76 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



MOUNT ZION PORPHYRY. 



This porphyry, when fresh and unaltered, is a gray rock resembling 

 fine-grained granite, and is made up mainly of quartz, feldspar, and mica ; 

 orthoclase being the predominant feldspar and biotite the original mica; 

 plagioclase feldspar is decidedly subordinate, and biotite but sparingly 

 developed. It is rarely found in an unaltered condition, however, and in 

 the various stages of alteration it passes through a rock in which the partly 

 decomposed biotite produces a slightly spotted appearance into a white rock 

 glistening with fine lustrous particles of muscovite which can hardly be dis- 

 tinguished from the White Porphyry. The muscovite results mainly from 

 the decomposition of the feldspar and also from that of the biotite. Larger 

 individuals of quartz and feldspar, as porphyritic ingredients, can frequently 

 be distinguished by the naked eye. Beside the above minerals the micro- 

 scope also detects zircon, magnetite, and apatite as accessory constituents 

 of the rock ; it shows, too, that the texture of the rock is quite gramilnr 

 throughout, with no amorphous material. 



Occurrence. This rock is of comparatively limited development, being 

 found thus far only on Mount Zion and on Prospect Mountain. It is gen- 

 erally in a less altered and therefore more typical condition on Mount Zion, 

 for which reason it has received that name; but the most entirely unaltered 

 specimens were obtained from some deep shafts on Prospect Mountain. On 

 the south slopes of Prospect Mountain it is generally very much decom- 

 posed and apparently grades off into White Porphyry, so that it is difficult 

 to draw a sharp dividing line between the two rocks. No rock that could 

 be definitely classed with this variety has been found south of Evans gulch, 

 and the body in the bed of the gulch above the mouth of South Evans has 

 been assigned to it somewhat doubtfully. 



WHITE PORPHYRY. 



The White or Leadville Porphyry is a generally white or granular, 

 compact, homogeneous-looking rock, composed of quartz, feldspar, and 

 muscovite. The quartz and feldspar are so intimately mixed together that 

 they can only occasionally be distinguished by the naked eye, the former 

 in small, double-pointed, hexagonal pyramids, the latter in small, white, rect- 



