We shall, however, probably be within the limits of truth in stating
that the number of great habitual volcanic vents upon the globe which
we have reason to believe are still in active condition, is somewhere
between 300 and 350. Most of these are marked by more or less
considerable mountains, composed of the materials ejected from them.
But if we include mountains which exhibit the external conical form,
crater-like hollows, and other features of volcanoes, yet concerning the
activity of which we have no record or tradition, the number will fall
little, if anything, short of 1,000.
The mountains composed of volcanic materials, but which have lost
through denudation the external form of volcanoes, are still more
numerous, and the smaller temporary openings which are usually
subordinate to the habitual vents that have been active during the
periods covered by history and tradition, must be numbered by thousands.
There are still feebler manifestations of the volcanic forces--such as
steam-jets, geysers, thermal and mineral waters, spouting saline and
muddy springs, and mud volcanoes--that may be reckoned by millions.
It is not improbable that these less powerful manifestations of the
volcanic forces to a great extent make up in number what they want in
individual energy; and the relief which they afford to the imprisoned
activities within the earth's crust may be almost equal to that which
results from the occasional outbursts at the great habitual volcanic
vents.
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