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 and other
vessels, have shown that the bottom of the deepest portion of the ocean,
far away from the land, is covered with volcanic materials which have
been carried through the air or have floated on the surface of the
ocean.
Fragments of scoria or pumice may be thrown hundreds or thousands of
feet into the atmosphere, those that fall into the crater and are flung
up again being gradually reduced in size by friction.
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