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.]