82 GEOLOGY AND MINING INDUSTRY/ OF LEADVILLE. 



a broad, general way it may be said that on the eastern slope of the range 

 Lincoln Porphyry extends from the northern edge of the map to Mosquito 

 gulch, Sacramento Porphyry from Mosquito gulch to the ridge south of 

 Little Sacramento gulch, and White Porphyry from there south to the limits 

 of the map. The only point observed which showed evidence of a feeding 

 channel from below was at the head of Little Sacramento gulch. 



PYRITIFEROUS PORPHYRY. 



This rock, though an extremely important element in the geology of 

 the immediate vicinity of Leadville, does not occur outside that region and, 

 like most of the eruptive rocks in the vicinity of the great ore concentra- 

 tions, is in such a universally decomposed condition that its original constitu- 

 ents cannot be definitely determined. It is generally of a white color, with 

 grayish-green or pinkish tints, comparatively fine grained, and with no 

 traces of large crystals. In it can be distinguished small grains of white 

 feldspar, quartz, biotite which is generally altered to a chloritic substance, 

 and pyrite. The last ingredient, from which it derives its name, is found 

 abundantly scattered through the rock in crystals, often so fine as to be 

 undistinguishable by the naked eye. They occur at times within the crystals 

 of quartz and biotite, and are hence supposed to be an original constituent 

 of the rock. They are frequently concentrated along cleavage planes, 

 sometimes associated with finely disseminated crystals of galena. Pyritif- 

 erous Porphyry is readily distinguished from the White Porphyry by its 

 crystalline constituents. It differs from the Sacramento and Gray Porphy- 

 ries by a relatively small amount of plagioclase feldspar and from the 

 former by the absence of hornblende Its most strikingly distinctive feat- 

 ure is the amount of pyrites which it contains, which is estimated to con- 

 stitute, on the average, 4 per cent, of its mass. The only further constitu- 

 ents disclosed by the microscope are minute crystals of zircon. Fluid but 

 no glass inclusions are found. 



Occurrence. The Pyritiferous Porphyry,. as stated above, is confined to 

 the area of the Leadville map, and is at present principally developed on 

 Breece hill and the slopes of Ball Mountain. Its original extent previous 

 to erosion was probably much greater than at present. It is a stratigraph- 



