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296 G E N E R A L V I E W OF
judice, and the volcanic characters of the
rocks in Auvergne are so clearly and indelibly
impressed, that they cannot be called
in question by any one who has examined
th em ; nor can these volcanic characters
be removed, except by some future revolution
of the globe.
To enable the reader to form a distinct
idea of the country round Clermont, it may
be useful to trace the outline of the principal
formations, and their relative position,
before I proceed to describe the different
volcanic mountains which I examined.
Ranges of granitic mountains*, which
run nearly north and south, extend in
breadth from the Saone at Lyons, to the
Allier near Clermont: the granite is continued
westward into Auvergne, and forms
the substratum on which the volcanic rocks
repose.
In the neighbourhood of Clermont a series
of strata of fresh-water limestone cover
the sides of the valley, and appear in the
surrounding hills to be interposed between
the granite and the volcanic rocks. Ascending
westward, all traces of the limestone
* These ai e called the mountains of Forez.
T n E G E O L O G Y OF A U V E R G N E . 297
disappear, and the volcanic rocks rest Immediately
on an elevated granitic plain,
the general level of which may be about
1600 feet above the valley of Clermont.
A number of conical and dome-shaped
hills or mountains rise from this plain to
the further height of from 1000 to 2000
f e e t; the highest of these is the Puy de
Dome. Some of these mountains preserve
the forms of well defined craters, and currents
of lava may be traced from them for
several miles, descending into the present
valleys. Other mountains have rounded
summits, without any appearance of a
crater : they appear to be principally composed
of semi-vitrified felspar, or felspathic
granite, called by the French trachyte,
which is, however, frequently intermixed
with scoriaceous or cellular lava, or with
basalt. Besides these, there are mountains
composed of volcanic tuffa, covered with
dark basalt, which is frequently columnar,
and forms isolated caps on their summits.
These mountains are supposed to be the
remains of more ancient beds oi lava, that
have been deeply furrowed with valleys,
by the same causes that have excavated
valleys in other parts of the world.
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