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



dently what it seems to be from field relations— a variation of 

 the alaskite or muscovite-biotite granite, the variation consist- 

 ing in the substitution of muscovite for feldspar. The pro- 

 portion of muscovite to quartz in this rock is such that the 

 rock is evidently nearly a chemical equivalent of the musco- 

 vite granite. Tiny grains of feldspar are found, generally 

 striated, and seem to be albite. 



Both in the hand-specimen and in the section this rock 

 appears perfectly fresh and all the constituents primary. The 

 freshness of the albite particles seems to remove all question 

 of decomposition, for there is hardly any trace of kaolinization. 

 A more thorough investigation, however, shows that muscovite 

 occurs in two distinct habits. In the first habit it forms large 

 irregular grains intergrown with the quartz, and sometimes 

 enclosing smaller grains of quartz — poikilitic structure. In 

 the second habit it occurs either in medium-sized blades or in 

 small sheaf-like flakes, often spherulitic in arrangement, which 

 are intergrown with quartz grains. 



Concerning the muscovite found with the first described 

 habit there is no question as to its primary nature. The last 

 described habit, however, suggests a secondary origin, in spite 

 of the freshness of all the minerals of the rock. Finally, a 

 thorough investigation has shown that these aggregates of 

 muscovite and quartz are plainly derived from orthoclase, some 

 fragments of which may yet be found in the midst, with blades 

 of muscovite and quartz grains penetrating them. It is plain 

 that the alteration of the feldspars has been confined to the 

 orthoclase and has not attacked the albite, which remains fresh. 



The amount of primary orthoclase which the aggregates 

 of secondary muscovite and quartz represent is so consider- 

 able as to put the primary rock among the fine-grained mus- 

 covite granites and to make it almost exactly similar to the 

 muscovite granite just described, with which it is so closely 

 connected in the field. 



Biotite granite. — The biotite granite, into which this great 

 fine-grained dike appears to merge south of Belmont, may be 

 briefly described. The rock is porphyritic, carrying numerous 

 crystals of orthoclase up to two inches in length. Under the 

 microscope these orthoclase crystals are found to be inter- 

 grown with shreds of orthoclase differently oriented from the 

 main crystal, but uniformly oriented among themselves. The 

 crj^stals also contain shreds of muscovite. The ground mass is 

 very coarse, and is remarkable for containing blotches of pure 

 quartz in granular aggregates similar to that described above 

 for the fine-grained muscovite-biotite granite of the great 

 dike. In this case the blotches of quartz are from one- 

 third of an inch to one-half an inch in diameter, and each 



