MONTPEZAT—VALLEY OF THE FONTAULIER. 23 
pezat and Burzet. It is a real cwp—the bottom being lower than the lowest edge 
of the crater by which the lava has poured out into the valley. A high and 
solid conoidal mass of lava and ashes commands the crater to the SW. so as to 
give the character of an unbroken top to the hill in that direction. The highest 
parts are composed of partly solid lava, and rise 100 feet above the bottom of the 
crater.* The principal lava-flow descends in the direction of Montpezat, at an angle 
(by estimation) of little less than 35°; yetit istolerably continuous. At its base, 
near the bridge across the Fontaulier (which is 1534 feet above the sea, and 1200 
below the summit of the Gravenne), we have a remarkable section, Plate IIL, fig. 
5, which is made transversely to the direction of the lava stream of the Gravenne, 
which forms the upper layer. The layer of basalt forming the base of the section, 
and which is separated from the other by a layer of granite boulders, cemented 
like that of Neyrac by the action of the acidulous water, is evidently the product 
of an altogether anterior eruption. The more modern eruption has bathed the 
whole valley with lava, which has formed a tolerably level prismatic bed, stretch- 
ing some way up the tributary stream of the Pourseuille, as well as up the 
valley of the Fontaulier, but not far in either case, and stopping considerably 
short of the town or village of Montpezat, which is built upon a mass of granite 
boulders. The junction of the Pourseuille and Fontaulier affords an excellent 
section below the picturesque site of the Castle of Pourchirol, Plate IIL, fig. 6. 
Here but one bed of basalt appears (I mean due to one eruption). 
The plateau formed in the valley of the Fontaulier by the lava of the Gra- 
venne is justly described by Mr Scrorr as having “its upper surface bristled 
with rocky and scoriform projections; which, however, by decomposition, resolve 
themselves into a rich soil, affording nourishment to very productive chestnut 
forests.” Many scenes of great beauty occur on this plateau. There is some 
appearance of a lake having been formed at the contraction of the valley at the 
Pont de la Motte, several miles farther down ; and both in this valley and that of 
Burzet (which we shall afterwards describe) there occur deposits of pozzuolana 
apparently stratified by water. At the junction of the Fontaulier with the tri- 
butary valley of Burzet, about half-way between the Gravenne and the Pont de 
la Beaume, the lava of Montpezat joins that of Burzet, or rather, overlies it,—the 
plan and section, Plate IV., fig. 1, marking plainly the more recent date of the 
eruption of the Gravenne. These streams remain superposed for some distance ; 
but at the Pont de la Motte (at Mayras) there is only a single stream, which is 
columnar, abounds in olivine, and reposes on a conglomerate, Iam unable to state 
positively to which of the two valleys this may be traced (although the determi- 
nation would be easy); but from its composition and other circumstances, I am 
inclined to think that it derived from the valley of Burzet. It continues all the 
* Messrs Lyett & Munrcnison (Edin. Phil. Journal, 1829, p. 27) speak of angular blocks 
of unaltered gneiss as occurring near the summit of the Gravenne. These appear to have escaped 
my notice. 
