the Allier, showing themselves at intervals throughout both valleys in prodigious accumulations, as in the Dent du Marais near the Lake Chambon, and the plateaux of Pardines and Nechers.

They universally contain blocks of every variety of trachyte and basalt, fragments of granite, pumice-stones, scoriæ, &c., and a large proportion of titaniferous iron in their sandy detritus. Masses of limestone occur in them where they cover or rest against the freshwater strata; and in similar circumstances the basalt has sometimes its cellular cavities filled with calcareous infiltrations. At the Montagne de Laveille, near Chidrac, some highly amygdaloidal portions occur, in which arragonite and carbonate of lime form nearly two-thirds of the mass. It is immediately beneath, or occasionally intercalated among, these tuffs and breccias that the celebrated bone-beds of Mont Perrier occur, in which Messrs. Croizet, Bravard, and Pomel have detected numerous mammalian remains belonging to several distinct assemblages of species, which the two former naturalists refer to successive tertiary epochs.*

The Dordogne, which for the first three or four miles of its course flows nearly from south to north, makes a sudden bend to the west a short distance below the village of the Baths, leaving to the right a massive portion of high table-land which exactly fronts the whole of its upper valley, and was perhaps originally separated from the central heights by some violent explosions, while the subsequent excavation of the channel of the Dordogne has widened the breach.

The base of this mountain consists of various conglomerates,

*See Sir C. Lyell's Manual, p. 552, of Organic Remains of Central France ed. 1855; and Quarterly Journal Geol. in Appendix infrà. Soc., vol. ii. p. 77. Also the Catalogue

enveloping beds of basalt; above these a band of clinkstone may be traced across the whole of its eastern and northern faces, surmounted by porphyritic trachyte—if indeed these two rocks do not, as might from many circumstances be suspected, pass into each other. Finally, the upper surface of the plateau exhibits more recent currents of basalt, which appear to have had their origin there.

Trachyte, however, occupies by far the most conspicuous place amongst these rocks, constituting the Puy Gros, an enormous flat-topped boss on the eastern summit, and descending from thence to the west in a wide unbroken platform, though occasionally covered by basalt, as far as the village of La Queuille, where the current terminates in a range of gigantic six-sided columns, some of which I observed to be not less than 15 feet in diameter; their height is not proportioned to so great a bulk, rarely exceeding 30 feet. The rock which composes them is a dark-coloured trachyte approaching to basalt (greystone), imbedding felspar and augite crystals, and exceedingly cellular. Its largest cavities often contain radiated crystallizations of arragonite.

Clinkstone, or the laminar and scaly species of trachyte, predominates to the north of the Puy Gros, where a thick bed of it seems to have been separated into detached segments by the torrent which flows from thence to Rochefort. The largest of the masses thus isolated are the Puy de Loueire, and the Roches Sanadoire and La Tuilière. At the first the phonolite is divided into compact tables; at the two last rocks, into very regular prisms. Those of Sanadoire are entangled into fantastic groups, in one spot diverging so regularly from a common centre as to resemble a circular fan. The prisms of La Tuilière are vertical or nearly so, and schistose, splitting into thin laminæ, which at the northern extremity of the rock are at right angles

to the axes of the prisms, but acquire gradually an increase of inclination, till at the other end their obliquity is such that their planes make an angle of but 15 or 20 degrees with the axes, and the agency of gravity co-operates with that of the weather in separating them: the rock on this side is in consequence completely in ruins. These plates are used as roofingslates throughout the neighbourhood, and hence the name of La Tuilière.

This clinkstone contains occasional crystals of felspar, and assumes in parts so much of the aspect of trachyte, that I am inclined to suppose it but an accidental variety of this rock, more particularly as the bed to which it belongs appears to merge in the great trachytic currents of the Puy Gros on one side, and the Puy de l'Aiguiller on the other.

The volcanic nature of the Roche Sanadoire was at one time strongly contested by naturalists, who examined the individual rock or its specimens alone, without consulting its evident connections with those around. But the frequent occurrence of cellular portions, and occasional scoria imbedded in its prisms, might have sufficed to convince even these sceptics of its igneous origin. Dr. Weiss of Leipsic discovered grains of haüyne in this rock, but they are almost microscopic, and very rare.

The basalt of the high plateau now under description appears to have been produced by repeated eruptions from a vent to the north-west of the Puy Gros, the site of which is marked out by two elevations entirely of scoriæ, called Chantouzet and Le Cros de Pèze.

Its currents have flowed both to the east and west, forming on one side the margin of the lake Guèry, on the other descending into the valley of the Dordogne, and exhibiting many prismatic ranges along its banks, as far as St. Sauve. They are accompanied by breccias, and may be observed at Murat le