Forked lightning flashes from the
column of cinders, and it is then easy to distinguish (as at the close of
the eruption of Mount Vesuvius, in the latter end of October, 1822) the
rolling thunder of the volcanic storm from the detonations in the interior
of the mountain. the flashes of lightning that darted from the volcanic
cloud of steam, as we learn from Olafsen's report, killed eleven horses and
two men, on the eruption of the volcano of Katlagia, in Iceland, on the 17th
of October, 1755.
Having thus delineated the structure and dynamic activity of volcanoes, it
now remains for us to throw a glance at the differences existing in their
material products. The subterranean forces sever old combinations of matter
in order to produce new ones, and they also continue to act upon matter as
long as it is in a state of liquefaction from heat, and capable of being
displaced. The greater or less pressure under which merely softened or
wholly liquid fluids are solidified, appears to constitute the main
difference in the formation of Plutonic and volcanic rocks. The mineral
mass which flows in narrow, elongated streams from a volcanic opening (an
earth-spring), is called lava. where many such currents meet and are
arrested in their course, they expand in width, filling large basins, in
which they become solidified in superimposed strata.
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