GEYSERS. 101 



4. PrestwicWs Theory. — If we assume the existence of a sub-crust 

 layer of liquid matter, then lateral crushing of the earth's crust, such 

 as undoubtedly occurs in mountain-making, would squeeze the liquid 

 upward into and through fissures to the surface, producing the quieter 

 lava-eruptions ; or else, coming in contact in its upward course with 

 subterranean waters, especially abundant in the fissured and cavernous 

 structure of recent lavas, would develop steam and therefore violent 

 explosive eruptions. 



Subordinate Volcanic Phenomena. 



These are hot springs, carbonated springs, solfataras, fumaroles, 

 mud-volcanoes, and geysers. They are all secondary phenomena, i. e., 

 formed by the percolation of meteoric water through hot volcanic ejec- 

 tions. Or perhaps in some cases the heat may be produced by slow 

 rock-crushing by horizontal pressure, as explained above, or else by 

 local chemical action. 



General Explanation. — Thick masses of lava outpoured from vol- 

 canoes remain hot in their interior for an incalculable time. Water 

 percolating through these acquires their heat, and comes up again as 

 hot springs; or, if in addition it contains lime, as lime-depositing 

 springs ; or, if it contains carbonic acid, as carbonated springs ; or, if 

 it contains sulphurous acid and sulphureted hydrogen, as solfataras. 

 If condensible vapors issue in abundance so as to make an appearance 

 of smoke, they are called fumaroles. If the hot water brings up with 

 it mud which accumulates about the vent, then it is a mud-spring or 

 a mud- volcano. If the heat is very great, so that violent eruption of 

 water takes place periodically, then it becomes a geyser. "We have 

 already spoken of carbonated lime-depositing springs (p. 77). We 

 shall again, under the head of the theory of mineral veins (p. 215), 

 speak of solfataras. The only one which need detain us now is geysers. 



Geysers. 



A geyser may be defined as a periodically eruptive spring. Gey- 

 sers are found only in Iceland, in the Yellowstone Park of the United 

 States, and in New Zealand. The so-called geysers of California are 

 rather fumaroles. Those of Iceland have been long studied ; we will, 

 therefore, describe these first. 



Iceland is a volcanic plateau, with a narrow marginal habitable 

 region sloping gently to the sea. The interior plateau is the seat of 

 every species of volcanic action, viz., lava-eruptions, solfataras, mud- 

 volcanoes, hot springs, and geysers. There are several hundred vents 

 of all kinds in comparatively small space, among which are many gey- 

 sers. One of these, the Great Geyser, has long attracted attention. 



Description. — The Great Geyser is a basin or pool fifty-six feet in 



