Some of these solid materials are evidently
fragments of the rock-masses, through which the volcanic fissure has
been rent; these fragments have been carried upwards by the force of
the steam-blast and scattered over the sides of the volcano. But the
principal portion of the solid materials ejected from volcanic orifices
consists of matter which has been extruded from sources far beneath the
surface, in highly-heated and fluid or semi-fluid condition.
It is to these materials that the name of "lavas" is properly applied.
Lavas present a general resemblance to the slags and clinkers which
are formed in our furnaces and brick-kilns, and consist, like them, of
various stony substances which have been more or less perfectly fused.
When we come to study the chemical composition and the microscopical
structure of lavas, however, we shall find that there are many respects
in which they differ entirely from these artificial products, they
consisting chiefly of felspar, or of this substance in association with
augite or hornblende. In texture they may be stony, glassy, resin-like,
vesicular or cellular and light in weight, as in the case of pumice or
scoria.
FLOATING PUMICE
The steam and other gases rising through liquid lava are apt to produce
bubbles, yielding a surface froth or foam. This froth varies greatly
in character according to the nature of the material from which it is
formed. In the majority of cases the lavas consist of a mass of crystals
floating in a liquid magma, and the distension of such a mass by the
escape of steam from its midst gives rise to the formation of the rough
cindery-looking material to which the name of "scoria" is applied.
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