(_a._) _General Structure of the Auvergne District._--From a granitic and gneissose platform situated near the centre of France, and separated from the western spurs of the Alps by the wide valley of the Rhone, there rises a group of volcanic mountains surpassing in variety of form and structure any similar mountain group in Europe, and belonging to an epoch ranging from the Middle Tertiary down almost to the present day. This volcanic group of mountains gives rise to several important rivers, such as the Loire, the Allier, the Soule (a branch of the Loire), the Creuse, the Dordogne, and the Lot; and in the Plomb du Cantal attains an elevation of 6130 feet above the sea. Its southern section, that of Mont Dore, the Cantal, and the Haute Loire, is characterised by magnificent valleys, traversing plateaux of volcanic lava, and exhibiting the results of river erosion on a grand scale; while its northern section, that of the Puy de Dôme, presents to us a varied succession of volcanic crater-cones and domes, with their extruded lava-streams, almost as fresh and unchanged in form as if they had only yesterday become extinct. A somewhat similar, but less important, chain of extinct volcanoes also occurs in the Velay and Vivarais, between the upper waters of the Loire and the Allier, in the vicinity of the town of Le Puy.[1] The principal city in this region is Clermont-Ferrand, lying near the base of the Puy de Dôme, and ever memorable as the birthplace of Blaise Pascal.[2] [Illustration: Fig. 17.--Generalised Section through the Puy de Dôme and Vale of Clermont, distance about ten miles. The general floor formed of granite and gneiss (G); D. Domite-lava of the Puy de Dôme; Sc. Cones of ashes and scoriæ; L. Lava-sheets; A. Alluvium of the Vale of Clermont and Lake deposits.] The physical structure of this region is on the whole very simple. The fundamental rocks consist of granite and gneiss passing into schist, all of extreme geological antiquity, forming a vast platform gradually rising in a southerly direction towards the head waters of the Loire and the Allier in the Departments of Haute Loire, Lozère, and Ardèche. On this platform are planted the whole of the volcanic mountains. (See Fig. 17.)