130 TERRESTRIAL DEPOSITS 



geysers. The silica is deposited partly by the evaporation of the 

 water and partly by the action of Algce (minute plants) which 

 nourish in hot water pools. Similar deposits are found in the 

 geyser regions of Iceland and New Zealand. 



Iron deposits are formed by the springs known as chalybeate, 

 which contain the carbonate of iron (FeC0 3 ) in solution. Con- 

 tact with the air speedily converts the soluble carbonate into the 

 insoluble F 2 3 , which forms brown stains and patches on the 

 channels leading from such springs, and considerable quantities of 

 it collect in pools. Here again, organic agency may supplement 

 the chemical work, for certain diatoms extract iron from the water, 

 as other Algae extract lime and silica. 



Certain mineral springs are of importance, as indicating the 

 way in which mineral veins were formed. (See p. 265.) The 

 Sulphur Bank Springs in the Coast Range of California are an 

 especially instructive example of this activity. Below the depths 

 to which the atmospheric influences penetrate, the fissures in the 

 rocks are filled with hydrated silica, which is as soft as cheese and 

 contains more or less cinnabar (sulphide of mercury). In other 

 places the silica is hardened to chalcedony, and deposits of cin- 

 nabar mixed with iron pyrites fill up the crevices. The hot waters 

 which build up these deposits are alkaline, charged with certain 

 acids and alkaline sulphides. Near Virginia City, Nevada, hot 

 alkaline springs rise through a series of fissures, in which they have 

 deposited linings of silica, amorphous and chalcedonic, with some 

 quartz, containing minute crystals of iron pyrites and traces of 

 copper and gold. On the surface the springs have formed a thick 

 layer of geyserite. 



Phosphate Deposits are the only strictly terrestrial organic for- 

 mations which require notice. These are principally derived from 

 guano, which is the accumulated excrement of birds (in caves, of 

 bats), and contains phosphates in large quantity. In rainless 

 regions, such as the Peruvian coasts and islands, the guano may 

 accumulate to great thickness without loss of its soluble matters, 

 but in rainy districts these are largely carried away by percolating 

 waters. Should the underlying rock be a limestone, it will be 



