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Plate VI.

VALLEY OF VILLAR AND PLATEAU OF PRUDELLE

south and Gressinier on the north, and terminating in an abrupt escarpment towards the east. It appears, however, to have once extended some way down the steep slope in this direction, for many portions of a similar basalt are to be found in situ amongst the vineyards which clothe its declivity.

The cone whence this current flowed still forms a projecting knoll at its western extremity, strewed with bomb-shaped scoriæ, the cavities of which contain delicate stalagmitic concretions of quartz, the fiorite of the French mineralogists. It is this hillock which occasioned the division of the current of Pariou (6).

At the point of contact of the granite and the basaltic bed it supports, is seen a layer of scoriæ, so far decomposed as to be cut with a knife; their cavities are filled with a white and brown bole of a waxy consistence. The basalt has separated on some points into very regular prisms of five or six sides, which exfoliate by decomposition in slaty lamina at right angles to their axes.

The accompanying engraving gives a view of the position of this plateau (on the left) hanging over the steep granitic gorge of Villar. Beneath is a Roman road, of which the old basaltic pavement is still entire (called le Chemin Ferré). It is formed upon the surface of the comparatively recent lava-current descending from Pariou, since the flowing of which, however, the ravine on the right has been worn away more than 150 feet in depth. It is evident that the entire gorge has been excavated since the basalt of Prudelle flowed upon the surface it now covers, which must then necessarily have been the lowest level of the vicinity. Here again is a most instructive collocation of the older and recent lava-streams, telling the same tale of the gradual erosion of the valleys of the district by causes still in operation.

Though the current of Prudelle has every appearance of being single, it consists of two very different species of basalt. That of the western extremity is remarkable for the numerous

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and large nodules of yellow and red olivine it encloses. the east has in place of this mineral confused crystals of augite and compact felspar, disseminated in a base which has all the characters of a fine-grained dolerite.

75. Puy Girou (A. E. 2888.). 76. Puy de Jussat. 77. Gergovia.-I consider these three eminences, all based on the freshwater limestone, to have originally formed a single plateau, capped by a current of basaltic lava proceeding from the neighbouring primitive heights, probably from the Puy de Berzé (79). (See Plate I.)

They are now partially separated from each other and from the granite; the first has wasted to a conical cluster of prisms converging towards the apex; * the second to a crested ridge; while the broad plateau of Gergovia still occupies an extended surface, which tradition and history unite to fix on as the site of that city of the Arverni which Cæsar and his legions so long and vainly besieged.†

Gergovia is as interesting to the geologist as to the antiquary. On the southern and western flanks thick strata of siliceous indusial limestone abound. The eastern face offers a distinct example of the alternation of basaltic currents with the calca

This peculiar disposition of the prisms, so evidently adapted to protect the mass they constitute from the destructive agency of rains and frosts, will be found on observation to be the cause which has preserved throughout all basaltic regions so many isolated and conical hills of this rock. I do not remember to have met with one of this nature in which it was not easy to trace such a structure; and this fact unites itself to the many others presented to the eye and understanding of the geologist by every great mass of mountains,

which tend to force upon him an overwhelming conviction of the vast amount of denudation of the supra-marine portions of the surface of the earth, that has been effected by the wasting powers of the meteoric agents, since its emergence from the ocean. The great height of the calcareous strata at the Puy Girou (2800 feet, A. E.) is remarkable.

Roman bricks, amphora, medals, and Gaulish axes and arrow-heads, in jade and serpentine, are frequently found on this plain.

reous strata of the freshwater formation. Two deep ravines have there laid open complete sections of all its beds. The base consists, like the plain from which it rises, of thin horizontal strata of white marly limestone. At about two-thirds of the whole

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7.

Eastern Face of Gergovia, showing its two Beds of Basalt (1, 2), and the stratified Calcareous
Peperino between them.

elevation occurs a massive horizontal bed of basalt, in some parts 40 feet in thickness, which appears to have moulded itself on the strata below, and upon the surface of which other calcareous strata have been again deposited.

These, however, though distinctly stratified, and with a general tendency to horizontality, are far from being as regularly disposed as the inferior limestone. They are frequently distorted and confused, and occasionally interrupted by narrow horizontal venous masses of basalt, which appear to be ramifications from the upper bed of great thickness, which surmounts the whole and forms the superficies of the plateau. The calcareous strata thus included between the two beds of basalt are thickly interspersed with volcanic ashes and scoriæ. A few thin strata of compact yellow limestone may be found apparently free from extraneous substances, but the general mass is rather a calcareous peperino than anything else. It is in parts veined with semiopal, and contains masses of siliceous limestone.

The lower bed of basalt projects far beyond the upper, appear

ing to have been originally more extensive; and those portions which cap the calcareous hills above Aubières, Perignat, and Channonat, must have belonged to this bed, which, whether erupted on the spot or at a distance, is clearly prior in date to the strata of calcareous peperino above it, as was remarked in an earlier page, and was produced at a time when the freshwater lake was still depositing its chalky sediment. It is in some parts very regularly prismatic, in others amorphous ; is of a dark colour, dense, hard, and sonorous; contains minute scaly crystals of glassy felspar, and some calcareous infiltrations.

The line of contact of this bed of basalt and the supporting stratum of marly limestone is well defined, and specimens of the smallest size may be taken from it, of which one half is compact, brittle, and black basalt, the other a white limestone effervescing strongly with acids, and apparently uninjured. The basaltic masses which occur in the intermediate calcareous strata are far from being so distinctly separated from the enveloping matter. The one substance, on the contrary, seems almost to pass into the other by a mechanical intermixture effected when both were of a very soft, if not fluid, consistence.

The upper plateau of basalt rarely presents a prismatic division on a large scale; but more frequently a tabular structure. It is on some points extremely cellular; the vesicles having been subsequently filled by crystallizations of carbonate of lime and arragonite, so as to give it the character of a very rich amygdaloid.

78. Montrognon.-A conical eminence of columnar basalt,, crowned by the ruins of a feudal fortress, and resting on the freshwater limestone. It is probably the sole remnant of a plateau formed by a branch from the current of Gergovia.

79. Puy de Berzé.-A salient eminence of the granitic platform; the site of an ancient volcanic aperture, for its summit is

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