342 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



In the second class mentioned tbe processes of decomposition have produced a 

 light-colored rock, in which the biotite is replaced by a light, straw-colored substance 

 with silvery luster, while hornblende and ore particles have almost entirely disap- 

 peared. The microscope shows that decomposition has from the beginning taken an 

 entirely different course from that just described, although here, as there, the tendency 

 has been to the formation of a particular mineral, that mineral being inuscovite instead 

 of epidote. Muscovite resulting from the decomposition of biotite has been described 

 (p. 324) in the case of the Mount Zion Porphyry, and the present instance is very simi- 

 lar The muscovite is filled with minute, pale yellowish needles and grains (rutile ?}, 

 which cause the macroscopically visible tinge of color. That this mineral is really 

 muscovite it may be difficult to prove beyond all dispute, but the feldspars in the same 

 specimen are almost entirely altered to an apparently identical substance, with some 

 calcite, while no chlorite or epidote is found, showing that conditions favorable to the 

 formation of muscovite certainly existed. In general it can be stated that those 

 (specimens in which the development of muscovite is most distinct occurred in masses 

 covered by drift or exposed in the workings of mines [86], while those containing 

 epidote are from more exposed'positions, usually above timber-line. The observations 

 are, however, too few to be considered as indicating any rule in the matter. 



Chemical data A silica determination proves that quartz must be prominent in 

 the groundmass, as the quartz macroscopically visible is much less than in the Lincoln 

 Porphyry, while the amount of silica found, 05.08 per cent., is but little less than that 

 in the latter rock, viz, 66.45 per cent. [85a]. The Sacramento Porphyrite also con- 

 tains 3.55 per cent, of soda to 2.57 per cent, of potash [85], which confirms the classi- 

 fication as a plagioclase rock. 



SELVERHEKLS POKPHYEITE. 



Occurrence and previous classification The intrusive sheets of eruptive rock occur- 

 ring in Mount Silverheels, two of which appear in the northeastern part of the Mos- 

 quito Range map, belong to a rock which is not easily classified. It is colored as 

 rhyolite upon the Ilayden Atlas of Colorado, and called ' (trachyte?)" by A. C. Peale 

 in his report upon the region. 1 Unfortunately, its relations were not at first correctly 

 understood by the present writer, and consequently the rock is colored upon the map as 

 quartz-porphyry, while he now regards it as a plagioclastic rock and as belonging to 

 the series of porphyrites. 



Description This rock is of a greenish or gray color and very fine grained, but it 



still exhibits a distinct porphyritic structure when not too much decomposed. Its 

 macroscopically visible constituents are feldspar, biotite, hornblende, and, sparingly, 

 quartz, all of them in very small individuals, seldom exceeding 3""" in diameter. The 

 groundmass is usually obscured by chloritic decomposition products. Microscopical 

 study shows the usual accessory minerals, including allanite and pyrite. 



With regard to the feldspar crystals it is difficult to decide which may have been 

 predominant from simple microscopical study, for many of them are entirely decom- 

 posed and the mixture of calcite and muscovite resulting in all cases does not give a 

 1 Annual Report United States Geological and Geographical Survey of Territories, 1873, pp. 214-21G. 



