to the depth of some feet. It then assumes an earthy aspect, and crumbles between the fingers; the mass is resolved into a meagre, ashy powder, in which are found crystals of hornblende, mica, and octohedral iron. The volcanic nature of domite has never been contested, and it is evidenced by the pumice stones which accompany and are enclosed by it; by the vitreous nature of its fel spathic crystals; by its being porous, impregnated with muriatic acid, coated with sulphur, and with sublimations of iron. Many contradictory opinions have been offered on the origin of these hills, isolated as they are in a region apparently bare of domite. Mr. Scrope, taking into consideration the fact that domite is, in all essential respects, the same as trachyte; and that trachyte forms the greater part of the Mont Dore, the Cantal, the Euganean Hills, the Monti Cimini, and the Lipari Isles; thinks that the Puy de Dôme and the four hills around it are to be assigned to a volcanic origin. And he accounts for their being in solid masses, without dispersion over a wide surface, by a very clear course of reasoning. It is evident, he says, that under similar circumstances of the surrounding levels and of propulsive force, the tendency of a mass of lava to quit the neighborhood of the orifice from which it is emitted will be in exact proportion to its fluidity; and when the fluidity is at its minimum, it will accumulate immediately around the orifice; one layer of the half-congealed and inert substance spreading over that which preceded it, till the whole assume the form of a dome or bellshaped hillock perforated in the centre by the chimney or vent, through which fresh matter may continue to be expelled, but which will at the end remain closed by that last sent up. Now the variety of trachyte which composes the Puy de Dôme, and the neighbouring domitic puys, consisting almost wholly of felspar, and therefore possessing the lowest possible specific gravity, and at the same time a very rude and coarse grain, and highly porous structure, is precisely that species of lava which we should expect a priori to have possessed the minimum of fluidity when protruded into the air; and we therefore can understand perfectly why, instead of flowing in thin and continuous sheets to a distance from its vent, it has accumulated in dome. and bell-shaped hillocks on the point where it was emitted. Of the chain of puys north of the Puy de Dôme, the most remarkable is the Puy de Côme, about 900 feet in height, and very regularly conical in form. The lava-current poured forth by this vent is of prodigious magnitude. It flows from the western base of the cone, which appears to have been thrown up after the eruption. At no great distance from its source the lava encountered an angular protuberance of granite, which separated the current into two branches. That to the right, the more considerable of the two, spread over a vast surface towards the west till it found an obstacle in a long line of hill, consisting of tufa from the Mont Dore, covered by an ancient plateau of basalt. Impeded in its progress, the lava followed the sweep of the hill in a north-east direction; and finding an issue at length, poured down on the present site of the castle and town of Pont Gibaud; immediately above which it seems to have met and flowed over a more ancient stream from the Puy de Lonchadière. Both then poured down the side of a hill which formed the border of the valley of the Sioule, usurping the channel of the river, down which they pursued their course to the distance of more than a mile. The Sioule, thus dispossessed of its bed, has been constrained to work out a fresh one between the lava and the granite of its western bank. But before this was accomplished, there is every appearance of its having formed a lake over the flat, al'uvial surface now forming the meadows of Pont Gibaud. In one part of this new channel, the excavation effected by the river has disclosed the internal division of the lava into vertical, jointed columns, the lower portions of which are straight and well formed, the upper twisted into various curves. The wall of lava is about fifty feet high, and the columnar division is prolonged incompletely to the extent of between 200 and 300 yards. The whole superficies of the plateau covered by the lava of the Puy de Côme cannot be estimated under ten square miles. Its thickness cannot be ascertained with certain ty, but is probably about thirty feet, on an average. It is a most rugged tract, presenting a succession of continual asperities, following each other like the waves of the ocean, with depressions between. It appears to consist of chaotic heaps, of rocky and angular blocks of compact basalt, tossed together in every variety of disorder; yet, in the deep and narrow intervals between these heaps, occur little particles of fresh and flowery turf, and knots of underwood spring from their clefts, in strange contrast with the desolation that prevails over this extensive wilderness. The Mont Dore is one of those remarkable mountainous excrescences which have covered the primary soil to the extent of many miles in diameter, and elevated themselves a proportionate height above the level. Though not the most consi derable mountain of this class in Central France, it attains the greatest absolute elevation. Its highest point, the Pic de Sancy, is given by Ramond, as 6,258 feet. Its figure is peculiar; as if seven or eight rocky summits were grouped together within a circuit of about three miles, where, as from the apex of a flattened cone, all the sides slope more or less rapidly, till they are gradually lost in the high plain around. This mass is deeply and widely eaten into, on opposite sides, by two principal valleys, (those of the Dordogne and the Chambon,) and also furrowed by many minor water channels, all having their sources near the central eminences, and diverting themselves to every point on the horizon. The Mont Dore and the Cantal share this peculiarity of construction, with Etna, the Peak of Tenneriffe, Palma, and other insulated volcanic mountains; and another more remarkable point of resemblance, is in the distribution of their rocks, which exhibit themselves in beds, every way dipping off from the central axis, and lying parallel to the external sloping flanks. This singular disposition would lead at once to the conclusion, that these mountains, are the remains of vast volcanos; and this conclusion is sustained, when the mountains are found to consist of prodigous layers of scoriæ, pumice stone, and their detritus, interstratified with layers of trachyte and basalt. No regular crater exists on the summit of Mont Dore. But the absence of one is readily accounted for by the evidences of the dilapidation which the mountain has suffered since the extinction of its fires. The fragmentary ejections of its vent, have gone to form the conglomerates that clothe its sides, and accumulate at its foot. The lava currents, and other more dura- be found without the limits of a ble products, have more successful- circle of ten miles radius. But what ly resisted the action of the ele these currents lose in length, they ments, and their highest extremi- make up in depth and width. The ties still bristle in elevated peaks, lavas of this class appear to have over a circular gorge, which occu- possessed an inferior degree of flupies the very heart of the moun- idity to those of basalt; probably, tain, and was probably the site of because of their inferior specific its central crater. gravity and greater coarseness of grain; and they have accumulated in prodigious volumes near the source. Nearly all the principal heights and central platforms o the mountain, are composed of trachyte, while basalt rarely shows itself, except in the outer slopes. If the materials of a volcanic mountain were arranged in any sort of uniformity, the valleys which have laid bare the Mont Dore, would exhibit its constitution in a complete manner, but the sections they offer disclose only vast irregular layers of tufa, and breccias, mingled with repeated currents of trachyte, clinkstone and basalt, and traversed with dykes of the same rocks. The opposite sides of each excavation offer correspondent sections, the same beds being visible at similar heights on both declivities. This is universally the case in all the narrower gorges, near the base of the mountain, where the diminished slope caused the currents 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 plateaux with a slight declination." The currents which compose these plateaux are found to consist of basalt, which has flowed on all sides, to the distance of fifteen, and twenty, and in some instances, of twenty-five and thirty miles, from the central height.* The plateaux of trachyte, on the contrary, rarely reach such an extent, and few portions of them deriving from the Mont Dore, are to The whole quantity of fragmentary matters, ejected by the vents of Mont Dore, must once have fully equalled that of its lava currents; but the nature of these conglomerates exposed them to more speedy destruction. They still exist, however, in immense quantities, in turn resting upon, supporting and enveloping the massive lava rocks of every kind. They may be divided into two species, according to the volcanic products predominating in their composition. Some consist wholly of triturated pumice, in which the fine silky filaments of this substance are to be recognized, as well as a few crystals of felspar. This occurs, either loose or arenaceous, by intimate mixture with water into a yellowish-white tufa, with a consistence resembling that of the tufa of the Phlegraean fields near Naples; occasionally, it has a lamellar structure. In general, however, this pulverulent substance envelopes various sized fragments of trachyte, basalt, and granite, forming a tufaceous conglomerate. As these coarser materials predomi *These dimensions have been parallelled by the lavas of modern volcanos. The current which reached Catania in 1669, was fourteen miles long, and in some places, six miles wide. Recupero measured the length of one on the Northern side of Ætna, and found it to be forty miles long Spallanzani mentions currents of fifteen, twenty, and thirty miles in length; and the current which issued from Skaptar Jokul, in Iceland, in 1783, covered a surface of ninety-four miles in length, by fifty in breadth. nate, a complete breccia is formed, rarer variety, compact, hard, and of in which the fragments are separated by occasional interstices or agglutinated by a cement, either of tufa or of iron-rust, derived from the partial decomposition of the fragments themselves, which are, in these instances, of a highly ferruginous basalt. In this condition, the conglomerate resembles the peperino of the Campagna of Rome. From the Pic de Sancy, as a central point of observation, the Mont Dore presents the following appearances. On each side of the Pic de Sancy, and connected with it by intervening ridges, rise craggy knolls, formed, like itself, of porphyritic trachyte, and more or less rounded by the action of the air and rains. One of these, Puy Ferrand, almost equals the Pic de Sancy in elevation. These two heights over look, on the right and left, two deep amphitheatral basins, one opening to the North, and encircled with a range of perpendicular precipices; the other, to the North-west. On the side, opposite to these basins, each eminence gives rise to an inclined plane, with a gradually decreasing slope, and widening, as it descends, into vast platforms, which reach the base of the mountain, and prolong themselves to some distance over the adjoining country. To the West, are two deep gorges, called Les Vallées de L'Enfer and de la Cour. Immediately opposite this latter gorge, on the East side of the valley of the Dordogne river, is a deep ravine, separating two craggy cliffs, called Cacadogne and Le Roc de Cuzan. It is strewed with colossal ruins from the rocks above, which consist of conglomerate, enveloping currents of trachyte and basalt, mingled in strange confusion. Among the blocks lying in the ravine, are many of a trachyte approaching to obsidian, with resinous lustre and fracture, and a black color; and another a brick-red color, with something of the gloss of pitch stone. Such is the nature of the area, overlooked by the central summits; and in these features, it is easy to recognize the traces of a vast and ruinous crater, not very dissimilar to the picture presented by the crater of Vesuvius, torn through the mountain by the eruption of 1822; which presented abrupt, precipitous escarpments like those of the gorges just mentioned, and composed of a conglomerate of scoriæ and volcanic fragments, enveloping horizontal beds of lava. These are the characteristics of most of the faces of the Mont Dore. On the South-western face, it presents a smoother and more uniform slope, than on the others. The currents of trachyte have proceeded but a short distance in that direction, from the central heights. They constitute two or three salient masses composed of a porphyritic rock, more or less porous, and in the vicinity of the supposed crater, even scoriform, and of a deep red color. Basalt, on the contrary, is extremely abundant on this side. It descends in extensive plateaux from the extremities of the trachytic beds: wherever these plateaux have been channelled by torrents, their sections offer ranges of columnar prisms of the greatest regularity. The limits of Mont Dore, on the South, are not clearly defined. The prolongation of its base meets that of the Cantal, and, with it, forms. a high and massive table-land, which divides the waters of the Dordogne and Allier. To the West, the inclination of this elevated table-land towards the Dordogne, is gradual, and its surface strewed with huge boulders of basalt and primitive rocks, attesting the force of the torrents from either mountain, and the frequent shifting of their beds. South of Mont Dore, are two more recent cones, called Montchal and Mont Sineire, which present one remarkable and peculiar feature, among these volcanic hills. Immediately at the foot of each of these cones, is a nearly circular hollow, very large and deep, and covered at the bottom with water. Both are bordered by nearly perpendicular rocks of ancient basalt. Their position shows them to be contemporary with the eruptions of the neighbouring cones, and it seems probable that they owe their formation to some extremely rapid and violent explosion. The volcanic remains, occurring within the departments Haute Loire and Ardèche, are of the second class, the products of a late epoch of volcanic activity, and almost uninterruptedly, cover a broad zone of the primary platform. They constitute a prolongation of the chain of Puys of Auvergne, but do not appear of so recent a date as the latest of those. The various points on which these eruptions have broken forth, are still marked by numerous volcanic cones of scoriæ, whose projection, as in Auvergne, accompanied the development of the volcanic phenomena. They are so thickly strewn along the axis of the granitic range that separates the Loire and Allier, as generally to touches each other by their bases and form an almost continue chain. On both sides of the granitic range, they are more sparingly distributed, a few being also found on the further side of each river. Throughout this tract, Mr. Scrope counted one hundred and fifty of these cones, and thinks he must have omitted many. Few of them present an entire or even a distinctly marked crater, and the generality have wasted to ridges and saddle-shaped hills, a form which volcanic cones have frequently been observed to assume by degradation. Their surfaces are scantily clothed with a meagre herbage, and occasionly a few stunted Scotch firs; but their dilapidation is incessantly going forward by means of frequent and shifting surface rents. The lava currents from these cones must have been exceedingly abundant. They appear to have directed themselves, on one side, into the bed of the Loire; on the other, into the bed of the Allier. The former have covered the whole Eastern slope of the range, (the granite which forms its nucleus, appearing only at distant intervals, or in ravines worn through the basaltic beds,) and are continued over the fresh water strata in a uniform sheet, forming a very extensive and but slightly inclined tract, which they seem to have completely deluged. The present bed of the Loire, and those of its tributaries have, therefore, been excavated through a vast mass of basalt and breccia, as well as through an uppermost layer, generally single of basalt alone, which undoubtedly derived from this chain of cones. In the puys of Monts Dôme, we are enabled by their comparatively rare occurrence and the intervals of primary rock which separate their currents, to trace every current to its spot of emission; but in this chain, the cones are more numerous and closer the volcanic energy seems to have been exerted far more furiously, and the lava currents united into one continuous and enormous crust, where all are confounded and mingled together.* A parallel instance is, that of the chain of volcanic cones thrown up in the Island of Lancerote, one of the Canaries, by the tremendous eruptions between the years 1750 and 1736. The formation of thirty distinct cones on a fissure of great length, within so short a space of time, leads to the supposition of a similar origin for the similar chain of cones in Auvergne and the Velay. |