rock surface has become cool enough for the growth of
certain plants upon it, these crevices still yield steam. It is
evident, in a word, that a considerable part of a lava mass, even
after it escapes from the volcanic pipes, is water which is intimately
commingled with the rock, probably lying between the very finest
grains of the heated substance. Yet this lava which has come forth
from the volcano has only a portion of the water which it originally
contained; a large, perhaps the greater part, has gone forth in the
explosive way through the crater. It is reasonably believed that the
fluidity of lava is in considerable measure due to the water which it
contains, and which serves to give the mass the consistence of paste,
the partial fluidity of flour and rock grains being alike brought
about in the same manner.

So much of the phenomena of volcanoes as has been above noted is
intended to show the large part which interstitial water plays in
volcanic action. We shall now turn our attention again to the state of
the deeply buried rock water, to see how far we may be able by it to
account for these strange explosive actions. When sediments are laid
down on the sea floor the materials consist of small, irregularly
shaped fragments, which lie tumbled together in the manner of a mass
of bricks which have been shot out of a cart. Water is buried in the
plentiful interspaces between these bits of stone; as before remarked,
the amount of this construction water varies. In general, it is at
first not far from one tenth part of the materials. Besides the fluid
contained in the distinct spaces, there is a share which is held as
combined water in the intimate structure of the crystals, if such
there be in the mass. When this water is built into the stone it has
the ordinary temperature of the sea bottom. As the depositing actions
continue to work, other beds are formed on the top of that which we
are considering, and in time the layer may be buried to the depth of
many thousand feet. There are reasons to believe that on the floors of
the oceans this burial of beds containing water may have brought great
quantities of fluid to the depth of twenty miles or more below the
outer surface of the rocks.

[Illustration: Fig. 15.--Flow of lava invading a forest. A tree in the
distance is not completely burned, showing that the molten rock had
lost much of its original heat.]

The effect of deep burial is to increase the heat of strata.