gpurr — Quartz-muscovite rock from Belmont, Nevada. 351 



Aet. XXXY. — Quartz-muscovite rock from Belmont^ 

 Nevada; the Equivalent of the Russian Beresite ;* by 

 J. E. Spurr. 



Occurrence. 



The rock which will be described in this paper occurs in a 

 large dike just east of Belmont, the seat of Nye County, 

 Nevada. It affords an interesting study in view of its pecu- 

 liar mineralogic composition ; its relation to other siliceous 

 rocks, into which it grades in the same dike ; its identity with 

 the Russian rock, beresite ; and its connection with ore deposits. 

 The dike in which the quartz-muscovite rock is found cuts 

 black, limy slates and gray, fine-grained, saccharoidal, crystal- 

 line limestones, which stand vertically. The formation is pre- 

 eminently a slaty one and often becomes schistose, this condi- 

 tion being due to the metamorphism occasioned by the dike 

 under consideration and by other connected dikes. The main 

 dike is half a mile wide and runs in a north and south direc- 

 tion ; on one side (the east) it is definitely bounded by the 

 slates, but on the west these slates form only a narrow band, a 

 few hundred yards in width, separating the dike from a large 

 mass of coarser intrusive siliceous granite which lies west and 

 south of Belmont. While the body of rock under considera- 

 tion is technically a dike, therefore, yet it is perhaps also to be 

 regarded as the marginal facies of the main intrusion. 



Near the contact of the dike, the shaly limestones become 

 transformed into jasperoid, as microscopic examination shows, 

 and the development of mica has occurred along certain planes, 

 so that the rock passes into a micaceous schist. The jasperoid 

 itself is often schistose and contains small bunches of yellow 

 and red metallic oxides which give it the aspect of a knotted 

 schist (Knotenschiefer). In the unaltered slates near here Mr. 

 Gilbertf found graptolites which mark the rocks as Silurian. 

 According to Mr. Walcott,^ the horizon probably corresponds 

 with part of the Upper Pogonip formation at Eureka. In this 

 limestone are found quartz veins which carry rich antimonial 

 silver ores. 



Composition of the Dike. 



The composition of this dike is not at all uniform, although 

 as a whole the rocks are moderately fine-grained and siliceous. 

 The essential constituents are quartz, feldspar and white mica, 

 but the proportions of these vary in the different localities by 



* Published with the permission of the Director of the IT. S. Geological Survey, 

 f TJ. S. Geographical Surveys, vol. iii, Geology, p. 180. 

 \ Monograph VIII, U. S. G. S , p. 2. 



