264 EOCENE PERIOD. [Ch. XIX. 
Phlegrasan fields and the flanks of Etna. They have given 
rise chiefly to currents of basaltic lava, whereas those of Mont 
Dor and the Cantal are in great part trachytic. There are 
perhaps about three hundred of these minor cones in Central 
France ; but a part of them only occur in Auvergne, where 
some few are found at the bottom of valleys excavated through 
the more ancient lavas of Mont Dor, as the Puy de Tartaret, 
for example, whence issues a current of lava which, flowing 
into the bed of the river Couze, gave rise to the lake of Cham- 
bon. Here the more ancient columnar basalts of Auvemne are 
seen forming the upper portion of the precipices which bound 
the valley. 
But the greater part of the minor cones of Auvergne are 
placed upon the granitic platform, where they form an irregular 
ridge about eighteen miles in length and two in breadth. 
They are usually truncated at the summit, where the crater 
is often preserved entire, the lava having issued from the base 
of the hill. But frequently the crater is broken down on one 
side, where the lava has flowed out. The hills are composed 
of loose scoriee, blocks of lava, lapilli, and puzzuolana, with 
fragments of trachyte and granite. 
The lavas may be often traced from the crater to the nearest 
valley, where they usurp the channel of the river, which has 
often excavated a deep ravine through the basalt. We have 
thus an opportunity of contrasting the enormous degradation 
which the solid and massive rock has suffered by aqueous 
erosion and the integrity of the cone of sand and ashes which 
has, in the mean time, remained uninjured on the neighbouring 
platform, where it was placed beyond the reach of the power of 
running water. 
Puy de Come.— We may mention the Puy de Come and 
its lava current, near Clermont, as one of the numerous illus- 
trations of the phenomenon here alluded to. This conical 
hill rises from the granitic platform at an angle of about 40 
degrees to the height of more than 900 feet. Its summit pre- 
sents two distinct craters, one of them with a vertical depth of 
