Vol. 4] 



Knopf-Thelen. — Geology of Mineral King. 



235 



widely divergent products of the same irruptive magma. While 

 often maintaining their persistence of character over several 

 square miles, a gradual transition through the medium of inter- 

 mediate types frequently takes place. The prevailing' rock upon 

 the eastern flank of the belt is a very coarsely granular aggre- 

 gate of orthoclase and quartz with subsidiary amounts of biotite 

 and hornblende. This granite readily becomes incoherent, and 

 disintegrates rapidly, forming great sand-slopes. The micro- 

 scope shows that the dominant potash feldspar is microcline, 

 usually to the almost complete exclusion of the orthoclase. As- 

 sociated with these are small amounts of acid plagioclase. and 

 occasionally, microperthite and microcline-mieroperthite. Be- 

 sides the usual accessories, magnetite, apatite, titanite and 

 zircon, the quartz is noted to contain capillary rutile. This acid 

 potash granite is splendidly exposed at Sawtooth Peak, at Silver 

 Lakes and at Florence Lakes, and reappears on Shotgun Creek. 



Passing south, the ferromagnesian minerals become more 

 abundant, and the amount of plagioclase increases. Toward the 

 southern extremity, the granite becomes noteworthy through the 

 development of large, imperfect phenocrysts of orthoclase scat- 

 tered somewhat sporadically throughout the matrix. They 

 often attain large dimensions, frequently measuring over four 

 centimeters in length and are broad in proportion. The ground 

 is a medium grained, eugranitic assemblage of orthoclase. plagio- 

 clase, quartz and biotite. The feldspars occur in nearly equal 

 proportions, placing the rock among the orthoclase-plagioclase 

 rocks as a porphyritic quartz monzonite. 



West of Mineral King the granite is prevailingly of the horn- 

 blende-biotite variety. The rock is a typical orthoclase granite 

 carrying a notable quantity of dark constituents. Toward the 

 north, however, an interesting modification is met with. The gran- 

 ite becomes surcharged with lustrous black hornblende prisms, 

 strongly idiomorphic, and frequently attaining a length of two 

 centimeters. Splendent foils of biotite become conspicuous; 

 sphene is macroscopically abundant, and quartz sinks to minor 

 importance. Systematic collection of hand specimens along a 

 normal to the contact showed a loss of the porphyritic habit with 



