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VOLCANIC PRODUCTS.
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and often so saturated that free crystals of the salt appear floating in it; in other cases it is some hydrocarbon; and sometimes it is liquefied
Fig. 2.—Minute Cavities, containing Liquids in the Crystals of Rocks.
carbonic acid. The presence of these liquids under such circumstances shows that the crystals have been formed under an enormous pressure.
The surface of fluid and semi-fluid lavas is covered with vast quantities of froth or foam which has been generated by the action of the escaping steam. If the lava consists of a mass of crystals floating in a liquid magma, this froth cools into the rough, cindery-looking mate-
