VOLCANIC PRODUCTS. 351 



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 lique- 



Fig. 2. Minute Cavities, containing Liquids in the Crystals of Rocks. 



fled carbonic acid. The presence of these liquids under such circum- 

 stances shows that the crystals have been formed under an enormous 

 pressure. 



The surface of fluid and semi-fluid lavas is covered with vast quan- 

 tities 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- 



