260 
EOCENE PERIOD. 
[Ch. XIX. 
formations of this class may belong some of the breccias imme- 
diately adjoining the dike in the hill of Gergovia ; but it 
cannot be contended that the volcanic sand and scoria? inter- 
stratified with the marls and limestones in the upper part of 
that hill were introduced, like the dike, subsequently by intru- 
sion from below. They must have been thrown down like 
sediment from water, and can only have resulted from igneous 
action which was going on contemporaneously with the depo- 
sition of the lacustrine strata. 
The reader will bear in mind that this conclusion agrees well 
with the proofs, adverted to in the seventeenth chapter, of the 
abundance of silex, travertin and gypsum precipitated when 
the upper lacustrine strata were formed : for these rocks, as 
we have pointed out, are such as the waters of mineral and 
thermal springs might generate. 
The igneous products above mentioned, as associated with 
the lacustrine strata, form the lowest members of the great 
series of volcanic rocks of Auvergne, Cantal, and Velay, which 
repose for the most part on the granitic mountains (see Map, 
above, p. 226). There was evidently a long succession of 
eruptions, beginning with those of the Eocene period, and 
ending, so far as we can yet infer from the evidence derived 
from fossil remains, with those of the Miocene epoch. The 
oldest part of the two principal volcanic masses of Mont Dor 
and the Plomb du Cantal may perhaps belong to the Eocene 
period, — the newer portion of the same mountains to the Mio- 
cene ; just as Etna commenced its operations during the newer 
Pliocene era, and has continued them down to the Recent 
epoch, and still retains its energy undiminished. There are 
some parts of the Mont Mezen, in Velay, which are perhaps 
of the same antiquity as the oldest parts of Mont Dor. Be- 
sides these ancient rocks, of which the lavas are in a great 
measure trachytic, there are many minor cones in Central 
France, for the most part of posterior origin , which extend from 
Auvergne, in a direction north-west and south-east, through 
Velay, into the Vivarais, where they are seen in the basin of 
