more favourable circumstances would have united into distinct crystals.

Small imbedded crystals of felspar are frequent, of augite or hornblende rarer. The lower parts of the bed are somewhat porous and occasionally vesicular: in the vicinity of the central summits highly cellular and even scoriform masses are abundant; they are usually of a reddish or reddish-blue colour, and a crystalline texture.*

BASALTS AND BRECCIAS.

I have already noticed basalt as having been largely produced by this volcanic system. The currents principally flowed towards the north-west, west, north-east, and south-east, led by the superior inclination of the primary platform in these directions.

Towards the south-east in particular an immense embranchment, composed of two or more successive beds of basalt accompanied by breccia, descends to a distance of more than thirty miles, rivalling in extent and volume that of clinkstone which we have described on the opposite side.

This current is not less interesting from its position than the magnitude of its dimensions. It appears to take its rise from between the scattered peaks of clinkstone which have been noticed above as grouped together on the south of the Mezen, and is prolonged with a gentle slope towards the south-east for

* I am aware that, in attributing the chain of clinkstone hills which stretch from the Mezen to Miaune to a single colossal current worn by the elements into fragments, I am differing from the greater number of local geologists, who consider these domes or puys to be each the product of a separate local outburst. I can only say that, on my recent visit to the district, all the ex

amination I was able to make confirmed me in my original opinion. And, moreover, I felt a strong suspicion that on many points of this range the clinkstone hummocks rest upon basalt which had previously flowed as lava in that direction over the granitic surface. I would strongly recommend the investigation of these two questions to such geologists as may visit this country.

M

the space of about twelve miles, resting on the high primitive platform of the Haut Vivarais, and forming an elevated ridge which separates the waters of the rivers Ardèche and Erieux. Near the line of junction of the primary and secondary formations it is broken off by a deep gap, through which passes the road from Aubenas to Privas. But on the opposite side of this depression the same bed is repeated at a precisely corresponding height on the summit of a mountain of Jura limestone. From hence it is continued with a similar gradual inclination to a direct distance of about twelve more miles. As long as this bed is based on granite its breadth is trifling, and it shows itself rather as a continuous flat-topped mountain-crest than in the usual form of a wide plateau. But on entering the limits of the secondary formation it assumes a different disposition, spreads itself to a width of five, seven, and nine miles, and covers an extensive and elevated table-land, which under the ancient régime went by the name of the Coiron. There cannot be the least doubt but that the whole of this lofty tract of Jurassic strata has been solely preserved from destruction by its volcanic capping. The remainder of the formation around has been eaten into in all directions by various mountain torrents, and gnawed down by meteoric abrasion to a far lower level. The Coiron alone beneath the shelter of its basaltic coating has effectually resisted, and juts out like a huge flattened headland from the margin of the high primary plateau into the southern plain. It has not, however, escaped uninjured. The agents of waste, to which the iron hardness of basalt itself ultimately yields, have intersected it by numerous transverse ravines, excavated sometimes to a considerable depth through both the volcanic and calcareous strata. They are separated by massive parallel embranchments, which derive from a straight longitudinal axis, exactly in the manner of ribs from the spine. On either side there are as

many as eight or nine of these, each crowned by a prodigious tabular load of volcanic products, presenting in its section a vast range of vertical cliffs resting on the secondary limestone strata. The perfect correspondence in position, structure, and dimensions, of these cappings, as well as their all branching off from the same stem, testify to their having been once united in a single continuous platform; and in all their aspects, but particularly from the side of Villeneuve, where the extremities of six or seven of these ramifications may be taken in at once by the eye,* their appearance is striking in the extreme.

The thickness of the volcanic mass is usually between 300 and 400 feet, which appeared to be made up, wherever I examined it, of two enormous distinct beds of basalt, separated by a layer, varying greatly in thickness, of scoriæ and volcanic fragments united into a breccia, or of loose scoriæ alone. Each bed, but particularly the inferior one, presents a sort of lower story of very perfect and well-matched vertical columns, surmounted by a still thicker mass, which is comparatively amorphous, but on a near approach is seen to consist of innumerable small columns both straight and curved, disposed in every possible direction and entangled into every variety of figure. These two portions are blended at their line of contact in such a manner that it is impossible to doubt their belonging to the same bed. The basalt of the uppermost surface is porous, slaggy, and scoriform.t

The almost architectural symmetry resulting on many points from this arrangement was probably the origin of the fable current among the peasants of the plain below, who still call

* See Plate XII. Also the General Map of Central France.

Exactly resembling in these re

spects the very recent lava-streams of the Vivarais to be described in a subsequent page.