326 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



the southwest base of Iron Hill [27p]. The specimen is no longer fresh, but it is not 

 in an advanced stage of decomposition. It was taken as a representative of the 

 main sheet near Leadville. 



The specific gravity of II was taken at 1G C. By special test in the White 

 Porphyry a very small amount of lead was found, = 0.003 per cent, of PbO (Part II, 

 Chap. VI). No CO 2 was found in I; that in II, taken together with the increased 

 percentage of CaO, indicates the presence of calcite, which is probably an infiltration 

 product, as there are dolomite bodies in the neighborhood. The close agreement of 

 these analyses is such as might have been expected from the preceding descriptions 

 and confirms the views expressed as to the close relationship of the two rocks. 



PYBITIFEEOUS PORPHYRY. 



This porphyry, so called on account of the remarkable amount of pyrite invari- 

 ably found disseminated through its mass, owes its importance principally to its sup- 

 posed connection with the ore deposits of Leadville. 



Its geographical extent is limited to the district shown upon the map of Leadville 

 and vicinity, where it seems to occupy a stratigraphical position, which to the north 

 is filled by the Gray and to the east by the Sacramento Porphyry. From the latter it 

 is distinguished in field appearance by its almost universally decomposed condition, 

 and in its constituents by a relatively small proportion of plagioclase ; from the for- 

 mer, in addition, by the absence of large crystals of orthoclase, and from both by the 

 want of hornblende. 



As a type, will be taken the unusually fresh rock occurring in White's gulch 

 between the Printer Girl and Golden Edge claims [87]. It has a distinct porphy- 

 ritic structure, showing numerous white feldspar crystals, with quartz, biotite, and 

 pyrite as other recognizable constituents. Altered feldspars are nearly indistinguish- 



