342 GEOLOGY AND ?iIINIXG IXDCTSTRY OP LEADVILLE. 



lu the second class mentioued tbe processes ci 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 differeut course from that just described, although here, as there, the tendency 

 has been to tbe formation of a particular mineral, that mineral being muscovite instead 

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

 (p. 324) in the case of the Mount Ziou 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 disi)ute, but the feldspars in the same 

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

 calcite, whde 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 

 .•-pecimens 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 grouudmass, as the (juartz macroscopically visible is much less than in the Lincoln 

 Porphyry, while the amount of silica found, (55.08 per cent., is but little less than that 

 in the latter rock, viz, G0.45 per cent. [85a]. The Sacramento Porphyrite also con- 

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

 fication as a i)lagioclase rock. 



SILVEEHEELS rORPHYKITE. 



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 llayden Atlas of Colorado, and called '• (trachyte"?)" by A. C. Peale 

 in his report upon tlie region.' Unfortunately, its relations were not at first correctly 

 understood by the preseut 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 coustitueuts are feldspar, biotite, hornblende, and, sparingly, 

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

 grouudmass 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 difScult 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 

 ' Annual Report United States Geological and Geographical Survey of Territories, 1873, pp. 314-21G. 



