=Hot springs and geysers.=—Thermal springs whose temperature approaches the boiling point of water are known as _hot springs_. A _geyser_ is a hot spring which intermittently ejects a column of water and steam. Both hot springs and geysers are to be found only in volcanic regions, and appear to be connected with uncooled masses of siliceous lava. In two of the three known geyser regions, Iceland and New Zealand, the volcanoes of the neighborhood are still active, and the lavas of the Yellowstone National Park date from the quite recent geological period which immediately preceded the so-called “Ice Age.” Wherever found, geysers are in the low levels along lines of drainage where the underground water would most naturally reappear at the surface. Their water has penetrated to considerable depths below the surface, but has been chiefly heated by ascending steam or other vapors. The water journey has been chiefly made along fissures, as is shown by the cool springs which often issue near them. Though some hot springs and geysers may disappear from a district, others are found to be forming, and there is no good reason to think that geysers are rapidly dying out, as was at one time supposed. The action of a geyser was first satisfactorily explained by the great German chemist Bunsen after he had made studies of the Icelandic geysers, and the mechanics of the eruption was later strikingly illustrated in the laboratory by an artificial geyser constructed by the Irish physicist Tyndall. In many respects this action is like that of the Strombolian eruption within a cinder cone, since it is connected with the viscosity of the fluid and the resistance which this opposes to the liberation of the developing vapor. In the case of the geyser, a column of heated water stands within a vertical tube and is heated near the bottom of the column. [Illustration: FIG. 202.—Cross section of Geysir, Iceland, with simultaneously observed temperatures recorded at the left, and the boiling temperatures for the same levels at the right (after Campbell).]