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tories into the plain of the Limagne. These terminate at the Allier, but on its further bank rise some few isolated cones of the same basalt, which mark the original extent of the currents, and prove that the river had not excavated its actual bed at the epoch of their descent. They are almost everywhere accompanied by beds of conglomerate equally derived from the central volcano, and probably for the most part drifted down to their present position by aqueous debacles, such as were referred to in a former page.

The two principal valleys that now drain this vast inclined plane, viz. those of Chambon and Besse, partly excavated through granite, and partly through the freshwater formation, but everywhere bordered by impending ranges of basalt and its conglomerates, are exceedingly interesting; and not the less so from the circumstance that the bottom of each has been occupied by a current of lava belonging to eruptions of a very recent date.* The upper basin of the first-mentioned valley exhibits in the overhanging cliffs porphyritic trachyte with conglomerates, resting on granite t and supporting basalt; the trachyte terminates above the village of Chambon, but the tuffs and breccias accompany the basaltic currents all the way to

*See Plate IX.

This is the highest point at which the granitic substratum of the Mont Dore shows itself, viz. 3714 feet from the sea. It re-appears on the north-east of the mountain below Murat le Quayre, in the valley of the Dordogne, at the absolute elevation of 3271 feet; and on the south east near Chastriex, of 3422 feet. If we take the mean of these three (3469 feet) for its probable elevation beneath the central summits of the mountain, we shall have 2748 feet as the depth of the volcanic products alone on that point, and above 1400 feet for

their average thickness throughout a central circle of 34 miles radius.

This volume, however considerable, is far inferior to that of the Cantal, perhaps scarcely exceeds that of Ve suvius, and sinks into nothing when compared with the colossal bulk of Ætna or the Peak of Teneriffe, which consist principally of volcanic matter from the level of the sea, and indeed from a great depth below this, to the height of 11,000 and 12,000 feet: or the still more stupendous trachytic formations of the Andes and Cordilleras.

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), im-bedding 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

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Plate VIII.

Basaltic Plateaux.

CLINKSTONE ROCKS, TUILIERE AND SANADOIRE, FROM THE PUY GROS (MONT DORE

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