etrated throughout its mass by steam bubbles, as it
usually but not invariably is found to be.

Nor is it difficult to see such a mechanism between volcanic ducts and
fissures conveying down water, as large and open pipes, for a large part
of their depth, as shall bring down water to foci of volcanic heat,
without the power of the water flowing back except as steam and through
the crater.

Indeed, the facts known as to geysers, and those of half-drowned-out
Volcanoes such as Stromboli--whose action is intermittent just as much
as that of a geyser--show that this is not merely probable. There is,
therefore, no need for the hypothesis of those who have supposed all the
huge volumes of steam blown off from Volcanoes in eruption to come from
vesicular water pre-existent in the minute cavities of crystalline or
other rocks before their fusion into lava: a fact not proved for many
classes of rock, and for none in sufficient quantity to account for the
vast volume of steam required and for the irregularity of its issue.

It is rather to anticipate, but I may state at once that, so far as the
admission of superficial waters to the interior, and to any depth to
which fissures or dislocation can extend, I believe no valid physical or
mechanical difficulties exist, taking into account _all_ the conditions
that may come into play together.

Another set of views has been suggested and supported by various
writers, which proposes to account for the rise of lava on purely
hydrostatic principles. The solid crust, fractured into isolated
fragments by tensions due to its own contraction, is supposed to sink
into the sea of lava on which it floats; and much ingenuity has been
expended in imagining the mechanism by which, in places, the liquid
matter is supposed to rise _above_ the surface of the crust.

I have no space for discussing these views further than to assert that,
in the existing state of our globe, and even admitting a solid crust of
only 60,000 metres thick, dislocation of the crust by _tension_ is not
possible. The solid crust of our globe, as I hope we shall see further
on, is not in a state of tension, and has not been so since it was
extremely thin, a mere pellicle as compared with the liquid nucleus, but
is, on the contrary, in a state of _tangential compression_.

However tenable, in other respects, may be the volcanic theory which
rests upon the assumption of a very _thin_ crust and a universal ocean
of fused rock