550 VOLCANOS OF AUVERGjSTE. [Ch. XXXII. 



range in a linear direction from Auvergne to the Vivarais, and they were 

 faithfully described so early as the year 1802, by M. de Montlosier. They 

 have given rise chiefly to currents of basaltic lava. Those of Auvergne 

 called the Monts Dome, placed on a granitic platform, form an irregular 

 ridge (see fig. 621, p. 462), about 18 miles in length, and 2 in breadth. 

 They are usually truncated at the summit, where the crater is often pre- 

 served entire, the lava having issued from the base of the hill. But fre- 

 quently the crater is broken down on one side, where the lava has flowed 

 out, The hills are composed of loose scoriae, blocks of lava, lapilli, and 

 pozzuolana, with fragments of trachyte and granite. 



Puy de Cbme. — The Puy de Gome and its lava-current, near Clermont, 

 may be mentioned as one of these minor volcanos. This conical hill rises 

 from the granitic platform, at an angle of between 30° and 40°, to the 

 height of more than 900 feet. Its summit presents two distinct craters, 

 one of them with a vertical depth of 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 dis- 

 tance 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 excavation has disclosed, in one spot, a wall of columnar 

 basalt about 50 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. Mean- 

 while the cone of Come remains unimpaired, its loose materials being 

 protected by a dense vegetation, and the hill standing on a ridge not com- 

 manded by any higher ground, so that no floods of rain-water can descend 

 upon it. There is no end to the waste which the hard basalt may undergo 

 in future, if the physical geography of the country continue unchanged, 

 no limit to the number of years during which the heap of incoherent and 

 transportable materials called the Puy de Come may remain in a station- 

 ary condition. In this place, therefore, we behold in the results of aque- 

 ous and atmospheric agency in past times, a counterpart of what we must 

 expect to recur in future ages. 



Lava of Chaluzet. — At another point, farther down the course of the 

 Sioule, w r e find a second illustration of the same phenomenon 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, towards the village of Chaluzet, 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 in some places of 400 feet. 



On the upper part of the precipice forming the left side of this ravine, 



* Scrope's Central France, p. 60, and plate. 



