QUARRIES OP VOLVIC. 355
m
nated with beds of syenite, and with a compact
greenish chloritic bed, and also with
another rock closely resembling basalt, probably
an intermixture of hornblende and
felspar. This latter rock alternated several
times with thegranite, and bore so close a
resemblance to some of the volcanic rocks
o f this district, that our guide, who made
some pretensions to be a mineralogist, pronounced
it to be lava when I showed it to
him. The granite was covered with a
stratum of black volcanic sand, which had
probably lain there undisturbed since the
period of its ejection : this volcanic sand
was covered with common sand, and vegetable
mould. Before arriving at Volvic, we
stopped to examine one of the quarries in
the lava. It is excavated under the surface,
forming a vast cavern or gallery,
which we were told extended nearly half a
league : it is about fifty yards wide, and is
lighted by apertures cut through to the day.
The width of the current of lava may be
here about three hundred yards : it fills up
the bottom of a narrow defile between rocks
of granite, and extends to a great depth, as
appears from some quarries which are lower
down than the great quarry ; the northern
side of the current is too scoriaceous to be
workable. Below Volvic, the lava enters
into the open country, and spreads out to
the breadth of three quarters of a mile. It
may be traced as far as St. Genest, where
it terminates. The spring which supplies
the town of Riom with water, rises up at
this place in an abundant stream, and is
conducted by a canal or aqueduct to Biom.
The water is very clear, having percolated
through the lava.
The surface and fissures of the lava of
Volvic are in many parts covered with
brilliant laminrn of specular iron ore, which
are larger than any I have seen in the lavas
from Vesuvius, but not so large as in the
lava of the Puy de Vache, south-west of
Clermont, or in the lavas from Stromboli.
It has been ascertained, by observations on
active volcanoes, that these metallic crystals
are formed by sublimation, the great
heat raising the metallic matter in a state
o f vapour, which condenses and crystallises
in cooling.
The day after we had been to the Puy
de Nugerre, we started early from Biom,
to visit the Puy de Chopine, passing through
Volvic, where we had engaged an old man,
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