28 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
tion of several ridges which have no approximation to a circular arrangement ; 
and they do zot form the entire circumference of the crater,* but only about one- 
half of it at its internal base, without allowing for the inequalities which even 
in that part are often filled up with volcanic slag, and the enormously thick 
coating of conglomerate above the granite on the eastern slope of the crater. 
Were we simply to project the points where the granite appears, it would make 
but a sorry circus. In fact, it may be regarded as a granitic valley choked with 
scorie, and plastered with Roman cement till it forms a nearly circular cavity. 
The occurrence of a crater in such a position only affords, in my opinion, a fresh 
proof of the singular perforation of non-volcanic strata (including granite or 
gneiss in that term), by subterranean explosions which have been so incredibly 
sudden and violent as to have occurred without any visible general disturbance 
(like a pistol-shot penetrating a board, to use a comparison which I have some- 
where read), which recall the extraordinary crater lakes of the Eyffel, pene- 
trating slaty rocks without deranging their strata, and seldom giving birth to any 
considerable volcanic stream. 
Fig. 4 of Plate IV. is taken from a rude eye-sketch of the crater of Pal. The 
chasm at the western entrance gives exit to the river Fontaulier, which takes its 
rise here. The temperature of the copious spring is 42°2, the height above the sea 
being 3900 feet.+ The existence of a spring of the natural temperature (for such 
it appears to be)t in such a position leads to an interesting reflection. Its great 
bulk, and probably uniform temperature, shew that it rises from a considerable 
depth; and it cannot be doubted that it must follow the course, or nearly so, by 
which the lavas amidst which it rises have made their way from the interior of 
the earth. The source of the spring is therefore in the direction of the volcanic 
focus. It is evident, then, that the mass of rock at that depth, which was 
once incandescent, has had time to cool to (sensibly) the temperature of the 
air. How old, then, must be the date of this geological recent eruption? It 
would not be difficult to submit it to calculation ; but the uncertainty of the date 
would render numerical results of little value. Every one comprehends that a 
mountain of granite, with a nucleus of incandescent lava, could not have been 
completely cooled unless in a period of time of very great length. 
* Tam sorry to say, that the view in M. Burat’s interesting work on Central France (Plate 
VII.), is altogether exaggerated and inexact. 
+ 1839, June 3. Spring 42°-0 (Therm. marked A. 1.). 1841, June 25. Spring 42°2 
(Therm. marked A. 3.). Now the correction of A. 3 is +0°2. That of A. 1 was 0°0 in 1838. 
By a singular coincidence the barometer on these two occasions marked the sume tenth of a millimetre. 
The temperature of the air was also within 1° Fahy. of being the same both days. 
¢ The mean temperature of Viviers on the Rhone, which is in the same Department, and only 57 
métres above the sea, is 55°'25 according to Corrs, as given in Dove’s Tables. The crater of Pal is 
at 1186 métres above the sea, or 1129 above Viviers. Now, in France generally (according to 
Martins), the decrease of temperature with height is 1° cent. for 180 métres, or 1° Fahr. for 
100 métres exactly; or 11°29 for 1129 métres, whence the mean temperature of Pal should be 
55°25 —11°:29=43°-96. The temperature of the spring was somewhat lower in the month 
of June, but the approximation is a fair one. 

nate net tetera tee . — 
