122 University of California Publications in Geology [Vol.7 



phenoerysts of green hornblende and whitish feldspar. The 

 groundmass is microcrystalline. The feldspar of the pheno- 

 erysts, making up about fifty per cent of the rock, is andesine, in 

 large part altered to muscovite, calcite, and quartz. The feld- 

 spar of the groundmass has about the same composition as that 

 of the phenoerysts and is also altered and silicified. Thirty 

 per cent of the rock is made up of the green hornblende of the 

 phenoerysts and groundmass, in fairly fresh condition. Mag- 

 netite phenoerysts are also present. 



The apophyses from the main mass differ from it both in 

 texture and color. The dike rock is dark bluish gray in color, 

 with an aphanitic groundmass and lath-shaped phenoerysts of 

 hornblende, some of which are a half inch in length. Small 

 masses of secondary pyrite can be seen in the hand specimen. 

 Under the microscope the rock is seen to be holocrystalline por- 

 phyritic with phenoerysts making up forty-five per cent of the 

 total mass. Green hornblende, in large measure altered to 

 chlorite, makes up forty per cent. Ten per cent is made of 

 phenoerysts of a monoclinic pyroxine, probably augite. Labra- 

 dorite feldspar of the groundmass comprises thirty-five per cent, 

 while the plagioclase of the phenoerysts makes up fifteen per cent. 

 The feldspars are largely altered to an aggregate of muscovite 

 and calcite. 



Acidic Igneous Rocks. — East of the intrusion of hornblende 

 diorite-porphyry the southern or main ridge of the El Paso 

 Range is made up of coarse-grained granite cut by dikes of 

 pegmatite and aplite. The relations of this granite to the meta- 

 morphic series and the hornblende-diorite porphyry was not 

 ascertained. A dike cutting the metamorphie series about one 

 half mile east of the lower narrows of Red Rock Canon contained 

 primary oligoclase feldspar, which had been subjected to crush- 

 ing, and quartz lenses representing either original quartz pheno- 

 erysts or amygdules. The original groundmass contained quartz 

 in large amount, which had been recrystallized. An original 

 crystal of titanite was crushed. A small amount of chlorite was 

 present in the quartz, and a vein-like mass of epidote, containing 

 veinlets of quartz, cut the rock. The rock should probably be 

 called an altered quartz porphyrj^. 



