so as to be glassy in texture externally,
these often have time to become perfectly crystalline within.

Gases and steam escaping from other similar masses may leave them
hollow, when they are termed bombs, or may pit their surfaces with
irregular bubble-cavities, when they are called scoriae or scoriaceous.
Such masses whirling through the air in a plastic state often become
more or less oblately spheroidal in form; but, as often, the explosive
force of their contained vapors shatters them into fragments, producing
quantities of the finest volcanic dust or sand. This fine dust darkens
the clouds overhanging the mountain, mixes with the condensed steam to
fall as a black mud-rain, or lava di aqua (Italian, water lava), or
is carried up to enormous heights, and then slowly diffused by upper
currents of the atmosphere. In the eruption of Vesuvius of A.D. 79, the
air was dark as midnight for twelve or fifteen miles round; the city of
Pompeii was buried beneath a deposit of dry scoriae, or ashes and dust,
and Herculaneum beneath a layer of the mud-like lava di aqua, which on
drying sets into a compact rock. Rocks formed from these fragmentary
volcanic materials are known as tuff.

VOLCANIC CONES HAVE SIMILAR CURVATURES

It is entirely of these cindery fragments heaped up with marvellous
rapidity round the orifice that the volcano itself is first formed. It
may, as in the case of Jorullo in Mexico in 1759, form a cone several
hundred feet high in less than a day. Such a cone may have a slope as
steep as 30 or 40 degrees, its incline in all cases depending simply on
the angle of repose of its materials; the inclination, that is, at which
they stop rolling. The great volcanoes of the Andes, which are formed
mainly of ash, are very steep. Owing to a general similarity in their
materials, volcanic cones in all parts of the world have very similar
curvatures; but older volcanic mountains, in which lava-streams have
broken through the cone, secondary cones have arisen, or portions
have been blown up, are more irregular in outline and more gradual in
inclination.

In size, volcanoes vary from mere mounds a few yards in diameter, such
as the salses or mud volcanoes near the Caspian, to Etna, 10,800 feet
high, with a base 30 miles in diameter; Cotopaxi, in the Andes, 18,887
feet high; or Mauna Loa, in the Sandwich Isles, 13,700 feet high; with
a base 70 miles in diameter, and two craters, one of which, Kilauea, the
largest a