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



the increase of one mineral to the exclusion of the others, the 

 changes being gradual and irregular; thus in some places the 

 rock becomes mainly quartz, in others mainly feldspar. Quartz 

 veins are abundant, but are of irregular form and are evidently 

 segregational, being a part of the results of crystallization con- 

 temporaneous in a general way with the crystallization of the 

 rest of the rock ; these veins often contain considerable mus- 

 covite. Biotite is sparingly present in many of the rock types 

 observed. Only two miles to the south this great, relatively 

 fine-grained dike runs into a mass of coarse, siliceous, biotite 

 granite with which it is apparently continuous, although it 

 may be that the dike is a slightly later intrusion connected 

 with the granite. The chemical and mineralogic composition 

 of this coarse biotite granite is much the same as those of the 

 finer-grained dike rocks above referred to, but the texture is 

 very different. Further south again this coarse granite is 

 overlain by flows of massive biotite rhyolite, which has nearly 

 the same chemical and mineralogic composition as the granite, 

 so far as can be discerned. 



It is probable that the coarse-grained granite and the rocks 

 of the finer-grained, siliceous, variable dike above described, 

 together with the quartz veins of the vicinity, are all nearly 

 contemporaneous, or closely consecutive allied phenomena, rep- 

 resenting a single period of intrusion and a single magma, the 

 variations in composition and period of injection being due to 

 differentiation. It is probable, also, that the similarly consti- 

 tuted biotite rhyolite is connected with the intrusive rocks, 

 which may possibly represent the roots or feeders of the 

 volcanic out-pourings. 



Microscopic exam ination. 



Biotite quartz-monzonite. — This specimen is of a type which 

 is very abundant in the dike, particularly near its central por- 

 tion. It is fine-grained and holocrystalline in the hand-speci- 

 men and contains occasional small phenocrysts of feldspar, 

 quartz and muscovite in a granular, saccharoidal groundmass. 

 There is a slight gneissic structure apparent in the hand speci- 

 men and the fracture follows this, but it does not appear in the 

 thin section ; it is evidently not a cataclastic phenomenon but 

 probably a flow structure. Under the microscope the ground- 

 mass is seen to be relatively fine-grained, granular and allotro- 

 morphic. It consists of a mosaic of quartz and feldspar, with 

 some biotite, the proportion of the different minerals being, in 

 the order named, about 15:10:1. The feldspar is both stri- 

 ated and unstriated. The unstriated feldspar was determined 

 once by the Fouque method as labradorite-bytownite. The 

 striated feldspars were determined in one case to be andesine ; 



