44 SYDNEY H. BALL 



At the Bullfrog- George prospect in the Slate Ridge molybdenite, 

 molybdite, and fluorite occur in a pegmatitic quartz vein. Molyb- 

 denite is present as metallic tablets and irregular scales of steel-gray 

 color which he between or are surrounded by the interlocking quartz 

 crystals of the vein. It evidently sohdified from the magmatic waters 

 contemporaneously with the quartz. Its alteration product, a bright- 

 yellow mineral in minute crystals and tufted aggregates, was 

 determined by W. T. Schaller to be molybdite. Fluorite, which 

 occurs in fractures in the quartz and also lines its vugs, is evidently 

 somewhat younger than quartz and presumably was deposited by 

 gases in the expiring stages of volcanism. 



In the Bullfrog Hills there is some evidence of two separate intru- 

 sions of pegmatitic material. The older pegmatite is a feldspar- 

 muscovite-quartz rock which is cut by a very siHceous pegmatite dike 

 of almost pure quartz. It is probable then that from the residium of 

 the granite magma a coarse-grained rock with the composition of a 

 granite first separated and solidified and at a later period a more 

 quartzose rock was deposited from the residual Hquid in fractures 

 in the older pegmatite. 



QUARTZ-MONZONITE PORPHYRY 



Dikes and sheets of quartz-monzonite porphyry, poor in ferro- 

 magnesian minerals, intrude Paleozoic rocks and granite of the Silver 

 Peak Range and Slate Ridge. This rock near Lida is apparently 

 cut by the diorite porphyry described below. 



The monzonite porphyry is a dense white or greenish-white rock 

 with abundant medium-sized phenocrysts, which however are sub- 

 ordinate in bulk to the ground-mass. They consist of whitish feld- 

 spars, some striated and others unstriated, silvery mica, and a few 

 quartz crystals. The central portions of the dikes and sheets are 

 more coarsely crystalhne than the borders and in instances approach 

 a granitoid texture. 



Microscopic examination proves the medium-grained microgranitic 

 ground mass to consist of orthoclase, microperthite, and anortho- 

 clase grains, plagioclase laths, and a few quartz anhedra. In some 

 thin sections the alkah feldspars poikihtically inclose the other min- 

 erals and in others quartz and orthoclase are in graphic intergrowth. 



