692 PLOMB DU CANT AX. [Ch. XXXII, 



and basalt are numerous, especially in the neighborhood of the large 

 cavity, probably once a crater, around which the loftiest summits of 

 the Cantal are ranged circularly, few of them, except the Plomb du 

 Cantal, rising far above the border or ridge of this supposed crater. 

 A pyramidal hill, called the Puy Griou, occupies the middle of the 

 cavity.* It is clear that the volcano of the Cantal broke out precisely 

 on the site of the lacustrine deposit before described (p. 229), 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 freshwater beds, and sometimes fragments 

 of flint, containing Lower Miocene 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 lacustrine strata has been observed, nor any tuffs 

 containing freshwater shells, although some of these tuffs include fossil 

 remains of terrestrial plants, said to imply several distinct restorations 

 of the vegetation of the mountain in the intervals between great erup- 

 tions. On the northern side of the Plomb du Cantal, at La Vissiere, 

 near Murat, is a spot, pointed out on the Map (p. 221), where fresh- 

 water limestone and marl are seen covered by* a thickness of about 800 

 feet of volcanic rock. Shifts are here seen of the strata of limestone 

 and marl.f 



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, Gantal, and Yelay, no vol- 

 canic pebbles had ever been detected, although massive piles of igne- 

 ous rocks are now found in the immediate vicinity. As this observa- 

 tion 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 depo- 

 sition of the freshwater 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 Lower Miocene 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, bat 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, 



* Mem. de la Soc. Geol. de France, torn. i. p. 175. 



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



X See Scrope's Central France, p. 21. 



