Its elevation varies from 850 to 400 feet above the waterchannels of the valleys on either side: and a branch of it, cut off and separated from the main current by a subsequent excavation, has assumed that conical form so general amongst basaltic remains, and to which the waste of ages tends to reduce all, and crowned by a ruined fortress, called Montredon, imitates exactly the Stolpens and Koenigsteins of other basaltic districts.* The current of La Serre originated in the granite, and the most considerable moiety of its extent rests upon that rock, the remainder on the freshwater limestone. Its western summit, called the "Tête de la Serre," or Puy de Nadailhat, measures 3461 feet from the sea-level. projecting tongue of hill, the partial separation from the rest. of Le Crest, at an absolute elevation of 2044 feet, giving a difference of level between the two extremities of 1417 feet, with a direct distance of rather more than six miles and a It terminates to the east in a point of which has suffered a On this point stands the village *This process has been repeated on numberless points of Auvergne, where almost at every step we meet with isolated and conical peaks, each consisting of an immense group of basaltic columns converging towards the summit. All of these were seized on in turn for the sites of fortresses, in those times of anarchy when an inaccessible position was the necessary condition of security to person or property. Such were the castles of Mont-rognon, Montredon, Mont-rodeix, Mont-celets, Vodable, Usson, Nonnette, Buron, Mozun, Murol, Vandeix, Bonnevie, Merçœur, Ibois, Mercurol, &c. &c. From the number of these strongholds and the almost impregnable nature of the greater part, the feudal tyrants of Auvergne outlasted those of the rest of France; and it was not until the ministry of Richelieu, and the vigorous reign of Louis XIV., that a final check was given to their career of violence and rapine. Many of them were judicially condemned and executed at Clermont by a special court held there and called "Les Grands Jours," a fate they well merited. Orders were then issued for the demolition of all the châteaux-forts of Auvergne, and little now remains of them but the foundations, and some fragments of their massive walls, which were generally constructed of basaltic prisms taken from the peak itself, and laid horizontally. Puzzolana was mixed with the mortar used in these constructions; and without the binding quality communicated by this ingredient, probably no cement would have taken effect on the smooth and iron surfaces of the prisms. quarter. The inclination therefore of this bed of basalt corresponds almost completely with that of the two very recent lavacurrents which have flowed down and now occupy the bottoms of the valleys on either side of it; the one proceeding from the Puy Noir, the other from the group of cones about the Lake Aidat: moreover, the distance to which it reaches is about the same as in their cases. The parallel between the older and newer basaltic currents visible in such close approximation is complete and highly interesting. The only essential difference is that of position; the one occupying the summit of a long hill, the others the bottom of its lateral valleys, but yet on many points even there forming plateaux which have acquired already a very considerable elevation above the actual river-channels. The surface of the plateau of La Serre is not a uniform slope, being broken by three declivities which have the appearance of steps. Two of these are over the granite, and another at the line of contact of this and the freshwater formation. They were probably occasioned by inequalities in the granitic surface here and there opposing a temporary check to the current. The portion which rests on the freshwater strata is smooth and nearly level. It is worthy of remark that on each side a transverse ravine is found near where the tertiary sedimental beds rest on the primary crystallines, and a depression exists on the surface of the plateau, along that line, the basalt having been already partially undermined there by an excavation sapped through the strata of friable sandstone which intervene between the calcareous formation and the granite. In time, no doubt, an entire separation will thus be effected, and the eastern portion of this hill will resemble the many others around, which have lost their connection with the primary heights. *See p. 92-3 suprà. The basalt of this plateau is from 50 to 100 feet in thickness. Though in general amorphous, or cleft by irregular vertical seams, yet on some points, particularly near Le Crest and at the Castle of Montredon, it exhibits very beautiful columnar groups. The current has here been eaten into by deep ravines which disclose its internal structure, and in all probability the apparent absence of this regular configuration in the remaining parts of the plateau is only superficial. Upon the whole, this hill is very instructive, as a type of the formation of basaltic plateaux in general; one of those valuable links which establish a relation between rocks apparently remote in geological position; one of the intermediate gradations through which a current of lava, on its first production, occupying the bottom of a valley, passes in the lapse of ages, and in virtue of the great resistance it offers (and lends umbrellalike to the strata beneath) to the downward wash of rain, into the massive and tabular capping of an isolated and lofty mountain. 72-73. Les Côtes de Clermont and Chanturgue.-These calcareous hills, once evidently united, and separated now but by a shallow ravine, are crowned with a bed of basalt, in nature and position very similar to that of Châteaugay; but its height above the plain is greater, and it has suffered more from waste, apparently in consequence of its superior antiquity. It has a gradual slope from the neighbourhood of the granitic escarpment towards the centre of the Limagne, and appears to have flowed as a lava-current from the heights which rise on the north-west of Durtol: perhaps it may have been once connected with those remnants of basalt which rest upon the, granite in the vicinity of the Puy Channat, but which have lost all traces of scoriæ or cellular parts. 74. Plateau de Prudelle.-A mass of basalt, which crowns a granite promontory impending over the valleys of Villar on the |