1917] Clark: Geology and Ore Deposits of the Leona Rhyolite 369 



FelsopJi yric Fades. — This is a light green to brown rock contain- 

 ing numerous dense, cloudy, indistinct patches. It has a subcon- 

 choidal fracture very similar to that of the rock just described. The 

 ground mass is composed of feldspar and quartz in various irregular 

 aggregates. The white clouded patches appear to be composed of 

 alteration products which are not determinable. In the finer part of 

 the matrix chlorite is present, producing a bluish-green stain in the 

 altered products. The phenocrysts are mostly plagioclase, but a few 

 aggregates of orthoclase and quartz occur in the matrix. The acces- 

 sory minerals are pyrite, magnetite, zircon, and apatite. 



Vitrophyric Fades. — This is a light blue to gray rock having a 

 cryptocrystalline ground mass which appears under the microscope to 

 grade into a hypocrystalline aggregate. Numerous crystals of feld- 

 spar, together with some quartz and irregular masses of magnetite, 

 constitute the phenocrysts. A few flakes of biotite partially altered 

 to chlorite are also present in the rock. The small aggregates which 

 make up the matrix are of two kinds : one is principally composed of 

 rounded grains with rare elongated forms, while in the other the 

 elongated crystals of feldspar and quartz form most of the ground- 

 mass and the few rounded grains present have been derived mostly 

 by alteration and addition of secondary quartz. The phenocrysts are 

 mostly oligoclase feldspar and quartz, with a few poorly developed 

 crystals of orthoclase. Some of the feldspar phenocrysts have been 

 partially silicified, so that they appear as a dense brownish colored 

 mass that does not always show the original crystalline structure. The 

 accessory minerals are pyrite, magnetite, apatite, and zircon. 



Microlitic Facies. — The microlitic facies is composed of two varie- 

 ties of glassy rock. 



1. The first has a cryptocrystalline ground mass which grades into 

 an undeterminable, brownish glass studded with minute elongated feld- 

 spars and grains of quartz. In portions of the rock these elongated 

 needles of feldspar are oriented in a single direction, giving rise to a 

 flow structure which is common in this facies of the rhyolite. Feld- 

 spar forms the most important phenocrysts, if we exclude certain 

 quite large aggregates of quartz and irregular masses of magnetite 

 in the matrix. Most of the feldspar is altered to kaolin and silica, 

 which form dense clay masses colored dark blue by chlorite. Certain 

 portions of the rock frequently show an intermediate stage between 

 the dense glassy base and the microlitic structure. 



