EXCAVATION OF THE LAVA OF THUEZ. 21 
and Sir R. I. Murcuison, in a paper published many years ago, on this very subject,* 
in which the excavation of the granite, as well as of the lava of Thuez, is cited in 
additional confirmation. But none of these authors, so far as I recollect, have 
referred to the singular manner in which this lake of lava has been, as it were, 
suspended in the middle of a valley which presents so great a declivity. The 
section just given in the Gueule d’Enfer seems to shew that that ravine must 
have been entirely excavated since the lava was consolidated!. There are few 
phenomena, geologically so recent, which appear more unaccountable, more dis- 
proportioned to the means by which apparently they must have been produced. 
The facts before us recal, in a striking manner, the parellel roads of Glen Roy in 
Scotland; lake terraces apparently of a similar age to the basalts of the Vivarais 
(that is, posterior to many of the ordinary river alluvia), and which must have 
required barriers far exceeding in dimension those which dammed up the lava of 
Thuez. But in Glen Roy, whatever were the barriers, they were certainly not 
composed of solid rock. Here, on the contrary, they would appear at least in a 
great measure to have been so. 
As we issue from the Gueule d’ Enfer, we find a tolerably wide and cultivated 
ravine, entirely based on the primitive rock; whilst the mural precipice of lava, 
in some places 200 feet high, extends for about a mile parallel to the river on 
the right. The contact of the lava with the ancient soil may almost everywhere 
be traced. The lower part is usually composed of vertical columns; the upper 
part is (as at Jaujac) only very imperfectly prismatic; but the whole is evidently 
the result of one eruption. The lower part of the stream, where it touches the 
soil, has in some places a very singular appearance, glistening and coaly; pro- 
bably composed of nearly pure augite mixed with carbonaceous matter, of which 
I found a singular proof in a portion of a vegetable stem, of which a cast has been 
made by the lava.+ 
The general phenomenon of the perpendicularity of the lava prisms to the 
surface of cooling is everywhere exemplified; but nowhere so beautifully as near 
the part of the cliff called the échelle du Roi (from a narrow steep passage formed 
by a dyke or vein in the lava, and by which the cliff may be ascended). Here there 
occurs beneath the prismatic lava a shapeless mass, apparently of old lava, slaggy, 
and not at all columnar; of which the new lava has formed an exact cast, and 
fringed it all round with columns perpendicular to its own most irregular surface. 
This is represented in Plate III. fig. 3, from a rather careful drawing made on 
the spot. I have often been inclined to think that the old lava, which must evi- 
dently have been cold when it was overflowed by the other, may be derived from 
the Gravenne of Montpezat; and this idea is confirmed by the consideration, that 
* Jameson’s Edinburgh Philosophical Journal, 1829. 
___ }_The specimen illustrating this curious fact, and others referred to in this paper, are now placed 
in the Museum of the Royal Society of Edinburgh. 
VOL. XX. PART I. F 
