6 ECONOMIC GEOLOGY 



maximum in value where the combined efforts of ascending 

 and descending solutions are the greatest; third, a decrease 

 in value with the greater depths. 



In the preceding discussion the enrichment of ore bodies has 

 been effected mainly through waters of meteoric origin. It 

 must not be forgotten that solutions which percolate through 

 fissures and enrich ore bodies are often of magmatic origin, 

 that is, they are derived directly from masses of igneous 

 material. 



In many cases these solutions have materially changed the 



FIG. 2. Section across a vein in the Hillside mine, Yavapai County, 

 Arizona, showing the ore scattered through clay. (After Richard.) 



nature and value of mineral deposits previously existing within 

 the fissure. 



Mineral Springs. Mineral springs as effecting ore deposits 

 may be divided into three distinct classes. 



(1) Carbonated Waters. Water charged with carbon dioxide 

 becomes a potent solvent for rock constituents. As the pressure 

 is lessened this supersaturated solution is relieved of a part of 

 its burden. The numerous varieties of travertine, as calcareous 

 tufa and Mexican onyx, are illustrations of this type of material. 

 Luray Caverns in Virginia, Mammoth Cave in Kentucky, and 

 Limestone Cave in Austria illustrate the solvent power of 

 carbonated waters. 



