278 IRVING. 



in a dense bluish groundmass, in which smaller feldspars may- 

 be easily distinguished. Macroscopic quartz is frequently pres- 

 ent. The rock has many inclusions of all descriptions, the com- 

 monest being slate fragments, and the most interesting, masses 

 of a coarse-grained binary granite. 



Microscopic Characters. — The microscope shows the rock to 

 consist of a groundmass of quartz and feldspar. In this are 

 embedded innumerable sanidine phenocrysts of varying size and 

 form. Most of them are but little resorbed, and the larger 

 number are square in outline showing only P and /. A marked 

 zonary banding is generally present. 



A considerable number of the feldspar phenocrysts are mi- 

 crocline and an acid plagioclase, which is probably albite. In 

 many cases the albite occurs included in the sanidine pheno- 

 crysts. Much pyrite is present, and is sometimes seen lining 

 the edges of myarolitic cavities, in w^hich are confined sulphur- 

 ous gases, so that the rock w^hen broken exhales an extremely 

 offensive odor. 



Foley Peak Type. 



Upon Foley Peak occurs a quartz-porphyry, which shows 

 a much coarser groundmass than that occurring on War 

 Eagle Hill. It is composed o a xenomorphic aggregate of 

 quartz and orthoclase in quite large masses. In this are phe- 

 nocrysts of quartz and plagioclase, the former being the most 

 abundant. 



In certain portions of the mass this rock show^s large and 

 quite numerous phenocrysts of quartz in addition to that pres- 

 ent in the groundmass. Otherwise it differs little from the War 

 Eagle Hill type except in the coarseness of the groundmass. 



Nevada Gulch Type. 



This type of quartz-porphyry occurs in a dike along the road 

 just below the junction of Nevada and Fantail Gulches. 



Megascopic Appearance. — The rock is of a light gray, porphy- 

 ritic character. The groundmass is dark gray, and very fine 



