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 Dome, 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 Dome, and ever memorable as the birthplace
of Blaise Pascal.[2]

[Illustration: Fig. 17.--Generalised Section through the Puy de Dome and
Vale of Clermont, distance about ten miles. The general floor formed of
granite and gneiss (G); D. Domite-lava of the Puy de Dome; Sc. Cones of
ashes and scoriae; 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, Lozere, and Ardeche.
On this platform are planted the whole of the volcanic mountains. (See
Fig. 17.)

The granitic plateau is bounded on the east, throughout a distance of
about 50 miles, by the wide and fertile plain of Clermont, watered by
the Allier and its numerous branches descending from the volcanic
mountains, and is about 25 miles in width from east to west in the
parallel of Clermont, but gradually narrowing in a southerly direction,
till at Brioude it becomes an ordinary mountain ravine. The eastern
margin of the plain is formed by another granitic ridge expanding into a
plateau towards the south, and joining in with that already described;
bu