RHYOLITES AND DACITES. 



41 



TOXOPAH RHYOLITE-DACITE. 



The Tonopah rhyolite-dacite occupies a large part of the area mapped. It 

 occurs in large unbroken areas in the northern corner and in numerous broken 

 and separated areas, bounded by faults, in the western corner. 



Appearance. The rock has many different aspects in the tield, gray, bright 

 red, black, and white being among the colors represented. Fine brecciation is 

 frequently observable, while in many cases the rock is glassy, dense, and charac- 

 terless, especially near the contacts of the intrusive masses, or in the thin sheets. 

 Under the microscope, however, the characters are much more uniform. 



Microscopic characters. Characteristically sparse and small phenocrysts occur 

 in a glassy, sometimes partly microcrystalline brown, gray-brown, or yellowish 

 groundmass. The rock often possesses flow structure, is rarely pumiceous or 

 slaggy, and frequently shows autobrecciation. Angular fragments of broken glass, 

 included in a cement of similar glass, and other phenomena indicate that the lava 

 moved while stiffening. 





Scale 



20 feet 



FIG. 2. Vertical sketched section of trench just west of Brougher Mountain, showing Tonopah rhyoliie-dnoite (6), intru- 

 sive into Fraction dacite breccia (at. 



The porphyritic crystals consist of feldspar, biotite, and quartz, which occur 

 in the order named. Small unstriated blunt crystals of orthoclase are always 

 predominant among the feldspars, though striated and more elongated crystals are 

 frequent. Optical determination of these shows that they range from andesine to 

 albite, andesine-oligoclase being the most frequent phase. Quartz crystals are 

 abundant in some phases, in others rare, and in many are wanting, especially in 

 the more glassy phases. Fresh biotite crystals are frequently present though 

 rarely abundant. They are usually small in size. 



A pseudomorph of iron oxide (specular iron?) after hornblende was observed 

 in one case, the original hornblende having been resorbed by the dacitic magma. 

 A single small crystal of augite was found in one of the slides, out of several 

 hundred examined. Small original crystals of specular iron are often observed. 



Alteration near contacts. Silicification and the production of secondary minerals 

 is widespread, especially near contacts where the rhyolite is intrusive into older 



