The limits of the original basin show themselves on the west, in the base of the granitic chain which separates the waters of the Loire and the Allier. The rise of the granite platform towards the Haut Vivarais bounded it on the south; and some irregular embranchments from the heights of St. Bonnet and La Chaise Dieu on the east and north. One of these granitic spurs indeed stretches across the lacustrine formation, and separates it into two, cutting off the small basin of Emblavès from the upper and larger one, in which lies the town of Le Puy. The river Loire passes now from the latter basin into that of Emblaves through the narrow, deep, and sinuous gorge of La Voute, and finally issues again from this by means of the similar defile of Chamalières. Both of these outlets appear to owe their origin to some of the most recent changes to which this singular district has been subjected. Certainly neither could have been in existence at the time that the massive sheets of basalt and clinkstone which severally cap the cliff-ranges above them were in a state of igneous liquefaction. The lower series of lacustrine beds consist, as in the formations already described, of sandstone, blue, green, and red variegated sandy marls, and clays. The sandstone is an excellent buildingstone; the clays are used for pottery.* The upper beds are chiefly of marly limestone, often highly siliceous, and enclosing * Doubts have been expressed with respect to these sandstones, &c., as well as the analogous beds which underlie the unquestionably freshwater and tertiary marls and limestone of the Limagne, whether they do not belong to the secondary formation of new red or variegated sandstone. The remains of plants which they occasionally contain indicate a marshy soil and atmosphere, consisting of large reeds, casts of cyclop teris and pecopteris, with some small seeds and fruits which appear referable to dicotyledonous plants. No shells have yet been found. In the corresponding sands of Auvergne a species of Cyrena is met with, and M. l'Abbé Croizet is, indeed, said to have found a bivalve apparently of marine origin; but the evidence of this isolated fact seems obscure, and not to be depended on, unless confirmed by future discoveries of the same character. See Appendix for the Fauna of the Tertiary strata. layers of flint passing into semi-opal, especially in the vicinity of volcanic rocks. At St. Pierre Eynac these siliceous strata evidently pass under the powerful clinkstone mass of Montplaux. Towards the middle of the basin the clayey marls alternate with beds of gypsum, several of which are sufficiently rich to be worked for agricultural and other uses. The shells contained in these beds are of lacustrine or marshy species; bones also occur in them, and the remains of fish, crustacea, birds, and their eggs. Above the marls with gypsum is usually a considerable thickness of calcareous and foliated marly strata, alternating with greyish limestone of the consistence of chalk, having tubular cavities attesting its palustrine origin, and numerous casts of planorbes, limnei, cyclostomæ, bulimi, cyprides, &c. Bones and teeth of animals, both terrestrial and aquatic, are also found in them abundantly, for a catalogue of which I must refer to M.Pomel.† In one site alone, the hill of Ronzon, and nearly in one bed, so large a number and variety of organic remains occur as almost to furnish a complete Fauna of the district at the period of its deposition, which MM. Pomel, Aymard, and Lyell unite in referring to the Lower Miocene. There is no clear evidence of the outburst of any neighbouring volcanos during the tertiary period in which these sedimentary beds were deposited in the freshwater lake of the Haute Loire. No alternation of volcanic matters with the sedimentary strata, such as those already described in the Limagne, have been detected within this basin. It is possible, therefore, that the earliest development of local volcanos occasioned such a disturbance as to cause the drainage of the lake; on the other hand, several of the basaltic breccias, which we shall have occa * M. Aymard mentions the following mammifers, as found by him in these beds:-Palæotherium primævum, Pal, sub gracile, Monacrum velaunum. sion to describe among the volcanic rocks of this district, bear a near resemblance to the peperino of the Auvergne lake-basin, and were perhaps, like that rock, the product of eruptions from within its area before the waters were wholly drawn off. IV. BASIN OF MONTBRISON. I have not myself visited this locality, nor am I acquainted with any detailed description of it. It occupies a valley-plain, about 20 miles long by 10 in width, encased between the granite and gneiss ranges of the Forèz and the Lyonnais on the south, west, and east, and the porphyry rocks and Devonian strata of the Tarare, through which the Loire finds an outlet to the north. The tertiary strata of this basin have so close a resemblance in character and position with those of the lower valley of the same river about Roanne, as to lead M. Raulin* to presume that the two basins were originally connected by a channel permitting their waters to maintain the same level. The same red and yellow sands, sandstones, clays, and green and white foliated marls are found at Marcilly, Boën, Sury le Comtal, and generally throughout the plain. Several eruptions of volcanic matter appear also to have taken place within this basin, and from the granitic heights to the west. But for the reason given above I am not aware of the precise circumstances under which the volcanic rocks present themselves. M. le Coq informs me that several basaltic dykes appear near the junction of the porphyry and granite; while others penetrate through beds of rolled pebbles, probably belonging to the lower term of the lacustrine series. A further examination of this basin seems very desirable. V. TRIPOLI BASIN OF MENAT. At Menat, on the road from Riom to Montaigu, occurs a sin * Bulletin XIV. p. 584. gular depression in the gneiss and mica-schist which here take the place of the granite of the primary or crystalline platform. It is nearly circular and about a mile in diameter, and discharges its waters into the Sioule through a narrow gulley worn in the schists not more than 12 feet wide and as many deep. Before this passage was effected they must have formed a lake over the whole valley, the surface of which is almost perfectly level. The excavations that have been made in the sedimentary beds beneath this surface show them to be composed to a considerable but unascertained depth of bituminous shale, or desiccated flaky clay of a muddy black colour, evidently the fine detritus of the micaceous and talcose rocks which enclose the basin impregnated with bituminous matter, and often containing vegetable remains in such abundance as to become a true lignite. It envelops many nodules of iron pyrites, globular or lenticular, sometimes assuming the flattened moulds of fish, chiefly a cyprinus, very like the Cyp. papyraceus of the lignite of the Siebengebirge. The thin folia of the shale and lignite exhibit on their surface innumerable impressions of leaves resembling those of the chesnut, sycamore, willow, lime, and aspen, which still grow in the neighbourhood, with others which certainly do not belong to European species, and resemble those of the liquid amber, Styraciflua and Gossypium arboreum.* Some flattened fruit, or seed-vessels, are also found resembling those of the hornbeam. These lignites appear to have undergone spontaneous combustion on some points (probably where pyrites abounded), and the shale has been thus converted into reddish tripoli, which is largely quarried for commercial use. It seems probable that the formation of this simple alluvial deposit is of no very distant date. * Bouillet, Vues et Coupes du Puy de Dôme.' 19 CHAPTER III. INTRODUCTORY ACCOUNT OF THE NOTICES WHICH HAVE BEEN HITHERTO PUBLISHED CONCERNING THE VOLCANIC REMAINS OF THE INTERIOR OF FRANCE. To those who now travel over the mountains of central France, and see on all sides marks of volcanic agency exhibited in the most decided manner, numerous hills formed entirely of loose cinders, red, porous, and scorified as those just thrown from a furnace, and surrounded by plains of black and rugged lava, on which the lichen almost refuses to vegetate, it appears scarcely credible that, previous to the middle of the last century, no one had thought of attributing these marks of desolation to the only power in nature capable of producing them. This apparent blindness is however very natural, and not without example. The inhabitants of Herculaneum and Pompeia built their houses with the lavas of Vesuvius, ploughed up its scoriæ and ashes, and gathered their chesnuts from its crater, without dreaming of their neighbourhood to a volcano which was to give the first notice of its existence by burying them under the products of its eruptions. The Catanians regarded as fables all relations of the former activity of Etna, when, in 1669, half their town was overwhelmed by one of its currents of lava. In the year 1751 two members of the Academy of Paris, Guettard and Malesherbes, on their return from Italy, where they had visited Vesuvius and observed its productions, passed through Montelimart, a small town on the left bank of the Rhône, and, after dining with a party of savans resident there, week.35 |