descended into the old river channels which drained
the granitic plateau. Thus the current which took its origin in the Puy
Gros descended into the valley of the Dordogne, while another stream
invaded the gorge of Champeix on the eastern side.

The more ancient lava-streams just described are invaded by currents and
surmounted by cones of eruption of more recent date, similar to those of
the Puy de Dome group lying to the northward. Such cones and currents,
amongst which are the Puy de Tartaret and that of Montenard, present
exactly the same characters as those of this group, to which we shall
return further on.

(_f._) _Volcanoes of the Haute Loire and Ardeche._--Separated by the
valley of the Allier and the granitic ridge of La Margeride from the
volcanic regions of Cantal and Mont Dore is another volcanic region of
great extent, which reaches its highest elevation in the central points
of Mont Mezen, attaining an elevation (according to Cordier) of 5820
feet, and formed of "clinkstone." The volcanic products of Mezen have
been erupted from one central orifice of vast size, and consist mainly
of extensive sheets of "clinkstone," a variety of trachytic lava, which
have taken courses mainly towards the north-west and south-east. These
great sheets, one of which appears to have covered a space more than 26
miles in length with an average breadth of 6 miles, thus overspreading
an estimated area of 156 square miles, has been deeply eroded by streams
draining into the Loire, along whose banks the rocks tower in lofty
cliffs; while it has also suffered enormous denudation, by which
outlying fragments are disconnected from the main mass, and form
flat-topped hills and plateaux as far distant as Roche en Reigner and
Beauzac, at the extreme distance (as stated above) of 26 miles from the
source of eruption.

But even more remarkable than the above are the vast basaltic sheets
which stretch away for a distance of 30 miles by Privas almost to the
banks of the Rhone, opposite Montlimart. These have their origin amongst
the clinkstone heights of Mont Mezen, and taking their course along the
granitic plateau in a south-easterly direction, ultimately pass over on
to the Jurassic and Cretaceous formations composing the plateau of the
Coiron, which break off in vertical cliffs from 300 to 400 feet in
height, surmounting the slopes that rise from the banks of the Ardeche
and Escourtais rivers near Villeneuve de Bere. This is probably