20 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
by a rough footway beneath the bridge across the Gueule d’Enfer. Here we are 
on the exact boundary of the immense lava plateau and the granite to the east,— 
that is, the granite is under our feet and to our left in descending, the lava is 
above us and to the right. The remarkable section (Plate III., fig. 2) of the 
Gueule d’Enfer shews plainly that the lava must have had a support which 
piled it up, when fluid, to the level which it still retains, and the position of 
the barrier is conclusively shewn by the direction of the stratum of basaltic 
columns, whose axes are as usual perpendicular to its surface, and which point 
out with mathematical accuracy the figure of the retaining wall now removed, 
and replaced by the deep ravine on the right of the spectator, who looks 
up the defile as in fig. 2. We are therefore compelled not only to admit 
the excavation of the Gueule d’Enfer since the lava was consolidated, but we 
must suppose that a barrier of some kind stretched across the valley of the 
Ardéche itself, in order to retain the prodigious lava flow at the great elevation 
which it has attained, and which causes its bared cliffs now to overhang the 
valley to a height of 250 feet, reckoning from the bed of the stream. A careful 
examination of the panoramic view will clearly prove the surprising dilemma 
in which we are placed. The almost perfect horizontality of the whole remain- 
ing surface of the lava proves that it consolidated tranquilly at that level; and 
yet we find to the right nothing but a wide open valley, which presents no trace 
of a support, and from which the lava itself has totally vanished; for the most 
scrupulous examination of the bed of the Ardéche has shewn me that there is 
nota volcanic vestige in its neighbourhood so far down as the environs of Neyrac; 
and, though it is as plain as the truth of hydrostatics that the basalt must have 
once filled up the whole bed of the Ardéche at this place, and abutted against the 
granite-hill opposite (from whence the panoramic view in Plate VI. is taken), there 
is now not a trace of it on the southern side of the river. How astonishing, then, 
must have been the excavating power which has not merely disintegrated the mass 
of lava which has disappeared, but has destroyed the barrier, by means of which it 
was accumulated to the level which it retains! It is certainly conceivable that this 
barrier might have been partly composed of the dejections of the volcano which, 
when much higher than at present, may possibly have extended its cone so as 
partly to close the valley ; yet the whole circumstances appear to shew that the 
forms of these volcanoes have not materially changed since the completion of their 
eruptions, and that certainly no vast or powerful streams of water, sweeping over 
the whole country after the manner of a debacle, can be invoked in explanation of the 
last excavation of the valleys; for the loose texture of the ashes, which repose upon 
every volcanic cone, would have given way at once before the action of a flood, 
however gentle. This argument has been effectively used by Mr Scrors, to prove 
that the removal of the lava beds can be ascribed only to the action of water 
following the channels of the present rivers ; and has been enforced by Sir C. LYELL 
