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.

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.