24. PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARATS. 
way to the Pont de la Beaume, and is no doubt the same which we have already 
described as underlying the great coulée of the valley of the Ardéche. 
Having now traced the course of the lava of the Gravenne, we shall point out 
some other volcanic appearances in the valley of Montpezat. And, first, we have 
a dyke of basalt, in the eranite of the left bank of the Fontaulier, near the hamlet 
of Les Plantas, about half-way between the Castle of Pourchirol and the entrance 
of the valley of Burzet. This is a phenomenon which we should naturally ex- 
pect to meet with very frequently in a country which, with a soil entirely primi- 
tive, has been pierced at so many points within a short space by volcanic ducts, 
which can hardly have been formed without an intense pressure from below, which 
might have been expected to rend the strata of gneiss in all directions, and then 
to have filled the rents with melted matter, constituting true dykes, such as those 
which occur in Monte Somma, near Naples. The reverse is, however, the case: 
and the dyke of Les Plantas is the only one which I have met with in this part of the 
Vivarais (amongst the older volcanic formations of Le Puy and the Coyrons they 
are numerous). No doubt, many may have escaped me; and I should never have 
known of this one, owing to its remote and concealed position, had I not been fortu- 
nately directed to it by a resident at Montpezat. It occurs in a small ravine (see 
the map), running in a direction nearly north and south, and is said to have been 
traced for a mile. Its plane is vertical. Its breadth varies from 1 to 4 feet ; and 
it sends forth small veins into the rock, and includes portions of granite in its 
substance. In composition, it resembles the lava of the district ; black in colour, 
with concretions of olivine. 
The village of Montpezat, charmingly situated on the rivulet Pourseuille, 
at the height of 1857 feet above the sea, does not stand upon the lava of 
the Gravenne, but about half-a-mile beyond where it terminates at the 
parish church of St Pierre. The valley is filled by a prodigious multitude 
of rolled blocks of granite; which here, as in the lateral valley of Thuez, 
appear to be out of all proportion to the extent of the ravine from which they 
must have been derived, and of which the countless multitude contrasts with 
the comparative rarity of basaltic blocks. At the head of the valley, near the 
source of the Pourseuille, rises a mass of volcanic cinders, well distinguished at a 
distance by its colour; and which is figured by Mr Scroprg, in his view of the valley 
of Montpezat (Plate XV. of his work), who describes it as an anonymous volcanic 
cone which has not produced any lava stream. He had evidently not visited the 
place ; for the true description is very different. I shall now detail the chief re- 
sults of a patient examination of this locality, both in 1839 and 1841; thereis no 
point in the Vivarais which I have more narrowly investigated. 
Montpezat, we have said, is placed upon an immense bed of granite boulders, 
which extend upwards, occupying the whole bed of the valley, to a great thick- 
ness; but near the village of Le Fau we find a mass of lava, which, with other 
