118 



ECONOMIC GEOLOGY 



massicot and minium. Native silver and embolite are both as- 

 sociated with the Leadville ores. The gangue minerals are quartz, 

 siderite, pyrite and gypsum. 



S. F. Emmons in Monograph XII of the United States Geologi- 

 cal Survey Reports states: 



" That the ores must have been formed beneath a thickness of at least 

 10,000 ft. of superincumbent rocks and an unknown amount of sea 

 water. If they had been deposited from hot ascending solutions as the 

 result of the relief of pressure, it would naturally be expected that the 



FIG. 76. Map showing approximate distribution of the principal 

 silver, lead and gold regions of Colorado. After Spurr. (By permission 

 of the Macmillan Company, from Ries' Economic Geology.) 



bulk of the deposit would have been found in the upper part of this mass 

 of rocks where the pressure was least instead of at its base. If the de- 

 posits had been made from ascending currents it would naturally be 

 expected that the process of deposition should have acted from the 

 lower surface of the beds upward, instead of from the upper surface 

 downward, as is shown in the case of the blue limestone which carries 

 the bulk of the ores. The few approximately vertical ore bodies that 

 have come under observation afford no evidence that their walls form 

 part of a channel through which the ore currents came up from below. 

 A downward current seems best to suit the facts thus far observed in the 

 Leadville deposits." 



