GEOLOGY AND EXTINCT VOLCANOS OF CENTRAL FRANCE. CHAPTER I. GRANITIC PLATEAU AND MARINE STRATIFIED FORMATIONS. THE parallel of 46:30, passing near the towns of Châteauroux and Châlons-sur-Saône, will be found to divide France into two nearly equal portions, of which the northern may be considered as a vast plain, whose waters flow gently towards the north and west through the Seine and the lower Loire. South of this line the surface continues to rise with a gradual slope, so as to form an inclined plane, which progressively acquires an elevation of more than 3000 feet above the sea in the Auvergne and Forèz, and a still greater in the Gevaudan and Vivarais, where it reaches the height of 5500 feet. Here it is abruptly cut down by the deep valley of the Rhone, which, running nearly due north and south, separates it from the ranges east of that river, in the departments Drôme, Isère, and Hautes Alpes. On the south-west also this high ground rapidly descends through broken and irregular embranchments to the basin of the Gironde. It may, in fact, be considered as a triangular B platform, tilted up at its south-eastern angle, and declining gradually to the north-west. The principal mass of this elevated district is composed of primary crystalline rocks, chiefly granite, overlapped on all sides by secondary strata belonging principally to the Jurassic system, which, at its southern extremity, attain a considerable elevation in the chain of the Cevennes. Abstraction is in this description made of the volcanic products which rise, like enormous protuberances, from the higher parts of the elevated granitic platform. It is also deeply indented by the valleys of the upper Loire and Allier. These, on some points, acquire considerable width; the first in the basins of Montbrison and Roanne, the latter in the plain of the Limagne. Several detached basins of carboniferous sandstone occur within this district, seeming to have been deposited in hollows or estuaries of the original island of primary rocks. And there are vestiges of four geographically distinct deposits of as many freshwater lakes belonging to the tertiary period, occurring severally in Auvergne, the Forèz, the Cantal, and the Velay. The granite of this district varies much in character, often within very narrow limits passing into gneiss, and sometimes, especially on its southern and western borders, into mica-schist, talcose-schist, or serpentine. The mica is sometimes replaced by pinite, either in amorphous nodules or crystallized in hexagonal prisms. It is here and there traversed by veins and dykes of fine-grained granite, of compact felspar, and of felspar porphyry. The felspar of the granitic rocks sometimes takes the form of large twin crystals, occasionally rose-coloured, like those of Baveno. The quartz often also presents beautiful crystallizations. The amethysts of Le Vernets, near Issoire, have long been known in commerce. In metals this primary district is not rich. Iron is very generally disseminated, but is only worked on a large scale at Alais (Gard), and at Rive de Gier, in the coal-basin of St. Etienne (Loire). Near Pont Gibaud argentiferous sulphuret of lead occurs, and has been wrought lately at a considerable expense, but, it is believed, with no great success. The same ore occurs near Villefort (Ardèche), and in the departments Aveyron and Lot, generally accompanied with manganese. The granite round Ardes (Puy de Dôme) and Massiac (Cantal), and the mica-schist of the Lozère, are rich in antimony. Copper is rare, but some veins in the Aveyron are supposed to have been anciently worked by the English when they were in possession of the country. Near Limoges and St. Yrieix the gneiss rock is decomposed into a kaolin of great purity, which has long supplied the china factories of Sèvres and Paris, and is even exported to the United States. Generally, the granite decomposes readily on the exposed surfaces, and presents therefore rounded outlines; while the gneiss, containing more quartz and mica, and having a schistose divisionary structure, exhibits peaked eminences and precipitous escarpments. The mica-schist passes on some points into clay-slate, as near Alassac (Corrèze). With the only other exception of the very limited district of Tarare, between the Rhône and Loire, where a quartziferous sandstone, probably Devonian, and accompanied with anthracite, has been penetrated by a large outburst of red porphyry, the entire region of Central France contains, I believe, scarcely any sedimentary strata more ancient than the carboniferous; the Cambrian, Silurian, and Devonian series being generally absent. The coal-measures are sometimes associated with triassic strata, but much more usually with lias, and other members of the Jurassic system. In connection with these strata, both arenaceous and calcareous, coal is found in detached patches nearly all round the granitic platform, and, as already observed, on many points within its limits, especially on one straight line crossing it from north-north-east to south-south-west, from near Moulins to Mauriac. The direction of this line is remarkably coincident with the apparent axis of the granitic dome, and with the neighbouring volcanic range. The most important of these coalfields are at Autun (Saône et Loire); Decize (Nièvre); Villefranche and Bert (Allier); Brassac, in the basin of the Allier, near Lempde; St. Etienne and Rive de Gier (Loire); Lavoulte, Prades, and Joyeuse (Ardèche); Alais and Garges (Gard); Creuze and Bedarrieux (Hérault); Sansac, Layssac, and Aubin (Aveyron); Brives (Corrèze); Bourglastic and Bassignac, in the basin of the Dordogne; Bort and St. Eloy (Puy de Dôme). An extensive series of limestone strata, belonging to the lias and oolite group, embraces (as has been before observed) the whole granitic platform like a frame; on its southern border especially, these calcareous rocks assume a remarkable development, forming a large proportion of the surface of the departments Aveyron, Lozère, Gard, and Ardèche. They constitute a vast elevated platform, sloping gradually from the primary range towards the south-west, and intersected by a few deep gorges, scarcely wider anywhere than the bed of the river that flows at the bottom of each. The stratification being nearly horizontal, though dipping to the south or south-west, this formation exhibits a series of flat-topped hills bounded by perpendicular cliffs 600 or 800 feet high. These plateaux are called "causses" in the provincial dialect, and they have a singularly dreary and desert aspect from the monotony of their form and their barren and rocky character. The valleys which separate them are rarely of considerable width; winding, narrow, and all but impassable cleft-like glens predominate, giving to the "Cevennes" that peculiarly intricate character which enabled its Protestant inhabitants in the beginning of the last century to offer so |