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prevented the ignition of the bituminous
matter, while the intermixture of
scoriæ and irregular masses of basalt, which
appears to have congealed from a state
of fusion, indicate that igneous eruptions
were ejected at the same time ; and as the
lava covers the tuffa, we may hence conclude
that the igneous eruptions were continued
a considerable time after the muddy
eruptions had ceased.
That the beds of tuffa, as well as the
other rocks of Auvergne, have in many
parts been deeply excavated by deluges or
torrents is very evident ; we cannot, therefore,
be surprised that in some situations
tuffa should be found transported from its
original position, and intermixed with
rounded fragments o f older rocks. This has
made some observers suppose, that all the
beds of tuffa were merely depositions of the
detritus of rocks that had been transported
by water, and intermixed with sand and
clay. On the contrary, there is every appearance
that the beds of tuffa or breccia
round Clermont, are original volcanic depositions.
Gergovia is about three miles south of
Clermont : it is said to rise 180 toises, or
1152 feet above the plain. It is a table
mountain nearly flat at the top, the length
from east to west at the summit rather exceeds
one mile, and its breadth from north
to south is about four or five hundred
yards. The ancient town of Gergovia, so
celebrated for its resistance to Cæsar, is
supposed to have stood near the western
end. The foundations of buildings are discovered
by digging, and antique vessels
are occasionally found. The whole of the
summit is cultivated, for though it is composed
o f basalt, it is covered with a good
vegetable soil. In the ploughed ground, I
observed fragments of pitch stone, and
rounded pebbles of greasy quartz. The
basalt of Gergovia rests on volcanic; tuffa,
which appears much decomposed where it
is exposed. In the tuffa, I observed
rounded masses of reddish compact phonolite
; but I do not consider the tuffa o f
Gergovia to be (as M. Montlosier supposes)
an alluvial bed of sediment and water-worn
fragments, deposited at the bottom o f an
ancient valley. Its position is at least five
hundred feet above the vale of Clermont,
and it appears to rest on fresh water limestone,
and marl.