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. 



Puy 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. 



