ey have three well marked
phases of action. The first of these is the state of permanent eruption,
as in case of the volcano of Stromboli in the Mediterranean. This state
is not a dangerous one, since the steam, escaping continually, acts as
a safety valve. The second stage is one of milder activity with an
occasional somewhat violent eruption; this is apt to be dangerous,
though not often very greatly so. The safety valve is partly out of
order. The third phase is one in which long periods of repose, sometimes
lasting for centuries, are followed by eruptions of intense energy.
These are often of extreme violence and cause widespread destruction. In
this case the safety valve has failed to work and the boiler bursts.

OFTEN REST FOR LONG TERMS OF YEARS

Such are the general features of action in the vast powers which
dwell deep beneath the surface, harmless in most parts of the earth,
frightfully perilous in others. Yet even here they often rest for long
terms of years in seeming apathy, until men gather above their lurking
places in multitudes, heedless or ignorant of the sleeping demons that
bide their time below. Their time is sure to come, after years, perhaps
after centuries. Suddenly the solid earth begins to tremble and quake;
roars as of one of the buried giants of old strike all men with dread;
then, with a fierce convulsion, a mountain is rent in twain and vast
torrents of steam, burning rock, and blinding dust are hurled far upward
into the air, to fall again and bury cities, perhaps, with all their
inhabitants in indiscriminate ruin and death.

CHAPTER XIX.

Theories of Volcanic and Earthquake Action.

Though the first formation of a volcano (Italian, vulcano, from Vulcan,
the Roman god of fire) has seldom been witnessed, it would seem that it
is marked by earthquake movements followed by the opening of a rent or
fissure; but with no such tilting up of the rocks as was once supposed
to take place. From this fissure large volumes of steam issue,
accompanied by hydrogen, nitrogen, carbon dioxide, hydrochloric
acid, and sulphur dioxide. The hydrogen, apparently derived from the
dissociation of water at a high temperature, flashes explosively into
union with atmospheric oxygen, and, having exerted its explosive force,
the steam condenses into cloud, heavy masses of which overhang the
volcano, pouring down copious rains. This naturally disturbs the
electrical condition of the atmosphere, so that th