xiii Action of Water on the Land 241 



supply may often be obtained even in rainless deserts, as 

 the deep layer of permeable rock may come to the surface 

 at a great distance in a rainy region (Fig. 50). 



315. Thermal and Mineral Springs. When the dip of 

 the permeable strata carries them far down into the Earth's 

 crust the water is greatly heated ( 291), and if it is brought 

 back to the surface its high temperature entitles the outflow 

 to the name of a thermal spring. Hot springs also abound 

 in volcanic regions and along the slopes of recently up- 

 heaved mountains, in which cases they are not necessarily 

 deep-seated ( 289). Hot water dissolves much more of 

 the rock substance than cold, and if it has traversed beds of 

 very soluble salts, such as the sulphates, carbonates, or 

 chlorides of the alkali metals or magnesium, it rises to the 

 surface as a mineral spring, often possessed of valuable 

 medicinal properties. When charged with carbonate of 

 lime, dissolved in the presence of carbonic acid under pres- 

 sure, the heated water on evaporating at the outlet deposits 

 carbonate of lime in large quantities. Calcareous deposits 

 from such springs often clothe whole hillsides with fantastic 

 sheets of rock, which under the name of tufa or travertine 

 furnish one of the most valuable building-stones in Italy. 



316. Geysers. Very hot water under high pressure 

 decomposes the silicates in granite and similar rocks, dis- 

 solving large quantities of silica, which are deposited as a 

 crust, termed siliceous sinter, when the heated water evapo- 

 rates on the surface. Some of the most fairylike scenery 

 in the world has been formed by such deposits of silica in 

 New Zealand, where the dazzling pink and white terraces 

 near Lake Tarawera were famous show- places until they 

 were destroyed by an earthquake in 1886. Many hot 

 springs depositing silica show the characteristic action of 

 geysers an Icelandic name expressive of the violent and 

 explosive gushes of steam and boiling water which alternate 

 with periods of quietness. At the bottom of the shaft of a 

 geyser the temperature is far above 212, but the water is 

 kept from boiling by the pressure of the column above, and 

 the uppermost layer is cooled by the air below the boiling- 

 point. After a time the surface water gets sufficiently 



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