CHAPTER VI.

REGION II.-THE MONT DORE.

§ 1. GENERAL OUTLINE OF THE MONT DORE.*

As yet we have found the primary soil concealed but by occasional masses of volcanic rocks, between which it crops out at no very distant intervals. But I have now to describe one of those mountainous excrescences which have covered its surface to an extent of many miles in diameter, and elevated themselves to a proportionate height above its level.

The Mont Dore, though not the most considerable of the three in bulk or extent, attains the greatest absolute elevation. Its highest point, the Pic de Sancy, is given by Ramond as 6258 feet, exceeding that of the Cantal by 128 feet. Its figure will be best understood by supposing seven or eight rocky summits grouped together within a circuit of about a mile in diameter; from whence, as from the apex of a flattened and somewhat irregular cone, all the sides slope more or less rapidly, until their inclination is gradually lost in the high plain around. Imagine this mass deeply and widely eaten into on opposite sides by two principal valleys, (those of the Dordogne and of Chambon,) and further furrowed by about a dozen minor water-channels, all having their sources near the central eminences, and directing

* The Mont Dore, anciently Mons Duranius, derives its appellation from the stream called Le Dore which rises on its summit, and is therefore im

properly written Mont d'Or. See Ramond, Nivellement des Plaines: Mém. de l'Inst. 1815.

themselves from thence to every point of the horizon. You will then have a rude but not inaccurate idea of the Mont Dore.

It is barely possible that some mountain, not volcanic, may, by long isolation, or accidental circumstances, have assumed somewhat of this form, but the additional peculiarity which the Mont Dore and Cantal share with Etna, the Peak of Teneriffe, Palma, and all other insulated volcanic mountains, is, that the rocks of which each is composed exhibit themselves in beds every way dipping off from the central axis, and lying parallel to the external sloping flanks. This singular disposition would induce us à priori to conclude these mountains to be the remains of vast volcanos. The idea is of course confirmed, when we discover on examination that they consist of prodigious layers of scoriæ, pumice-stones, and their fine detritus, interstratified with beds of trachyte and basalt, which bear the stamp of an igneous origin, and descend often in uninterrupted currents, till they reach and spread themselves over the platform around the base of the mountain.

It is true that no regular crater remains on this summit. It would be irrational to expect one in a volcanic mountain which exhibits so many other proofs of having been long and violently attacked by the agents of dilapidation since the extinction of its fires.

The fragmentary ejections of its vent have gone in great part to form the immense conglomerates that clothe its sides and accumulate at its foot. Its more durable productions, its lavacurrents and some consolidated breccias, have more successfully resisted the wear and tear of ages, and their highest extremities still bristle in elevated peaks over a circus-like gorge, which occupies the very heart of the mountain, and was probably the site of its central crater, but which now, branching out into deep and short recesses, forms the upper basin of the principal valley,

and the recipient in which two mountain rills, the Dore and Dogne, unite, at the source of the noble river which from thenceforward bears their joint names.

If the materials of a volcanic mountain were arranged in any sort of uniformity, the valleys which have reduced the Mont Dore to a mere skeleton would exhibit its constitution in the most satisfactory manner; but as might be expected, the sections they offer disclose only vast and irregular layers of tuff and breccias, mingled with repeated or alternating currents of trachyte, clinkstone, and basalt, and traversed by numerous dykes of the same rocks.*

The opposite sides of each excavation generally offer corresponding sections, the same beds being visible at similar heights on both declivities, but varying occasionally in thickness. This is universally the case in all the narrower gorges near the base of the mountain, where the diminished slope caused the lavacurrents to increase in width as much as in length; and in these situations the same bed or series of beds often extends over a surface of many square miles, forming a succession of vast platforms, with a slight, and, towards their termination, scarcely perceptible declination.

On examining the currents which compose these distant

Were the causes which occasion the activity of Etna to cease, this volcanic mountain would before the lapse of many centuries assume the chief characteristic features of the Mont Dore. Even now, its sides are furrowed by deep and vast valleys produced by earthquakes and the rapid descent of torrents of rain. The beds of lava of different epochs may be seen forming numerous pseudo-strata one above the other, and corresponding on the opposite sides of these valleys; the most remarkable of which in this

respect is that of Trisoglietto. See Ferrara, Descrizione dell'Etna, 1818.

It appears that the flanks of the Peak of Teneriffe are yet more deeply intersected by rents and ravines; and M. Escobar is said to have counted above 100 strata of different lavas and beds of pumice on the sides of the valley of las Guanchas, N.W. of the Peak. See too the description of the Islands of Palma and Madeira in Sir C. Lyell's Manual, ed. 1855, p. 498 et seq.

plateaux, they are found to consist of basalt, which has flowed on all sides to the distance of 15 and 20, and in some instances, on the east and north, of 25 or 30 miles from the central heights.* Though the continuity of some of these sheets of basaltic lava has been destroyed, we may remount many of them without meeting any interruption, till at no great distance from the summit of the group we arrive at a spot, which, from the torrefied and vesicular nature of the basalt, and the number of scoria and bombs still adhering to its surface, appears to be the source of the current, the vent from which it was expelled.

The plateaux of trachyte, on the contrary, rarely reach to such an extent, and few portions of them deriving from the Mont Dore are to be found without the limits of a circle of 10 miles radius. But what these currents lose in length they make up in height and width. The lavas of this class appear to have possessed an inferior degree of fluidity to those of basalt; probably owing to their inferior specific gravity+ and greater coarseness of grain; and in consequence they have accumulated upon one another in prodigious volumes in the vicinity of the source. They thus become the most conspicuous if not the most considerable portions of the edifice which they have reared in common with the others. Trachyte constitutes nearly all the principal heights and central platforms of the mountain, while basalt rarely shows itself but on its outer slopes or in the lateral escarpments and at the bottoms of its valleys.

A few isolated fragments of basaltic currents may, however, be

*These dimensions are far from being unparalleled by the lavas of modern volcanos. Sir W. Hamilton reckoned the current which reached Catania in 1669 to be 14 miles long, and in some parts 6 wide. Recupero measured the length of another, upon the northern side of Etna, and found

it 40 miles. Spallanzani mentions currents of 15, 20, and 30 miles (Voy. en Sicile, i. 219); and Pennant describes one which issued from a volcano of Iceland in 1783, and covered a surface of 94 miles by 50! (North Globe, vol. i.) † See Considerations on Volcanos, pp. 86, 92, et seq.

seen at a considerable elevation, and resting, without interposition of other substances, on some of the trachytic plateaux; while instances of so clear a superposition of trachyte to basalt are less common. Hence it has been supposed that the eruptions which produced the lavas of this latter sort took place after the cessation and final extinction of the volcano which gave birth to those of trachyte. But facts are far from warranting this supposition; and I shall have to adduce several examples of the evident alternation of the two rocks.

Nor, indeed, are they always to be accurately distinguished from each other. Amongst the infinitely diversified varieties of trachyte to be found at the Mont Dore, where no two beds are alike, and even the same frequently changes its aspect in a considerable degree, several have the laminar structure, foliated texture, and scaly grain which characterizes clinkstone. When these, as they occasionally do, contain a large proportion of augite, they approach closely to, and in fact are undistinguishable from, basalt. Where the texture is not scaly, but the quantity of augite considerable, the trachytes often assume the exact appearance of some of the recent lavas of the Monts Dôme, are extremely cellular, of a dark grey colour, and crystalline texture. The trachyte used as a building-stone at Mont Dore les Bains is almost identical with the lava of the Puy de Nugère quarried at Volvic.

The whole quantity of Fragmentary matters ejected by the principal and subsidiary vents of the Mont Dore must once have fully equalled that of its lava currents; but the loose nature of these conglomerates has exposed them, of course, to more speedy destruction. The volume of those which remain is, however, prodigious. They in turns rest upon, support, and envelop the massive lava-rocks of every kind. They are found at every