The lava itself, as left in huge deposits upon the surface, assumes
various forms, some crystalline, others glassy. The latter is usually
found in the condition known as obsidian, ordinarily black in color,
and containing few or no crystals. It is brittle, and splits into
sharp-edged or pointed fragments, which were used by primitive peoples
for arrow-heads, knives and other cutting implements. The ancient
Mexicans used bits of it for shaving purposes, it having an edge of
razor-like sharpness. They also used it as the cutting part of their
weapons of war.
CHAPTER XX.
The Active Volcanoes of the Earth.
It is not by any means an easy task to frame an estimate of the number
of volcanoes in the world. Volcanoes vary greatly in their dimensions,
from vast mountain masses, rising to a height of nearly 25,000 feet
above sea-level, to mere molehills. They likewise exhibit every possible
stage of development and decay: while some are in a state of chronic
active eruption, others are reduced to the condition of solfataras, or
vents emitting acid vapors, and others again have fallen into a more or
less complete state of ruin through the action of denuding forces.
NUMBER OF ACTIVE VOLCANOES
Even if we confine our attention to the larger volcanoes, which merit
the name of mountains, and such of these as we have reason to believe to
be in a still active condition, our difficulties will be diminished, but
not by any means removed. Volcanoes may sink into a dormant condition
that at times endures for hundreds or even thousands of years, and
then burst forth into a state of renewed activity; and it is quite
impossible, in many cases, to distinguish between the conditions of
dormancy and extinction.
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