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 scoriae 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, andean 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 



