nsists 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. But
when the lava contains no ready-formed crystals, but consists entirely
of a glassy substance in a more or less perfect state of fusion,
the liberation of steam gives rise to the formation of the beautiful
material known as "pumice." Pumice consists of a mass of minute glass
bubbles; these bubbles do not usually, however, retain their globular
form, but are elongated in one direction through the movement of
the mass while it is still in a plastic state. The quantity of this
substance ejected is often enormous. We have seen to what a vast extent
it was thrown out from the crater of Krakatoa. During the year 1878,
masses of floating pumice were reported as existing in the vicinity of
the Solomon Isles, and covering the surface of the sea to such extent
that it took ships three days to force their way through them. Sometimes
this substance accumulates in such quantities along coasts that it is
difficult to determine the position of the shore within a mile or two,
as we may land and walk about on the great floating raft of pumice.
Recent deep-sea soundings, carried on in the Challenger a