latter groups might be adduced from the extent of the erosion which
has taken place in the sheets of lava having their sources in the vents
of the Plomb du Cantal and of Mont Dore, owing to which, magnificent
valleys, many miles in length and hundreds of feet in depth, have been
cut out of these sheets of lava and their supporting rocks, whether
granitic or lacustrine, and the materials carried away by the streams
which flow along their beds. These points will be better understood when
I come to give an account of the several groups; and in doing so I will
commence with that of the Cantal.[6]

(_d._) _The Volcanoes of the Cantal._--The original crater-cones of
this group have entirely disappeared throughout the long ages which have
elapsed since the lava-streams issued forth from their internal
reservoirs. The general figure of this group of volcanic mountains is
that of a depressed cone, whose sides slope away in all directions from
the central heights, which are deeply eroded by streams rising near the
apex and flowing downwards in all directions towards the circumference
of the mountain, where they enter the Lot, the Dordogne, and the Allier.
The orifice of eruption was situated at the Plomb du Cantal, formed of
solid masses of trachyte, which, owing, as Mr. Scrope supposes, to a
high degree of fluidity, were able to extend to great distances in
extensive sheets, and were afterwards covered by repeated and
widely-spread flows of basalt; so that the trachyte towards the margin
of the volcanic area becomes less conspicuous than the basalt by which
it is more or less concealed from view, or overlapped. Extensive beds of
tuff and breccia accompany the trachytic masses.

Magnificent sections of the rocks are laid open to view along the sides
of the valleys, which are steep and rock-bound. Except towards the
south-west, about Aurillac, where lacustrine strata overlie the granite,
the platform from which rises the volcanic dome is composed of granitic
or gneissose rocks. Accompanying the lava-streams are great beds of
volcanic agglomerate, which Mr. Scrope considers to have been formed
contemporaneously with the lava which they envelop, and to be due to
torrents of water tumultuously descending the sides of the volcano at
periods of eruption, and bearing down immense volumes of its fragmental
_ejecta_ in company with its lava-streams.[7] Nowhere throughout this
region do beds of trachyte and basalt alternate with one anoth