*

compact rock enclosing small crystals of glassy felspar and augite, having a tendency to a prismatic rhomboidal divisionary structure, and stained throughout by the oxide of iron in stripes of a brown colour exactly imitating in their arrangement the undulating and parallel zones of a fir-plank. On other points all fragments are absent, and the tuff is indurated, obviously by water, into a laminated shale, full of impressions of leaves and branches of trees, and sometimes passing into an earthy lignite, which is used by the peasants as fuel.

These different modifications of composition, as well as the position and appearance of the great beds of conglomerate, are reconcileable with no other mode of production than that to which was attributed the same order of rocks in the Mont Dore. Their formation was obviously contemporary with that of the lava-currents they envelop; but their nature and extent preclude all idea that they are owing solely to the projections of the volcano; while the great elevation which they attain round the central heights refutes the supposition of their being merely alluvial deposits from any body of water which may have existed at the foot of the mountain during its eruptions. It only remains then to conclude them the result, for the most part, of torrents of water tumultuously descending the sides of the volcano at the periods of eruption, and bearing down immense volumes of its fragmentary ejections, in company with its lava-streams.

These vast beds are usually surmounted by currents of basalt; sometimes, as in the valleys of St. Paul de Salers and Falgoux, repeated five or six times, and separated from each other only by a layer of their own scoria. I noticed no instance of beds of trachyte alternating with basalt, as was observed in the Mont Dore. Throughout the Cantal the production of the former

* Analogous to the regenerated trachyte observed in Hungary by M. Beudant.

rock, with its associated tufas and breccias, seems to have ceased before the eruptions of basalt commenced.

Such, at least, is the general order observed in the sections afforded by the principal valleys, at some distance from the summit of the mountain. On a further ascent, the confusion of products becomes greater, indicating the vicinity of the vent of eruption. The conical form of the whole mountain, and the divergence of all its currents from the neighbourhood of a few central heights, leads, as in the case of the Mont Dore, to the presumption that the volcano of the Cantal had one principal and central crater; and many circumstances unite to fix the situation of this upon the double basin in which the upper sources of the rivers Jourdanne and Cer are first collected.

It is from the outer circuit of this area, encircled by several culminating peaks and ridges of trachyte, that the chief volcanic currents appear to originate. In the centre rises the Puy Griou, separated from a mass of clinkstone, the Plomb du Cantal, by a considerable depression, across which passed the old road from Murat to Aurillac, now superseded by a tunnel bored through the connecting ridge at a level 900 feet lower. The Plomb, the highest point of the whole mountain (6258 feet A. E.), is of basalt, and thence probably proceeded the enormous basaltic currents which have flowed towards the south-east.

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In piercing the tunnel mentioned above, a great number of more or less vertical dykes were met with, of trachyte, clinkstone, and basalt, traversing a breccia containing cellular and scoriform fragments of these rocks, as well as veins of green pitchstone porphyry. This structure, so similar to that of the centre of the Mont Dore, is just what we should expect to find in the eruptive chimney of a volcano.†

See a paper by M. Ruelle, Bulletin xiv. p. 106.

The eminence called the Col de Cabre overlooks the source of the Jourdanne on the north, giving rise to a mountainous embranchment principally consisting of trachyte, which extends in a north-easterly direction, and separates the valleys of Murat and Dienne; while to the N.W. of the same area the Puy Mari stands at the head of powerful and repeated currents of basalt, which, accumulating on one another, form the Montagnes de Salers, and spread from thence over a high plain towards the Mont Dore.

Indeed, with the exception of the masses already designated, and a vast but considerably degraded current of the same nature which terminates in an enormous and elevated plateau above the town of Bort on the west of the Dordogne, resting on a layer of pebbles, and apparently occupying the former bed of the river which now flows more than 1000 feet below it ;-with these exceptions, the trachytic lavas of the Cantal are far from conspicuous on the exterior of the mountain, and are greatly exceeded in number, volume, and extent by its beds of basalt, which stretch in all directions from the central eminences over its sloping skirts, and on the east and south-west reach to distances of 25 and 30 miles.

To the south-east they form an extensive and uniform high plain, called La Planeze, reaching to the base of the primitive range of La Margeride, but little furrowed by watercourses, and of a singularly dreary aspect from its total nudity. At its extremity is seated the departmental capital St. Flour; and here, as wherever else this plateau is broken through, successive and parallel beds of basalt may be seen surmounting one another in very regularly columnar ranges.

A series of similar plateaux extends from the mountains behind Salers to the town of Mauriac, and thence some way northward. They consist principally of a light-coloured basalt, with a highly

crystalline grain, sprinkled with large cellular cavities. It exhibits on different points the columnar, the tabular, and spheroidal concretionary structures; and wherever it has yielded to the erosion of torrents, is seen to rest on a tuff conglomerate.

But the basaltic beds through which the Alagnon has excavated its valley in the immediate neighbourhood of Murat are the most remarkable of all for their regular columnar configuration, no less than for their bulk.

They are associated with trachyte, accompanied and in part enveloped by accumulations of breccia; but on particular points colossal portions of basalt have been stripped of these coverings and isolated from the remainder of the current to which they belong. Such are the Montagnes de Bonnevie and Chastel. The former, at whose foot stands the town of Murat, has been long celebrated for the beauty of its columns. It is a large conical rock, about 400 feet in height from its base, composed of a single enormous bundle of prisms converging from all sides towards the apex; those of the exterior being slightly curved, the central ones straight and vertical. These last are the most perfect, and have been exposed by dilapidation on the eastern side of the rock. They are smooth, long, and slender, usually six-sided, rarely exceeding 8 or 10 inches in diameter, with a height often of 50 or 60 feet unbroken by joints or flaws. or flaws. museums of Paris and Lyons, as well as many private cabinets, have been enriched with columns extracted from hence; but it is a work of extreme delicacy to separate them whole from the rock, and still more difficult to convey them uninjured to any distance. This basalt is brittle, sonorous, hard, compact, finegrained, of a dark colour, and free from any visible crystals. It is remarkable that the western face of the rock is entirely amorphous.*

The

* See Plate X.