cone of very large size formed on a granitic ridge which separates the rivers Fontaulier and Ardèche. It has a very regular bowl-shaped crater, gently inclined to the north; in this direction the lava poured over its lowest lip into the basin of Montpézat, which it entirely filled to an average depth perhaps of 150 feet, and a width of nearly half a mile. Hence it descends the valley to the confluence of the Fontaulier and Burzet, where it appears either to have stopped, the volcano having exhausted its efforts, or to have mingled with a similar current which reached the same point from above the village of Burzet. The mass of basalt thus deposited, as well as the subjacent granite, has been since cut through to depths varying from 100 to 200 feet, by the powerful action of the rivers whose channels it usurped; its present disposition, and the beautiful columnar ranges discovered by this excavation, may be imperfectly judged of from the annexed sketch taken near the junction of the torrents Fontaulier and Pourseille, about a mile below Montpézat.* Similar ranges extend all the way to its termination at Aulière, where . they are still more symmetrical, many columns being straight, vertical, and entire from top to bottom of the escarpment. The engraving published by Faujas, if its faulty execution be allowed for, will give a tolerably correct idea of the architectural regularity of this façade. The basalt may be observed on many points to rest on granite, with the intervention of a stratum of rolled pebbles. Its uppermost surface is bristled with rocky and scoriform projections, which, however, by decomposition resolve themselves into a rich soil affording nourishment to very productive chesnut forests. Some way above Montpézat, and perched just below the verge of the high primitive platform, is another cone of puzzolana and * See Plate XIV. scoriæ, also seen in the engraving, which does not appear to have given birth to any lava-stream. 2. Volcano of Burzet.-A bed of basalt occupies the bottom of the valley of Burzet, and follows all its windings as far as the point where it opens into the Ardèche, a distance of eight miles. It was produced from a point of eruption considerably above the village of Burzet, and at about the same elevation as the last-mentioned cone. It is chiefly remarkable for imbedding numerous nodular masses of olivine of a brilliant light green, and often as large as the fist. It is also very regularly columnar, and I observed it has not unfrequently happened that the seam separating two proximate columns cuts through one of the large imbedded knots of olivine, leaving a segment on either side. This fact seems to prove that the columnar divisionary structure was in its origin attended by a powerful contractile force, and also that it did not take place till the lava was so far consolidated, and the knots of olivine consequently so firmly compacted in its crystalline substance, as to separate along the line of the seam even when it divided them in two, sooner than quit their matrix.* In its disposition, or, more properly speaking, its aspect, this bed differs from the one last described; for the river, instead of cutting a deep channel through the mass, and consequently exhibiting vertical sections of it on either side, generally flows over its surface, the upper and amorphous part of which it has worn away, and thus disclosed on many points a plane horizontal section, in which the polygonal extremities of the columns are united into a sort of pavement (called by the natives, as with us in the north of Ireland, Pavés de Géans, or Giants' Causeways), not *In the face of such a fact as this, it is difficult to deny the reality of a contraction, or to speak of the columnar structure being occasioned by the "mutual pressure of spherical concretions." -See Scrope on Volcanos, p. 135 et seq. unlike those of the Roman roads in Italy, but arranged with far greater neatness and accuracy of design. The columns here, as throughout the Bas Vivarais, are usually hexahedral, often of five sides; those of four occur rarely, of seven still seldomer; I met with none of eight or nine; and of three, only when interposed between larger columns in the manner of a pyramidal wedge. The columns are habitually of small diameter, not often exceeding ten or at most twelve inches. They are sometimes divided by very frequent joints; at others attain a length of sixty feet without any separation. It struck me as a remarkable, and perhaps not a fortuitous coincidence, that, while the basaltic lava of Burzet is thickly sprinkled with knots of olivine, the granite, from the interior of which it has flowed in such abundance, contains an equal proportion of similarly shaped and sized nodules, composed of granular pinite with interspersed mica and quartz; this character prevailing only in a certain district near the site of the eruption. If the basaltic lava was derived from the fusion and recrystallization of granite, may we imagine the knots of pinite to have been converted during the process into olivine? 3. Volcano of Thueyts.-A volcanic cone to the east of the village of Thueyts is connected with that of Montpézat by a small portion of the primary ridge on which both eruptions broke forth, probably at the same epoch. It is much inferior in size to the other, and without a regular crater; but has vomited an abundant flood of lava into the bed of the Ardèche. Thueyts is built upon its surface, which is cellular and scoriaceous. The river has gnawed out a new channel between the bounding cliffs of this plateau and the granite of its southern bank, and exhibited a majestic colonnade of basalt, about one hundred and fifty feet in height, and extending with few breaks for a mile and a half along the valley. * 4. The cone of Jaujac, called La Coupe de Jaujac, from its cup-shaped crater, has this peculiarity, that it rises from a coal formation, occupying the bottom of a long transverse valley, between elevated ranges of granite and gneiss, and would thus appear to countenance the exploded notion that volcanic fires are alimented by immense beds of coal. The primitive fragments, however, frequently found enveloped by its scoria and basalt, sufficiently prove, if proof were wanting, that the source of the erupted matters existed, at least, below the sandstone which encloses the coal strata. The crater is very large and regular, but breached towards the north. Its figure is elliptical; the longer axis being directed north and south. Its sides as well as those of the cone are thickly clothed with chesnut-woods; and it is remarkable here as elsewhere, that those trees which grow on the volcanic are much larger and more productive than those on the primitive soil around. The earth resulting from the trituration and decomposition of recent basalt seems to be peculiarly favourable to the vegetation of the Spanish chesnut. Those of the woody region of Ætna are a well-known instance of prodigious luxuriance. At the foot of the cone gushes a mineral spring strongly impregnated with carbonic acid gas. From the northern breach of the crater may be traced a vast current of basalt, which occupies and descends the valley of the Alignon to a distance of between two and three miles. The village of Jaujac stands upon this bed, on the brink of a mural precipice, which is continued to the termination of the current, and everywhere presents a columnar range of almost unexampled beauty, about 150 feet in height. The river has excavated its channel between this and the *See Frontispiece.. |