356 Spurr — Quartz -muscovite rock from Belmont, Nevada. 



resenting the final product of the residual solution of the gen- 

 eral magma. In these quartz veins the metallic minerals are 

 scattered in bunches or disseminated particles, rarely in banded 

 form. These metallic minerals consist principally of stetefeld- 

 tite, which is an argentiferous ore of antimony, containing 

 besides silver, lead, copper and iron. Dana* notes that this 

 mineral in Peru has been regarded as probably arising from 

 the decomposition of chalcostibite, a sulphide of antimony and 

 copper. Chalcostibite occurs at Wolfsberg in the Hartz in 

 nests embedded in quartz. 



The metallic minerals being, from their habit, plainly con- 

 temporaneous with the quartz veins which enclose them, it is 

 evident that the deposition of these minerals, the formation of 

 the quartz veins, the metamorphism of the country-rock to 

 jasperoid and muscovite schist, and the endomorphism of the 

 muscovite granite to quartz muscovite rock were contempo- 

 raneous occurrences, all brought about by the same agencies, 

 which were the solutions representing the end product of the 

 differentiation of the granitic intrusive rock. 



TJie identity of the quartz-mnscovite rock with the beresite of 



Russia. 



The rock called beresite occurs in the vicinity of Beresovsk 

 in the Urals, where it is intimately connected with veins of 

 auriferous quartz. The beresite itself forms distinct dikes, 

 varying from two to twenty meters, and reaching forty meters 

 in width. These veins have a general uorth and south direc- 

 tion, but vary locally and interlace. They were first described 

 by Gr. Rosef in 1837. He describes the constituents of the 

 rock as orthoclase, plagioclase, quartz, and muscovite in exceed- 

 ingly varying proportions. The feldspar diminishes and often 

 completely disappears, leaving the rock composed of quartz 

 and muscovite. Some varieties are like sericite schists; others 

 like micaceous sandstones. Rutile is occasionally found as 

 accessory. Rose was inclined to regard the Beresovsk veins as 

 apophyses from the neighboring granite of Schartassh,;}; which is 

 a fine-grained rock of semiporphyritic nature consisting chiefly 

 of quartz and feldspar (the latter in part albite), with small 

 flakes of biotite. The exact relation of the beresite to the 

 neighboring granite, however, has not been ascertained. § The 

 beresite is intrusive into vertical or highly dipping schistose 

 strata, either chloritic (listvenites), or talcose and argillaceous. 



* System of Mineralogy, sixth edition, 1896, p. 204. 

 fReise nach dem. Ural, vol. i, p. 186; vol. ii, p. 55T. 

 jOp. cit., vol. i, p. 189. 

 § Op. cit., vol. ii, p. 559. 



