Ch. XXXIL] EOCENE VOLCANIC rocks. 553 



lacustrine deposit before described (p. 204), which had accumulated in 

 a depression of a tract composed of micaceous schist. In the breccias, 

 even to the very summit of the mountain, we find ejected masses of the 

 fresh-water beds, and sometimes fragments of flint, containing Eocene 

 shells. Valleys radiate in all directions from the central heights of the 

 mountain, increasing in size as they recede from those heights. Those 

 of the Cer and Jourdanne, which are more than 20 miles in length, are 

 of great depth, and lay open the geological structure of the mountain. 

 No alternation of lavas with undisturbed Eocene strata has been ob- 

 served, nor any tuffs containing fresh-water shells, although some of 

 these tuffs include fossil remains of terrestrial plants, said to imply seve- 

 ral distinct restorations of the vegetation of the mountain in the inter- 

 vals between great eruptions. On the northern side of the Plomb du 

 Cantal, at La Vissiere, near Murat, is a spot, pointed out on the Map 

 (p. 195), where fresh-water limestone and marl are seen covered by a 

 thickness of about 800 feet of volcanic rock. Shifts are here seen in 

 the strata of limestone and marl* 



In treating of the lacustrine deposits of Central France, in the fifteenth 

 chapter, it was stated that, in the arenaceous and pebbly group of the 

 lacustrine basins of Auvergne, Cantal, and Velay, no volcanic pebbles had 

 ever been detected, although massive piles of igneous rocks are now found 

 in the immediate vicinity. As this observation has been confirmed by 

 minute research, we are warranted in inferring that the volcanic eruptions 

 had not commenced when the older subdivisions of the freshwater groups 

 originated. 



In Cantal and Velay no decisive proofs have yet been brought to 

 light that any of the igneous outbursts happened during the deposition 

 of the fresh-water strata ; but there can be no doubt that in Auvergne 

 some volcanic explosions took place before the drainage of the lakes, 

 and at a time when the Upper Eocene species of animals and plants still 

 flourished. Thus, for example, at Pont du Chateau, near Clermont, a 

 section is seen in a precipice on the right bank of the river Allier, in 

 which beds of volcanic tuff alternate with a fresh-water limestone, which 

 is in some places pure, but in others spotted with fragments of volcanic 

 matter, as if it were deposited while showers of sand and scoriae were 

 projected from a neighboring vent.j- 



Another example occurs in the Puy de Marmont, near Veyres, where 

 a fresh-water marl alternates with volcanic tuff containing Eocene shells. 

 The tuff or breccia in this locality is precisely such as is known to result 

 from volcanic ashes falling into water, and subsiding together with 

 ejected fragments of marl and other stratified rocks. These tuffs and 

 marls are highly inclined, and traversed by a thick vein of basalt, which, 

 as it rises in the hill, divides into two branches. 



Gergovia. — The hill of Gergovia, near Clermont, affords a third 

 example. I agree with MM. Dufrenoy and Jobert that there is no 

 * See Lyell and Murchison, Ann. de Sci. Nat., Oct. 1829. 

 •J- See Serope's Central France, p. 21. 



