94 ECONOMIC GEOLOGY 



ville porphyry with a regular and well-defined upper limit to the 

 ore body. The ore occurs in the blue limestone of Carboniferous 

 age. The lower surface of the ore body is irregular and often ill 

 defined. It sometimes occupies the entire thickness of the lime- 

 stone formation. The ore sometimes occurs near the contact of 

 the gray porphyries with the blue limestone, sometimes in both 

 the calcareous and siliceous beds, sometimes in the porphyries 

 themselves either near contact surfaces or along joint and fault 

 planes. As a rule the argentiferous lead ores occur in the blue 

 magnesian limestone while the auriferous pyrites and the copper 

 ores are more frequently found in the quartzites and porphyries. 



Leadville Minerals: Native gold in flakes or leaflets; the silver 

 minerals are argentiferous galenite, cerargyrite, embolite and 

 native silver; the lead minerals are galenite, cerussite, anglesite, 

 massicot minium, and wulfenite; the accessory minerals are 

 sphalerite, calamine, stibnite realgar, bismuthinite, malachite, 

 chrysocolla, wulfenite, a vanadate of lead and zinc, pyrite, and 

 hydrous and anhydrous oxides of iron. The gangue minerals 

 are quartz, pyrite, siderite, barite, gypsum and hydrous silicates 

 of aluminum. 



Origion of the Ores: According to S. F. Emmons, the ores 

 were derived from a descending aqueous solution. The ores de- 

 rived their metallic content from the neighboring eruptive rocks. 

 Mr. Emmons further contends that the metals must have been 

 formed beneath a thickness of at least 10,000 ft. of superincum- 

 bent rocks and an unknown amount of sea water; that if they had 

 been deposited from hot ascending solutions as the result of the 

 relief of pressure it would naturally be expected that the bulk 

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

 mass of rocks where the pressure was the least, rather than at the 

 base; that at the time of deposition the sedimentary beds were 

 horizontal and relatively undisturbed; that if the deposits had 

 been made from ascending currents the process of deposition 

 would have acted from the bottom 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; that in the region of 

 the greatest ore development there is a noticable absence of 

 channels extending downward through which ascending solu- 

 tions might have come; that the vast majority of irruptive bodies 

 are in the form of horizontal sheets parallel with the stratifica- 

 tion; and that the few approximately vertical bodies afford no 



