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330 V O L C A N I C T U F F A .
upper than the lower part o f the hill ; but as
the strata have evidently been much disturbed
since their original formation, their
intermixture with the limestone may admit
of a satisfactory explanation. I f the harder
bed, which resembles a dyke, was formed
by subterranean fire, filling up a fissure in
tbe hill, it might also force up portions of
the subjacent limestone.
Whatever opinion we adopt, the present
position of the beds in the Puy de Cruelle
incontestibly proves that they have been
subjected to some more powerful agency
than the gradual erosion o f water, and we
gain a knowledge of one important fact,
namely, that after the more ancient volcanic
rocks of this district were formed,
they have been more or less changed in position
; a change which probably took place
at the epoch that immediately preceded
the eruption of the more recent volcanoes.
The labour of fifty men, employed in clear-
in away the debris in some o f these mountains,
and ascertaining the exact line of
junction of the granite, and the older volcanic
rocks, and also of the limestone with
these rocks, would do more to decide the
doubtful points in the geology of Auvergne,
V O L C A N IC T U F F A . 331
than volumes o f the most elaborate reasoning
.O
In the Puy de Cruelle, the bed of volcanic
tuffa is not covered by basalt as at
Montadoux, Gergovia, Canturges, and other
mountains round the plain o f Clermont; this
constitutes the principal difference between
them. It is also more completely insulated :
the geological position of this hill is represented
by that of the lowest mountain on
the right hand in the section (page 298).
The volcanic tuffa intermixed with bitumen
is in some parts of Auvergne more than
three hundred feet in depth, and bears the
evident marks of being the product of an
aqueous or muddy eruption, intermixed
with an eruption of lava and scoriæ, which
increased and covered the tuffa with compact
lava or basalt. Where I had the opportunity
of examining the tuffa near its
junction with basalt, I observed that the
quantity of the basaltic masses in the tuffa
increased as it approached the basalt ; until
the whole formed a mass of compact basalt,
but more or less intermixed with scoriæ.
The earthy base of the tiiffa, and its intermixture
with bitumen, indicate that it was
ejected in a state of mud, and that the_
hil - X