GROUND-WATER 213 



of saline springs is usually extracted from beds of salt beneath the 

 surface. Lime carbonate, one of the commonest substances in 

 solution in ground-water, is dissolved from limestone, or derived 

 by chemical change from rocks containing some calcium compound. 

 Thus lime feldspars, by carbonation, give rise to lime carbonate. 

 Chalybeate waters often arise from the oxidation of iron sulphide 

 (a mineral which is common in many sedimentary rocks) and the 

 solution of the resulting sulphate. Medicinal springs are those 

 which contain some substance or substances which have, or are 

 supposed to have, curative properties. 



Mineral matter in solution. The number and variety of mineral 

 substances in spring water is very great, and the amount of solid 

 matter in solution varies widely. Some of the hot springs of the 

 Yellowstone Park contain nearly three grams (2.8733) of mineral 

 matter per kilogram. 1 The springs of Leuk (Switzerland) bring 

 to the surface annually more than 2,000 tons of calcium sulphate 

 (gypsum) in solution, and in the same time the springs of Bath 

 (England) bring up an amount of mineral matter in solution suffi- 

 cient to make a column 9 feet in diameter, and 140 feet high. 2 



Geysers. Geysers are intermittently eruptive hot springs. 

 They occur only in volcanic regions (past or present), and in but 

 few of them, being known only in the Yellowstone National Park, 

 Iceland, and New Zealand. There are said to be more than sixty 

 active geysers in the Yellowstone Park. 



The cause of the eruption is steam. The surface-water sinks 

 down until, at some unknown depth, it comes into contact with 

 rock sufficiently hot to boil it. The source of the heat is not open 

 to inspection, but it is believed to be the uncooled part of extruded 

 or intruded lava. From what was said earlier in this chapter it 

 is clear that geysers do not have their origin in water which sinks 

 down to the zone of great heat, where the downward increment of 

 heat is normal. 



The water of a geyser issues through a tube of unknown length. 

 Whether the tube is open down to the source of the heat is not deter- 

 minable, but water from such a source finds its way to the tube. 



1 Gooch and Whitfield. Bull. 47, U. S. Ge'ol. Surv. 



2 Geikie. Text-book of Geology, 3d ed., p. 367. 



