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300 G k O L O G Y O F A U V E R G N E .
currents of scoriaceous and cellular lava,
G. G, may be traced for several miles, descending
into the valleys.
This is an enunciation of the present
appearance, and the position of the principal
volcanic and other rocks in the vicinity of
Clermont, unconnected with any hypotheses.
Except that the relation of the
rock E to the subjacent rock H, is unknown,
as I have before mentioned; and I
do not think it absolutely certain th at the
limestone B. B extends under the volcanic
rocks that appear to rest upon it, though
the limestone may be traced nearly round
them, as at Gergovia and the Puy de
Cruelle, which I shall afterwards decribe.
It is possible that the limestone might
have been deposited, after the mountains
which seem to cover some part of it
were formed; for the calcareous strata may
have filled the valley to a certain height, and
have been subsequently excavated, leaving
a portion of limestone round the bases of
these mountains. This limestone might
have been protected by the volcanic matter
that had fallen upon it, as represented in
the figure, above the section, where th e
calcareous strata B are supposed to extend
g e o l o g y o f A U V E R G N E . 301
no farther than the mountain I, but to be
covered by tuffa at C, which had fallen
upon it. It is also possible that the mountain
I, may have been forced up through
the strata at B, and certain appearances at
the Puy de Cruelle indicate that some
disturbance of the strata has taken place.
I think it right to state these doubts respecting
the position of the limestone,
though I incline to the opinion of the
French geologists, that the limestone was
deposited prior to the formation of the
volcanic beds ; and therefore that its true
position is represented in the annexed
section. In some situations farther south,
basalt is seen covering the fresh-water limestone.
I f we admit that the matter which forms
the mountains D. D. D, C. C. C, has once
flowed as lava, we must admit also, that
the tuffa and basalt in each were once
portions of continuous beds, whatever
may be the causes by which they were separated.
I shall not stop to inquire, at
present, whether they were detached by
the gradual erosion of water, or by the
partial subsidence or elevation of the crust
of the globe. It is deserving of notice.
F J l
ÀXI
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