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316 M O N T A D O U X .
sion of eruptions from the same opening
must form a volcanic hill, with a crater. It
ought always to be recollected, that the
volcanic cone or mountain is not itself the
seat of subterranean heat, or, in other
words, it is not the furnace, but merely the
chimney, through which the volatile and
melted matter is ejected from an unknown
depth. Nor is it formed by the first eruption,
but by a succession o f eruptions from
the same aperture, when the rent or fissure is
nearly closed. In England we have instances
of fissures of great width and many miles
in length, filled with basalt, which cut
through the strata, but we have no decided
volcanic crater.
In our second excursion to the volcanic
mountains on the south-west of Clermont,
we first visited Montadoux, a conical mountain,
composed of volcanic tuffa, capped
by a mass of basalt. The tuffa is very variable
in quality; in some parts it is hard
and porphyritic, with crystals of felspar;
in others soft, containing balls of indurated
and fine grained b a sa lt: in some instances
these balls of basalt appear to have run
into irregular forms in a state of fusion,
among the softer materials. The tuffa
nearest the basalt contains the greatest quan-
tity of scoriaceous balls and masses, which
increase till the whole becomes basalt. This
indicates that the lower bed passes by gradation
into basalt, and that the basalt and
tuffa have a similar origin. It has been supposed
by some who have visited Auvergne,
that the tuffa is principally an alluvial
deposition o f rounded fragments of volcanic
rocks, intermixed with sand and
clay, but I think the whole is an original
product, formed by an intermixture of lava,
with an eruption of mud. The basalt is
black, and extremely hard, resembling that
of Arthur’s Seat, near Edinburgh, but
contains a larger portion o f olivine. The
basalt which covers Montadoux descends
nearly half way down on the south-east ;
but whether it is in its original position, or
has fallen from above, may be doubtful.
On a second visit to Montadoux, I examined
the basalt and basaltic balls on
the southern side : the tuffa contained
distinct crystals of felspar, and some of
the balls o f basalt were very close grained,
approaching to the quality o f dark horn-
stone.
About two miles west of Montadoux,