Ch. XIX.J 
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 scoria?, 
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. 22G. t Scrope's Central France, p. 98. 
