Spurr — Quartz-muscovite rock from Belmont, Nevada. 355 



consists of a mosaic of intergrown grains. Besides this quartz, 

 the groundmass consists of an allotriomorphic, granular inter- 

 growth of quartz and feldspar, with subordinate biotite. The 

 feldspar is mostly orthoclase. A faintly striated feldspar was 

 determined to be anorthoclase, or microcline-anorthoclase. The 

 feldspars sometimes show zonal structure. 



Differentiation of the Great Dike. 



This has not been carefully studied, but in general the 

 darker colored rock, which has been shown in one specimen to 

 be a quartz-monzonite, occupies the central portion of the dike, 

 while the rock near the margin consists of the siliceous mus- 

 covite granite, or quartz-muscovite rock. This seems to indi- 

 cate, at this locality at least, that the borders of the dike are 

 more siliceous than the center. 



Origin of Quartz-muscovite rock and connection with ores. 



Although the quartz-muscovite rock is evidently a product 

 of alteration, yet it is not due to surface weathering, since the 

 condition of the rock is quite fresh and hard, and the albite 

 shows no trace of decomposition. The process must be 

 regarded as one of endomorphism and as connected and prob- 

 ably contemporaneous with the exomorphism indicated by the 

 alteration of the siliceous limestone of the wall-rocks to jas- 

 peroid and mica schist. In both the intrusive and the intruded 

 rock the result of the metamorphism has been the same, pro- 

 ducing quartz and muscovite at the expense of the orthoclase 

 on the one hand, and of the calcite and subordinate minerals 

 on the other. In the case of the wall-rock the metamorphism, 

 being apparently from its distribution dependent upon the 

 intrusion, evidently took place after this intrusion and was 

 brought about by the solutions which accompanied the igneous 

 rock, or were residual from its solidification. Within the dike 

 the similar alteration was probably contemporaneous with that 

 in the country rock. 



In the immediate vicinity of this intrusive mass are ore 

 deposits, which in the time of Nevada's prosperity made this 

 region one of considerable wealth, although at present the 

 mining industry is perfectly dormant. The writer had not 

 opportunity to study these, but according to Mr. Emmons* 

 the ores generally occur in white quartz veins which are often 

 several feet in width. These quartz veins are probably con- 

 temporaneous with those already described as occurring in 

 irregular form within the dike rock itself, and as evidently rep- 



* Geological Explorations of the 40th Parallel Mining Industry, vol. iii, p. 393. 



