

SPRINGS AND GEYSERS 49 



mineral and common. Mineral springs, in the popular sense of the 

 term, are of two types: (i) Those which contain an unusual amount 

 of mineral matter, and (2) those which contain some unusual min- 

 eral. All springs which are not mineral are common. This classi- 

 fication is not very significant, for all springs contain more or less 

 mineral matter, and many springs which are "common" contain 

 more mineral matter than some which are "mineral." Mineral 

 springs are themselves classified according to the kind and amount 

 of mineral matter they contain. Thus saline springs contain salt; 

 sulphur springs contain compounds (especially gaseous) of sulphur; 

 calcareous springs contain abundant lime carbonate, etc. Medicinal 

 springs are those which contain some substance which has, or is 

 supposed to have, curative properties. 



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. 



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

 down until, at some unknown depth, it comes in 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. 

 Water may enter the tube from all sides and at various levels. 

 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 water were converted into steam as fast 

 as it enters the tube, steam would escape continuously, and there 

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

 water in the tube to the .boiling-point after some lapse of time, 

 and after a good deal of water has accumulated, an eruption is 

 possible. 



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

 known, but a definite conception of the principles involved may 



