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the Lipari islands, or in Iceland. By the
kindness of a friend, I received before leaving
England, a cask filled with specimens
of the lavas from all the most remarkable
eruptions of Vesuvius, and with the exception
of one mineral (leunite) peculiar to
this volcano, all the others may be identified
with the lavas, from what are considered
the more recent volcanoes in
Auvergne.
Though the evidence of the volcanic
origin of the lavas in Auvergne is as strong
and complete as it is possible to be, yet
some English writers have affected to doubt
the testimony of competent observers, because
it tended to invalidate their favourite
theories respecting the aqueous formation
of basalt. For, in many instances, currents
of undoubted lava are observed to
pass by gradation into compact basalt, resembling,
in all its characters, the basalt
so abundantly occurring in the northern
part of our own island ; hence we are compelled
to admit the former agency of subterranean
fire in Britain, though we have
no remaining vestiges of well characterised
volcanoes. There are many parts o f Au-
315
vergne also, where, though the rocks are
evidently volcanic, there is no appearance
whatever of a volcanic crater. This is
the case particularly at Mont d’Or, and
has very much perplexed geologists, but
a little reflection would have removed
the difficulty. The first breaking out of a
volcano in a new situation, is not preceded
by the formation of a crater j it is nothing
more than a fissure or rent, which, if it be
sufficiently wide, will admit the lava to
flow freely, but if it be partially choked
up, the open parts will become so many
mouths, round which the ejected matter
will accumulate. In this manner, nine
volcanic hills were formed in one line on
the side of Etna, along a fissure which became
partially choked up during an eruption.
In certain parts of Mont d’Or,
where the occurrence of scoriæ on the surface
indicates the intense action of volcanic
fire, no trace of a crater can be observed,
as Montlosier informs us, except
a projecting vitrified knoll, which is imperforated.
Here we may suppose that the
matter was ejected from a fissure which
terminated at this knoll, and that there
had been only one eruption ; for a succès