368 University of California Publications in Geology [Vol. 10 



facies show amygdaloidal structure, but flow structure is not common, 

 except in certain glassy facies. It is everywhere jointed and frac- 

 tured and under weathering processes yields a soil which contains 

 numerous angular fragments. The chemical alteration of the pyrite 

 has produced iron oxide that stains the soil to a characteristic brown 

 or red color, so that the regolith of the lava is readily recognized in 

 the field. 



PETROGRAPHICAL CHARACTERS 



The Leona rhyolite may be divided into four facies, according to 

 texture. The first, which shows the most advanced degree of crystal- 

 lization, has a felsitic texture and is composed of quartz and feldspar, 

 which only rarely attain phenoerystic development. This will be 

 known as the felsite facies. 



The second facies is similar to the above, but consists of pheno- 

 crysts of feldspar and quartz set in a dense groundmass that in places 

 is microcryptocrystalline, but is generally cryptocrystalline. This will 

 be designated the felsophyric facies. 



The third facies is characterized by well developed quartz and acid 

 plagioclase phenocrysts set in a glassy ground mass and will be desig- 

 nated the vitrophyric facies. 



The fourth, or microlitic facies, may well be divided into two 

 varieties, one the glassy and the other the spherulitic. The glassy 

 variety has a hypocrystalline ground mass which grades into a dense 

 glass. The crystalline constituents of the ground mass are mostly 

 quartz, but minute needles of feldspar are sparingly included in the 

 matrix. The spherulitic variety has a glassy ground mass which shows 

 radiolitic areas as distinct from the more dense glass. This grades 

 into devitrified areas which show a cryptocrystalline character. The 

 spherulites are sporadically distributed through the matrix, and also 

 occur sparingly embedded in the phenocrysts. 



Felsite Facies. — This is a brownish white rock, composed of feld- 

 spar and quartz, having a subconchoidal fracture and a felsitic texture. 

 A very few phenocrysts of oligoclase are present in the ground mass, 

 which is cryptocrystalline and altered. The quartz crystals are fre- 

 quently rounded by corrosion, but in spots secondary quartz is present 

 in quite large aggregates along fractures. The accessory minerals are 

 pyrite, magnetite, leucoxene, and small needles of apatite. 



