Spurr — Quartz-muscovite rock from Belmont, Nevada. 353 



in another, oligoclase-albite. There is probably also consider- 

 able orthoclase. The rock, therefore, appears to belong to the 

 monzonitic family and to the quartz-monzonite group. 



Siliceous muscovite-hiotite granite. — This rock has in general 

 a fine grain, like that of the quartz-monzonite just described. 

 In general, however, it is much lighter colored than this rock, 

 since the mica which it contains is sporadic, leaving the rest of 

 the rock composed of quartz and feldspar. Much of the rock 

 observed in the field contains practically no mica, so that it 

 passes into alaskite. The specimen studied is remarkable as 

 containing frequent bunches of pure granular quartz, about a 

 quarter of an inch in diameter, and resembling exactly vein- 

 quartz. In thin section the rock is seen to be in general of 

 medium grain, while it varies locally from very coarse to very 

 fine. The habit of the minerals is allotriomorphic granular. 

 Just as the texture varies so enormously, so the arrangement 

 of the constituent minerals is irregular, some areas being 

 entirely of quartz in irregular interlocking grains, while others 

 consist of quartz and feldspar. The feldspar is mostly 

 untwinned and was determined as orthoclase, while some 

 twinned feldspar was determined as albite. There are in the 

 section occasional tiny accessory grains of muscovite and bio- 

 tite, and a large broken crystal of zircon. The feldspar in 

 one section encloses primary muscovite, while in another case 

 the muscovite encloses quartz. This would seem to make the 

 order of crystallization quartz, muscovite, and feldspar, but the 

 irregular intergrowth of the whole section shows that the 

 crystallization of all these minerals was essentially contempo- 

 raneous. The quartz and feldspar are often intergrown in 

 micrographic fashion. The feldspar is slightly kaolinized and 

 in places there are very small flakes of secondary muscovite, 

 which must be held separate from the rest of the muscovite in 

 the rock, which is plainly primary. 



Quartz-muscovite rock. — This rock is the one of chief 

 interest in the dike, and occurs in large masses. It changes 

 gradually and irregularly into the alaskite or muscovite biotite 

 granite above described, and from its field relations is evi- 

 dently a variation of this rock. The specimen examined, 

 which is typical, has the exact appearance of a white or light- 

 gray, medium-grained, micaceous quartzite. It is found espe- 

 cially near the margins of the dike and cuts into the sedimentary 

 slates, which are noticeably metamorphosed into jasperoid and 

 mica-schist near the contact. Under the microscope the rock 

 is seen to be medium-grained, with a fairly even, allotrio- 

 morphic, granular structure. The chief minerals are quartz 

 and muscovite, the former predominant. The rock is evi- 



