rsou are composed,
may have had their origin in the granite itself, melted and rendered
viscous by intense heat. Dr. E. Gordon Hull has suggested that the
domite hills (owing to their low specific gravity) may have filled up
pre-existing craters of ashes and scoriae without rupturing them, as in
the case of the heavier basaltic lavas, and then still continuing to be
extruded, may have entirely enveloped them in its mass; so that each
domite hill encloses within its interior a crater formed of ashes,
stones, and scoriae. In the case of the Puy de Dome there is some
evidence that the domite matter rests on a basis of ashes and scoriae,
which may be seen in a few places around the base of the cone. It is
difficult without some such theory as this to explain how a viscous mass
was able to raise mountains some 2000 or 3000 feet above the surrounding
plain.[13]

(_j._) _Sketch of the Volcanic History of Central France._--It now only
remains to give a brief _resume_ of the volcanic history of this region
as it may be gathered from the relations of the rocks and strata to the
volcanic products, and of these latter to each other.

_1st Stage._--It would appear that at the close of the Eocene period
great terrestrial changes occurred. The bed of the sea was converted
into dry land, the strata were flexured and denuded, and a depression
was formed in the granitic floor of Central France, which, in the
succeeding Miocene period, was converted into an extensive lake peopled
by molluscs, fishes, reptiles, and pachyderms of the period.

_2nd Stage._--Towards the close of the Miocene epoch volcanic eruptions
commenced on a grand scale over the granitic platform in the districts
now called Mont Dore, Cantal, and the Vivarais. Vast sheets of trachytic
and basaltic lavas successively invaded the tracts surrounding the
central orifices of eruption, now constituting the more ancient of the
lava-sheets of the Auvergne region, and, invading the waters of the
neighbouring lake, overspread the lacustrine deposits which were being
accumulated therein. These volcanic eruptions probably continued
throughout the Pliocene period, interrupted by occasional intervals of
inactivity, and ultimately altogether ceased.

_3rd Stage._--Towards the close of the Pliocene period terrestrial
movements took place, owing to which the waters of the lake began to
fall away, and the sheets of lava were subjected to great denudation.
This process, probably accele