A geyser is a natural fountain which spouts a column of steam and hot water at intervals. Geysers are connected with volcanoes, and represent a stage when volcanic activity is declining. A geyser consists of a shaft or fissure sufficiently deep to communicate with hot water and vapors beneath the ground. In the part of the shaft the temperature is considerably above that at which water boils, but the water is kept from boiling by the pressure of the column above, whose uppermost layer, in contact with the air, is cooled below boiling point. Periodically the surface water is sufficiently heated from beneath to boil; the pressure on the layers beneath is relieved, and the superheated water changes to steam, ejecting the entire upper column of boiling water and steam with great violence. A geyser in the Yellowstone National Park, throws up a column of water and steam 250 ft. high; then follow several weeks of quiet. Another geyser, 'Oíd Faithful,' throws a column of 150 ft. at regular intervals of about an hour.
