ases from the molten rock, but the lava being in a highly liquid
state, the steam and gases dissolved in it escape quietly and without
explosions. If, however, the molten rock is less completely fluid, or
in a viscous condition, the vapors and gases contained in it find
difficulty in escaping, and may be retained until, becoming concentrated
in large volume, they break their way to the surface, producing violent
explosions. Volcanoes in which the lava extruded is viscous, and the
escape of steam and gases is retarded until the pent-up energy bursts
all bounds, are of the explosive, type. One characteristic example is
Vesuvius.

"When steam escapes from the summit of a volcanic conduit--which, in
plain terms, is a tall vessel filled with intensely hot and more or less
viscous liquid--masses of the liquid rock are blown into the air, and on
falling build up a rim or crater about the place of discharge. Commonly
the lava in the summit portion of a conduit becomes chilled and perhaps
hardened, and when a steam explosion occurs this crust is shattered and
the fragments hurled into the air and contributed to the building of the
walls of the inclosing crater.

"The solid rock blown out by volcanoes consists usually of highly
vesicular material which hardened on the surface of the column of lava
within a conduit and was shattered by explosions beneath it. These
fragments vary in size from dust particles up to masses several feet in
diameter, and during violent eruptions are hurled miles high. The larger
fragments commonly fall near their place of origin, and usually furnish
the principal part of the material of which craters are built, but the
gravel-like kernels, lapilli, may be carried laterally several miles
if a wind is blowing, while the dust is frequently showered down on
thousands of square miles of land and sea. The solid and usually angular
fragments manufactured in this manner vary in temperature, and may still
be red hot on falling.

"Volcanoes of the explosive type not uncommonly discharge streams of
lava, which may flow many miles. In certain instances these outwellings
of liquid rock occur after severe earthquakes and violent explosions,
and may have all the characteristics of quiet eruptions. There is
thus no fundamental difference between the two types into which it is
convenient to divide volcanoes."

MOUNTAINS BLOW THEIR HEADS OFF

"In extreme examples of explosive volcanoes, the summit portion of a