2G4 



EOCENE PERIOD. [Ch. XIX. 



Phlegraean 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 do 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 Auvergne 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 arc 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 scoriae, 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. 



PUIJ dc C6me. 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 S(K) feet. Its summit pre- 

 sents two distinct craters, one of them with a vertical depth of 



