crystalline nature of the rock
composing them, which has been formed by lava cooling slowly under great
pressure.

It has been suggested that the frequent rending of volcanic cones during
eruptions may be connected with the gradual and successive upheaval of
the whole mass in such a manner as to increase the inclination of the
beds composing the cone; and in accordance with the hypothesis before
proposed for the origin of Monte Nuovo, Von Buch supposes that the
present cone of Vesuvius was formed in the year 79, not by eruption, but
by upheaval. It was not produced by the repeated superposition of scoriae
and lava cast out or flowing from a central source, but by the uplifting
of strata previously horizontal. The entire cone rose at once, such as
we now see it, from the interior and middle of Somma, and has since
received no accession of height, but, on the contrary, has ever since
been diminishing in elevation.[529]

Although I consider this hypothesis of Von Buch to be quite untenable, I
may mention some facts which may at first sight seem to favor it. These
are recorded by M. Abich in his account of the Vesuvian eruptions of
1833 and 1834, a work illustrated by excellent engravings of the
volcanic phenomena which he witnessed.[530] It appears that, in the year
1834, the great crater of Vesuvius had been filled up nearly to the top
with lava, which had consolidated and formed a level and unbroken plain,
except that a small cone thrown up by the ejection of scoriae rose in the
middle of it like an island in a lake. At length this plain of lava was
broken by a fissure which passed from N. E. to S. W., and along this
line a great number of minute cones emitting vapor were formed. The
first act of formation of these minor cones is said to have consisted of
a partial upheaval of beds of lava previously horizontal, and which had
been rendered flexible by the heat and tension of elastic fluids, which,
rising from below, escaped from the centre of each new monticule. There
would be considerable analogy between this mode of origin and that
ascribed by Von Buch to Vesuvius and Somma, if the dimensions of the
upraised masses were not on so different a scale, and if it was safe to
reason from the inflation of bladders of half-fused lava, from fifteen
to twenty-five feet in height, to mountains attaining an altitude of
several thousand feet, and having their component strata strengthened by
intersecting dikes of solid lava.

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