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TERRESTRIAL DEPOSITS 



mediately escapes, and more or less of the CaC0 3 is deposited as 

 travertine in the neighbourhood of the spring, often in masses of 

 considerable extent and thickness. The process is not always 

 entirely chemical. The beautiful calcareous terrace formed by 

 the Mammoth Hot Springs, in the Yellowstone Park, is, in part 

 at least, due to the separation of the lime salt from the water by a 

 jelly-like plant, which grows in the hot water and is spread in 



Fig. 49. — Travertine terrace of the Mammoth Hot Springs, Yellowstone Park. 



bright coloured layers over the surface of the terrace. The parts 

 of the terrace where deposition is no longer in progress, can be at 

 once distinguished by their white colour. 



Siliceous deposits are much less common than the calcareous, 

 because of the rare conditions under which silica is dissolved 

 in any considerable quantity, hot solutions of alkaline carbon- 

 ates being necessary for this purpose. In the Yellowstone Park, 

 especially on the Plrehole River, are great terraces and flats of 

 hard white siliceous sinter, or geyserite, which have been formed 

 and are still being added to by the innumerable hot springs and 



