel intervening between the basalt and the granite soil. It is evidently 
formed by the concreting action of calcareous matter, held in solution by the 
carbonic acid which occurs abundantly in the neighbourhood upon the detritus 
of the valley, which it has compacted to such a degree as to form a kind of sand- 
stone. The carbonic acid occurs in a spring rising in a neighbouring meadow, 
and having a temperature of no less than 78°°5, whilst a small spring, between it 
and the river, marked only 51°. The elevation of the mineral spring above 
the sea is 1359 feet. In the side of a granite hill, close to the hamlet of Neyrac, 
is a dry discharge of carbonic acid gas, producing a suffocating atmosphere, 
similar to that of the Grotto del Cane near Naples, the effects of which have been 
elaborately described by Fausas and other older writers on the Vivarais. 
Continuing to ascend the valley from Neyrac, either by the road or by the 
river, objects of interest multiply. The slopes on the right are occupied by an im- 
Immense mass of red cinders, and slaggy lava and ejected bombs, indicating the 
close proximity of a crater, round the outside of which, indeed, the road winds 
for some distance,—whilst the valley seems absolutely barred in advance by im- 
mense cliffs of basalt, which tower over rugged rocks of granite, amongst which 
the river struggles. Across it has been thrown an extremely slender and elegant 
arch, which bears the name of Pont du Diable, or Pont d’Enfer. The basaltic cliffs 
rise to a height of nearly 300 feet above the level of the stream, and upon the 
plateau which they form is the mean but charmingly-situated village of Thuez. 
For variety of outline, luxuriance of vegetation, rich colouring, and romantic 
forms of ground, few spots can be compared to it; and, on my last visit, disre- 
garding the very indifferent accommodation which it affords, I made it the prin- 
cipal centre of my excursions. A short description will, I hope, tend to give a 
distinct idea of its situation, and the chief points of picturesque as well as of 
geological interest. 
The reader must imagine the bed of a rapid stream (the Ardéche) to have 
been worked out through the lapse of ages, by natural operations, to a great 
depth in a soil of granite,—that near the junction with a tributary stream on the 
left, a powerful volcano suddenly opened, emitting a torrent of lava which filled 
VOL. XX. PART I. E 
