POST-JURASSIC ROCKS OF NEVADA 43 



the aplite and granitoid rocks differ in the relative proportions rather 

 than in the kind of minerals present. The apHtes are more siliceous 

 than the older granitoid rocks, those of the granites usually having 

 alaskitic affinities and those of the quartz-monzonite, granitic affinities. 

 Upon weathering the aplites protrude from the surface of the granitoid 

 rocks. 



PEGMATITE 



Pegmatite is usually associated with the granitoid rocks and is the 

 only representative of the granular siHceous series at Bare Mountain, 

 and is the predominant intrusive of the Bullfrog Hills. At most 

 places it cuts the granitoid rocks with sharp contacts, and normally 

 the intrusion of aplite intervened between that of granite and of peg- 

 matite, although graduations from pegmatite to granite also occur. 

 At one place in the Gold Mountain Ridge an aphte dike passes along 

 its strike into a dike with narrow apHtic border and broad pegmatitic 

 center. 



The pegmatite usually occurs in well-defined dikes but throughout 

 the more easterly of the two granite masses in the Pahute Mesa 

 ellipsoidal masses of coarse quartz-feldspar-pegmatite are inclosed 

 in the granite. The contact between the two rocks is at some places 

 sharp, at others gradational. The ellipsoidal form and the absence 

 of apparent channels from one mass to another in the plane of obser- 

 vation suggest that the pegmatite formed in place from the residual 

 fluids of the granitic magma. 



Chemically the pegmatites are more acid than their granitoid 

 associates although in the Panamint Range the pegmatite of the 

 quartz-monzonite has also undergone a considerable enrichment in 

 soda. In texture the pegmatites are either coarse, irregular, granular 

 aggregates or the intimate intergrowth commonly called graphic 

 granite. The latter rock in the Gold Mountain Ridge is a transition 

 phase between granite and coarsely granular pegmatite. 



The pegmatites of southwestern Nevada contain but few rare 

 minerals. Pyrite occurs as an original constituent of the pegmatite in 

 the Belted Range and in the Bullfrog Hills. At the former locaUty it 

 is reported to carry values in gold and silver and in 1905 was being 

 prospected. The gradation from either a muscovite- or biotite- 

 pegmatite to quartz veins is seen in most of the larger granite areas. 



