96 RUNNING WATER 



Thermal Springs are those whose temperature is notably higher 

 than that of ordinary springs in the same region, and they range 

 from a luke-warm to a boiling state. This increase of temperature 

 maybe caused in either of two ways: (i) In volcanic regions, 

 water coming into contact with uncooled masses of lava is highly 

 heated and reaches the surface as a hot spring. Of this class 

 are the innumerable thermal springs of the Yellowstone Park. 

 (2) Wherever the disposition of the rocks is such that water may 

 descend to great depths within the earth and yet return to the 

 surface by hydrostatic pressure, thermal springs appear. These 

 conditions are found only in regions where the rocks have been 

 much folded and fractured. In this case the temperature of the 

 water is raised by the interior heat of the earth, which, as we 

 have seen, increases with the depth. Springs of this class occur 

 numerously along the Appalachian Mountains, and in larger num- 

 bers and of higher temperatures they accompany the various 

 ranges of the Rocky Mountains and Sierra Nevada. 



Geysers are thermal springs which periodically erupt, throwing 

 up hot water in beautiful fountains, accompanied by clouds of 

 steam. Though of great scientific interest, geysers a$e not im- 

 portant geological agents, because of their rarity, since they occur 

 only in Iceland, the Yellowstone Park, and New Zealand. 



The destructive effects of thermal springs are principally ac- 

 complished below the surface, and have already been considered 

 under the head of underground waters. The high percentages of 

 dissolved materials which such springs usually contain are evidence 

 of the important work of rock disintegration which they perform. 



3. Rivers 



The destructive work of rivers, including in that term all surface 

 streams, is far less extensive, in the aggregate, than that of the 

 atmospheric agencies, but because the work of a stream is concen- 

 trated along its narrow course, it appears much more striking and 

 impressive. 



The chemical disintegration performed by rivers is of no great 



