the middle of the series, and are distinguishable from them at a distance by their whitish tint where the rock is exposed, and by their rounder contours.

With the exception of the Puy Chopine they consist entirely of that rock to which the most considerable of them has given the name of domite; each appearing as one enormous mass of this substance, in which it is not easy to discover traces of any definite structure; in some parts nearly compact, solid, and moderately hard; light, earthy, friable, or completely pulverulent, in others. The substance of one mountain differs but in accidental and insignificant characters from that of another. The colour of the rock is generally a greyish or brownish white; but on many points it has acquired various tints of red and yellow from the action of acid vapours. It absorbs moisture with the greatest avidity, and this action is accompanied by a hissing noise and a considerable disengagement of air-bubbles. Its texture is rudely granular; and on examination with a lens it appears to be an aggregation of imperfect microscopic crystals of glassy felspar, sprinkled with still smaller grains of augite, and occasional pallets of mica. These elements are partly separated from each other by minute pores, which render the substance rough to the touch, light, spongy, and bibulous. The larger imbedded minerals are, the glassy variety of felspar, generally cracked and mealy; mica in hexagonal or rhomboidal pallets, either bronzed or black; hornblende in acicular hexagonal foliated crystals, generally of a deep black, and highly lustrous along the planes of cleavage; and sphene; as well as specular and titaniferous iron in dispersed grains, blade-shaped laminæ, or regular octohedrons.

That these different substances crystallized nearly at the same period, would seem from the intimate manner in which their crystals are interwoven. Yet in general it appears that those

of mica and hornblende were entirely formed before the complete crystallization of the felspar; for perfect crystals of both these minerals may be frequently seen enveloped or as if suspended in the centre of a large crystal of glassy felspar, and the cracks produced in the mica or hornblende are invariably penetrated by felspar. Very commonly the component crystals appear to have been broken, bent, or split, probably by the movement which the enveloping substance suffered when propelled upon the surface of the earth from the volcanic focus.

This rock is extremely liable to decomposition, which affects it often to the depth of some feet. Its parts are then disaggregated, it assumes an earthy aspect, and crumbles between the fingers; the crystals of felspar become carious, lose their lustre ; and finally the whole mass is resolved into a meagre and ashy powder, in which the crystals of hornblende, mica, and octohedral iron are found uninjured.

The volcanic nature of domite has never been contested; and indeed it is sufficiently evidenced by the pumice-stones which accompany and are enclosed by it; the vitreous nature of its crystals of felspar; by its being porous, impregnated with muriatic acid, coated with sulphur, with sublimations of iron, &c., not to mention its similarity to the trachyte of the Mont Dore, which has still stronger proofs of a volcanic origin.

Surrounded and partly embraced by cones of scoria and lapilli, and remote from the rocks of the Mont Dore, which alone they resemble in substance, these hills have offered a perplexing problem to all the cursory visitors of Auvergne, and at the same time an ample field for conjecture to theorists. Hence the contradictory opinions that have been started on their origin. Desmarest considered the rock which composes them as a granite calcined in situ by a volcanic conflagration environing it;-Saussure, a petrosilex which had undergone the same ex

traordinary operation;-Dolomieu, Mossier, Montlosier, and De Buch, a granite triturated or liquefied by volcanic agency below the earth, and then by a sudden expansion of gas propelled through different apertures, over which it was consolidated in the form of huge bubbles.

Ramond objected to these hypotheses in their full extent, and argued that to imagine a mountain of the magnitude of the Puy de Dôme baked throughout upon the spot, is a stretch of imagination which can only be equalled by the supposition of its having swelled up like a bubble without bursting.

He denies, moreover, that each of the domitic hills is independent of the rest, the insulated production of a single and local operation, and asserts that this rock shows itself partially on many other points in the vicinity-a fact tending to show that these separate masses of the same substance were once, if they are not still, united, and constituted together a large bed, covering an extensive surface of the plateau, which succeeding • volcanic eruptions and other mechanical injuries have in great part destroyed, concealed, and reduced to the seemingly insulated remnants which now alone show themselves. M. Ramond concludes that this bed is a ramification from the trachytic currents of the Mont Dore. M. d'Aubuisson also has professed the same opinion; so that the authorities on either side seem nearly balanced.

It is, I believe, now generally recognised that domite is but a variety of trachyte; the same rock, in all essentials, which constitutes the greater part of the Mont Dore and Cantal, the Euganean Hills, the Monti Cimini, and the Isles of Lipari and Ponza.* It is therefore wholly unnecessary, nor should we be

* Vide Brocchi, Cat. Rag. passim.-Brieslak, Institutions Géologiques, vol. iii., &c. &c.

warranted in the attempt, to account for its production by any other mode of formation than that which appears common to similar rocks in other places; unless such an explanation should be in this instance opposed by any manifest improbability.

There is every reason to conclude the trachytes of the Mont Dore and Cantal, as well as the clinkstone of the Mezen, to have been propelled from a volcanic orifice in a state of incomplete liquefaction-in short, as lavas-and to have followed the inclination of the ground they occupied, flowing in a manner differing only from that of basaltic lavas in proportion to their different consistence and very inferior fluidity, or the accidental circumstances which may have concurred to modify their disposition.

It is evident that, under similar circumstances of the surrounding levels and of propulsive force, the tendency of a mass of lava to quit the neighbourhood 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 assumes the form of a dome or bell-shaped 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 à priori to have possessed the minimum of fluidity when protruded into the air; *

*See Considerations on Volcanos, p. 92-96.

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and we therefore can understand perfectly why, instead of flowing in thin and continuous sheets or streams to a distance from its vent, like the basaltic lavas produced about the same time and from the same fissure, it has accumulated in dome and bell-shaped hillocks on the point where it was emitted. That this was the mode of production of these masses of trachyte, certainly that they were thrown up on the spots they now occupy, seems to me proved by their rising in every instance either from the middle or the side of a regular crater and cone of scoriæ.*

If it could be imagined possible for the volcano of the Mont Dore to have sent forth a vast current of trachyte in this direction, of which these hills have been supposed the remaining segments, in spite of the fact that the great elevation of the granite ridge upon which they rest above the surrounding country renders it the last of all directions which such a current could have taken, and in spite of the improbability that a rocky bed of which the Puy de Dôme, a mass rising 1600 feet above its base, is merely a detached remnant, should have left no traces of its existence in the interval between that mountain and the Mont Dore, a distance of 7 or 8 miles; yet a still stronger objection to this hypothesis remains behind, viz. the improbability that the position of each of these fragments should severally coincide exactly with that of the vent of a separate recent eruption; that the only points on which any considerable remnants of this supposed bed are to be found should be precisely

*See Plate III., and the map of the Monts Dôme. In Iceland M. Robert describes ('Voyage en Islande,' Paris, 1840) the Mont Baula as a pyramidal dome of yellowish-white very porous trachyte, partly columnar, at the foot of which is seen a contiguous crater

of eruption. The resemblance of this to the Puy de Dôme, rising from the crater of the Petit Puy, is complete. In Hungary also, and elsewhere, trachytic domes are described as rising in the middle of crateriform hollows.