with respect to ourselves. I am not aware of any organic remains having been observed under these lava-beds, which might throw any light on the geological age to which they belong.

There is one instance, in the immediate vicinity of Aubenas, in which a volcanic eruption appears to have taken place within, but at little distance from, the limits of the secondary limestone formation. The hill upon which this town is built consists of strata dipping to the south at an angle of 10°. Within a hundred yards north of the town walls, and scarcely so much from the sandstone on which they rest, these limestone strata are abruptly broken through by an enormous vertical dyke of basalt, which protrudes about 30 feet above the summit of the hill, and may be traced down its side into the valley of the Ardèche, pursuing an easterly direction. At the lower part of the hill the dyke measures from 12 to 20 feet between its cheeks; higher up it increases in bulk, and at the summit is 90 feet thick. From thence it makes a sudden curve to the north, returns to the north-east, and is again to be traced down the same face of the hill about a hundred yards from the other branch; thus including a large portion of the limestone strata, which preserve their parallelism and general inclination. This second branch of the dyke has dislocated and shattered the limestone within a space of from 40 to 50 yards, enveloping blocks and fragments of all sizes, with which it is mixed confusedly, and presents the appearance of a loose limestone breccia traversed or cemented by veins of basalt. Two smaller veins may also be observed cutting perpendicularly the undisturbed limestone strata enclosed by the two arms of the principal dyke. The one is 15, the other 10 inches thick.

The basalt itself is of a very dark grey colour, compact, fine-grained, hard, and tough; its fracture tends imperfectly to the conchoidal, and it breaks into curved and angular pieces.

It encloses numerous large crystals of greenish-black augite, and a brilliant sea-green olivine. It is almost universally penetrated by calcareous infiltrations, which line all the cavities of the cellular parts with snow-white calc-spar, and are even found in the interior of some of the crystals of augite. Small fragments also of limestone are enveloped by the basalt, to which they adhere firmly, the two substances appearing soldered together by an intimate though partial mixture. Occasionally the whole enclosed fragment of limestone is hardened, of a dark grey colour, a granular or crystalline texture, and a siliceous or cherty aspect; in some instances, on the contrary, it is white, earthy, and effervesces readily with acids; while in others the basaltic and calcareous particles seem to have united and separated again, the latter into small grains or patches resembling white porcelain imbedded in a dark grey basaltic base. The large masses of limestone in contact with the veins exhibit few or no signs of alteration.

The whole appearance of this dyke and its subordinate ramifications announces its having been forcibly propelled from below through the calcareous strata; but at what epoch, or whether connected in any way with the neighbouring eruptions whose products have been described above, it is difficult even to conjecture.

This is the most southern instance of any volcanic formation which attaches itself to the primary mountain-group of Central France. The interval from hence to the neighbourhood of Béziers, Agde, and Pezenas, or Aix and Toulon,-all points upon which volcanic remains are also to be found,-is, I believe, totally exempt from such substances; and it seems a very fair conclusion, that the enormous mass of secondary strata which from this spot southward covers the fundamental granite, and consequently the focus of all volcanic energy, stifled and impeded its ulterior development.

CHAPTER IX.

CONCLUDING REMARKS.

If we now turn to take a wide and general view of the interesting tract whose particular formations have been described in the preceding pages, some of its most prominent features will be found to suggest considerations of no mean geological importance.

1. We first remark the peculiar position of the great mass of primary or Plutonic rocks, piercing, like a vast protuberance, through the secondary strata which surround it on every side, and appearing to have formed an island in the ocean from the commencement of the secondary period, and at no subsequent time to been covered by the sea; for,

2. It is observable that no marine deposits later than the Jurassic system are to be found within the area of this elevated district, or nearer to it than the low chalk hills of Champagne and Touraine on the north, and the still lower basin of Languedoc on the south; while in lieu of these a very massive calcareous, and in part arenaceous formation, the accumulated sediments of one or more extensive freshwater lakes, occupies the principal depressions in the primitive table-land, and prolongs itself from thence northwards as far as Nevers and Moulins.

3. Having noticed at least three of these calcareous freshwater deposits within the mountainous district to which our examination has been confined, the question naturally rises in the

mind, whence could such immense accumulations of carbonate of lime be derived? Other examples of analogous formations are usually found in cavities surrounded by chalk or secondary limestone. The calcareous formations of Auvergne, the Cantal, the Haute Loire, and Montbrison are entirely cased in granitic rocks. This circumstance wholly prevents our supposing the carbonate of lime to proceed from the detritus of other limestone strata; and it is therefore to the calciferous springs of St. Alyre, St. Nectaire, Rambon, Chalucet, and the numerous others of a similar nature which we can hardly doubt to have been more productive when the subterranean forces of this district were in greater activity, that we can alone look for its origin. A large proportion of the lime emitted from such of these sources as were below the level of the lakes was first probably secreted by the chara and other aquatic plants growing from their bottom (of which numerous impressions are still found), and thence taken as food into the substance of innumerable mollusks, which by the gradual accumulation of their shells gave rise to the marly strata of these lake-basins; while on those spots where the springs produced the matter in abundance, or in the open air, beds of more or less compact semi-crystalline limestone, or travertin, were formed by its precipitation. This view is strongly confirmed by the admixture with the lime both of siliceous matter, which is known to be frequently deposited from thermal springs in a volcanic country, and also of gypsum for it has been shown that eruptions were habitually taking place during the deposition of the calcareous strata of the Limagne and the Cantal, if not of the Haute Loire: and if sulphuretted hydrogen was evolved, as is usual during these phenomena, through the soft marly beds at the bottom of the lakes, the sulphuric acid uniting with the lime would necessarily produce that mineral: of which a part may have

been precipitated on the spot, and the remainder carried away by the water into the lower lake-basins to be ultimately deposited there (Gypsum of Paris?). The strong odour of sulphuretted hydrogen emitted from the marly limestone of Le Puy seems indeed to prove that this gas was produced at the epoch of its deposition.

In fact, the calcareous freshwater formations of the centre of France differ but in this one respect (viz. the presence of gypsum and silex, which the thermal volcanic springs will account for) from the recent shell-marl deposits of the Bakie and other lochs in Scotland, described by Sir C. Lyell. In both are found Limneæ, Planorbes, Helices, a species of Cypris, the remains of Chara and the Gyrogonites. In both, marly strata in which all traces of shells have disappeared, but which occasionally contain bones of mammalia and birds, alternate with others of a yellowish travertin limestone, often semi-crystalline, tubular, containing remains of vegetables, insects, &c., and with beds of sand. The basins of both still contain springs charged with carbonate of lime, and both occur in the neighbourhood of trap or volcanic rocks.*

4. With regard to the rocks of volcanic origin, they are distinguished primarily into, 1st. The products of three great habitual vents-the Monts Dore, Cantal, and Mezen, which appear to have been in activity towards the same time, and to have raged at intervals and with intense energy during a period of considerable duration. 2ndly. The products of single occasional eruptions from a vast number of separate apertures,

* I retain these remarks as they were printed in the first edition (1826), although aware that to the greater number of my readers such arguments will be familiar from the admirable develop

ment and confirmation they have since received in the works of my friend Sir Charles Lyell. See his Manual, p. 197-203, ed. 1855.