388 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The wedge-shaped block of ground between the Emmet and Iron 

 faults may be considered a portion of the formation which, by compression 

 between the adjoining blocks, has been lifted up relatively and compressed 

 into an anticlinal fold. Actual outcrops of Blue Limestone are found near 

 the bed of California gulch, opposite the Globe shaft. The Columbia tunnel 

 was run in apparently on the very crest of the fold and developed consid- 

 erable ore on the contact. From the line of the tunnel the formation dips 

 gently to the eastward and very steeply to the westward, so that in the 

 Crescentia shaft,, a little west of it on the slopes of California gulch, at a 

 depth of 335 feet the limestone had not yet been reached, but the shaft was 

 in the Gray Porphyry beneath the White. 1 Section G, Atlas Sheet XXV, 

 represents graphically the structure thus described. 



Ore deposits. The principal deposition of ore has taken place along the 

 contact-plane between the Blue Limestone and overlying White Porphyry, 

 and extended to greater or less depth into the mass of the limestone. In 

 several instances large deposits have been formed within the body of the 

 limestone, being probably on the line of some natural cleavage or joint 

 plane which caused a deviation of the ore currents from their normal 

 course. 



The vein material or gangue consists of hydrated oxides of iron and 

 manganese, silica, and clay. The iron varies from a hard, compact, more or 

 less silicious brown hematite to a simple coloring matter of the clay. Man- 

 ganese is found sometimes in fine, needle-like crystals of pyrolusite, but 

 mainly occurs as a sort of wad, a black clayey mass known to the miners 

 as " black iron." Silica occurs either as a blue-black chert or as a granular, 

 somewhat porous mass, hardly distinguishable from quartzite. Clay is 

 found in greatly varying degrees of impurity, from a white kaolin down, 

 and is a product of the decomposition of porphyry. It occurs either in 

 place or as an infiltrated mass. Besides this should be mentioned the 

 Chinese talc of the miners, found mainly at the actual contact. 



The ore is principally argentiferous galena and its secondary products 

 are carbonate of lead, or cerussite, and chloride of silver. As accessory 



1 Late developments in the lower Garden City shaft show that the Blue Limestone is considerably 

 mineralized and that the formation dips very steeply to the southwest. 



