Sometimes, indeed, this process may be
retarded for an indefinite period by the accession of fresh supplies of heat;
for we find that the lava in the crater of Stromboli, one of the Lipari Islands,
has been in a state of constant ebullition for the last two thousand years; and
we may suppose this fluid mass to communicate with some caldron or reservoir of
fused matter below. In the Isle of Bourbon, also, where there has been an
emission of lava once in every two years for a long period, the lava below can
scarcely fail to have been permanently in a state of liquefaction. If then it be
a reasonable conjecture, that about 2000 volcanic eruptions occur in the course
of every century, either above the waters of the sea or beneath them (Ibid.
Volcanic Eruptions.), it will follow that the quantity of Plutonic rock
generated or in progress during the Recent epoch must already have been
considerable.
But as the Plutonic rocks originate at some depth in the earth's crust, they can
only be rendered accessible to human observation by subsequent upheaval and
denudation. Between the period when a Plutonic rock crystallises in the
subterranean regions and the era of its protrusion at any single point of the
surface, one or two geological periods must usually intervene.
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