| OCR Text |
Show 260 EOCENE PERIOD. [Ch. XIX, formations of this class may belong some of the breccias immediately adjoining the dike in the h.ill of Gergovia ~ ~ut it cannot be contended that the volcamc sand and sconre mterstratified with the marls and limestones in the upper part of that hill were introduced, like the dike, subsequently by intrusion 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 deposition 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 clu Cantal may perhaps belong to the Eocene period,-the newer portion of the same mountains to the Miocene; 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 lV[ont Dor. Besides these ancient rocks, of which the lavas are in a great measure trachytic, there are many minm· 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, in~o the Vivarais, where they arc seen in the basin of Ch. XIX.] AUVEIWNE-MON1' DOU. 261 the Ardechc. This volcanic line docs 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 Miocene period; but there may probably be found, among these cones and their accompanying lavas, rocks of every intermediate 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 Canta1, and then pass on to the train of ncwc1· cones, examining the evidence at present obtaineu 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 volcanic 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 laye1·s of scorire, 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 t. Conglomerates also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various masses arc 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. 1'he 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. |
