Ch. XIX.] 



AUVERGNE MONT DOR. 261 



the Ardeche. This volcanic line does not pass by the Plomb 

 du Cantal; it was formed, as nearly as we can conjecture in 

 the present imperfect state of our knowledge, during the Mio- 

 cene period; but there may probably be found, among these 

 cones and their accompanying lavas, rocks of every interme- 

 diate age between the oldest and newest volcanic formations 

 of Central France. 



We shall first give a brief description of the Mont Dor and 

 the Plomb du Cantal, and then pass on to the train of newer 

 cones, examining the evidence at present obtained respecting 

 their relative ages, and the light which they throw on the 

 successive formation of alluviums and on the excavation of 

 valleys. 



Mont Dor. Mont Dor, the most conspicuous of the vol- 

 canic masses of Auvergne, rests immediately on the granitic 

 rocks standing apart from the fresh- water strata *. This 

 volcano rises suddenly to the height of several thousand feet 

 above the surrounding platform, and retains the shape of a 

 flattened and somewhat irregular cone, all the sides sloping 

 more or less rapidly, until their inclination is gradually lost in 

 the high plain around. It is composed of layers of scoriae, 

 pumice-stones, and their fine detritus, interstratified with beds 

 of trachyte and basalt, which descend often in uninterrupted 

 currents, till they reach and spread themselves around the 

 base of the mountain f . Conglomerates also, composed of 

 angular and rounded fragments of igneous rocks, are observed 

 to alternate with the above ; and the various masses are seen 

 to dip off from the central axis, and to lie parallel to the 

 sloping flanks of the great cone, in the same manner as we 

 have described when treating of Etna. 



The summit of the mountain terminates in seven or eight 

 rocky peaks, where no regular crater can be traced, but where 

 we may easily imagine one to have existed which may have 

 been shattered by earthquakes, and have suffered degradation 

 by aqueous causes. Originally, perhaps, like the highest 

 * See the Map, p. 226. t Scrope's Central France, p. 98. 



