Ch. XIX.] RAVINES EXCAVATED THROUGH LAVA. 
265 
250 feet. A stream of lava takes its rise at the western base 
of the hill, instead of issuing from either crater, and descends 
the granitic slope towards the present site of the town of Pont 
Gibaud. Thence it pours in a broad sheet down a steep 
declivity into the valley of the Sioule, filling the ancient river- 
channel for the distance of more than a mile. The Sioule, 
thus dispossessed of its bed, has worked out a fresh one between 
the lava and the granite of its western bank ; and the excava- 
tion has disclosed, in one spot, a wall of columnar basalt about 
fifty feet high % 
The excavation of the ravine is still in progress, every winter 
some columns of basalt being undermined and carried down 
the channel of the river, and in the course of a few miles rolled 
to sand and pebbles. Meanwhile the cone of Come remains 
stationary, its loose materials being protected by a dense vege- 
tation, and the hill standing on a ridge not commanded by 
any higher ground whence floods of rain-water may descend. 
Buy Rouge. — At another point, farther down the course of 
the Sioule, we find a second illustration of the same pheno- 
menon in the Puy Rouge, a conical hill to the north of 
the village of Pranal. The cone is composed entirely of 
red and black scoriae, tuff, and volcanic bombs. On its western 
side there is a worn-down crater whence a powerful stream 
of lava has issued and flowed into the valley of the Sioule. 
The river has since excavated a ravine through the lava and 
subjacent gneiss, to the depth of 400 feet. 
On the upper part of the precipice forming the left side of 
this ravine, we see a great mass of black and red scoriaceous 
lava ; below this a thin bed of gravel, evidently an ancient 
river-bed, now at an elevation of 50 feet above the channel of 
the Sioule. The gravel again rests upon gneiss, which has 
been eroded to the depth of 50 feet f . It is quite evident in 
this case that, while the basalt was gradually undermined and 
* Scrope's Central France, p. 60, and plate. 
f See Lyell and Murchisou on the Excavation of Valleys, Edin. New Phil 
Journ., July 1829. 
