550 A TREATISE ON METAMORPHISM. 



carbonate, forming hematite or limonite, its solution is very slow, and thus 

 the upward transfer of the compounds at the time of drought may more 

 than compensate for the solution at periods of humidity, and actual 

 segregation of certain 'materials at the surface take place. 



Furthermore, it is to be noted that in humid regions vegetation is 

 abundant, and therefore the upward transfer of material through the 

 agency of life is at a maximum. This compensates in part for the relative 

 inefficiency of capillarity in humid regions as compared with that in arid 

 regions. It therefore appears that whether a region be arid or humid the 

 vertical transfer of soluble material through the agency of life and by 

 capillarity takes place effectively. 



CONCENTRATION BY CIRCULATION EXTENDING INTO BELT OF CEMENTATION. 



It has been pointed out, on pages 128, 156, that waters continuous^ join 

 the belt of cementation and continuously issue from it. In general the 

 waters which issue from the belt of cementation are not saturated, but not 

 infrequently the waters of springs as they near or reach the surface pre- 

 cipitate material by lowering of the temperature, by evaporation, or by 

 chemical change. 



Perhaps one of the most common deposits thus produced is limonite. 

 The iron precipitated is usually carried underground as carbonate, but when 

 it reaches the surface, where oxygen is abundant, oxidation takes place, the 

 carbon dioxide is liberated, the ferric oxide unites with water, and limonite 

 is thrown down. Thus bog deposits of iron ore are built up. Another 

 deposit which is not infrequently produced is calcium carbonate. This may 

 be thrown down as a result of lessening pressure and temperature, or by 

 evaporation. Such deposits are known as tufa or travertine. They are 

 found in many parts of the world. One of the best illustrations is fur- 

 nished by the travertine deposits of the Yellowstone National Park, at 

 Mammoth Hot Springs and on Terrace Mountain. The latter represents 

 the deposit of springs now dead. A third class of deposits thus built up 

 by issuing spring water is siliceous sinter. These are best illustrated by 

 the deposits of geyserite in the Yellowstone National Park. It is inter- 

 esting to note that the travertine and the siliceous sinter deposits in the 

 Yellowstone Park are in the same region, and one in which the spring waters 

 are hot. To the latter fact may be attributed the magnitude of the deposits. 



