236 GEOLOGY. 



stone Park contain nearly three grams (2.8733) of mineral matter per 

 kilogram/ 



The composition of various spring and well waters is shown in the 

 accompanying table, which gives some idea of the range of mineral sub- 

 stances commonly in solution in ground-water. 



Geysers. — Geysers are intermittently eruptive hot springs. Thej^ 

 occur only in volcanic regions (past or present) and in but few of them. 

 Active geysers are virtually confined to the Yellowstone Park and 

 Iceland, though they formerly existed at other places. Those of New 

 Zealand have but recently become extinct. The great geyser region 

 of the world is the Yellowstone Park, where there are said to be more 

 than sixty active geysers. 



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

 until, at some unknown depth, it comes into contact with rock suffi- 

 ciently hot to boil it. The source of the heat is not open to inspec- 

 tion, but it is believed to be the uncooled part of an extrusive lava flow, 

 or of an intrusive lava mass. From what was said on pp. 216 and 217 it is 

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

 to the zone of great heat, where the 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. Water 

 may enter the tube from aU sides and at various levels from top to 

 bottom. The heating may precede or follow its entrance into the 

 tube, or both. So far as the water is heated after it enters the tube, 

 the point of most rapid heating may be at the bottom of the tube or 

 at some point above. If the temperature of the source of heat were 

 high enough to convert the descending water into steam as fast as it 

 enters the tube, the steam would escape continuously, though there 

 would be no geyser; but if the rock is only hot enough to bring the 

 water to the boiling-point after some lapse of time, and after some 

 water has accumulated, an eruption is possible. 



The exact sequence of events which leads up to an eruption is not 

 known, but a definite conception of the principles involved may j^erhaps 

 be secured by a definite case. Suppose a geyser-tube filled with water, 

 and heated at its lower end. As the water is heated below, con- 



1 Gooch and Whitfield. Bull. 47, U. S, Geol. Surv. 



