1917] Clark: Geology and Ore Deposits of the Lcona Rhyolite 371 



matrix. In this facies the phenocrysts are much less common, and 

 those present are of small dimension. The rhyolite southeast of 

 Haywards has been classed as felsite. In this portion of the outcrop 

 alteration has been so intense that fresh rock is very hard to obtain. 

 However, the rock shows a rather uniform felsitic texture, with a 

 feeble development of small phenocrysts. 



SUMMAKY OF THE MICEOSCOPIC STUDY 



The petrographical study of the rhyolite shows that it may be 

 divided into four facies largely based upon textural differences. The 

 series includes a range of facies from those which are mostly glassy 

 to those which are holocrystalline containing only a few small pheno- 

 crysts. 



The rock is comparatively simple and rather uniform in compo- 

 sition. Orthoclase, plagioclase, mostly oligoclase, and quartz are the 

 essential constituents of the original rock. The accessory minerals 

 are pyrite, magnetite, apatite, zircon, and leucoxene. Only the first 

 three mentioned are in abundance. This study seems to support field 

 evidence that the different facies are probably products of a single 

 lava flow and came from a common source. 



CHEMICAL CHARACTERS 



The chemical composition as given in three different analyses is as 

 follows : 10 



(1) (2) (3) 



SiO, 71.00 72.12 71.60 



Ti0 2 17 .12 



ALO, 12.70 11.49 11.93 



Fe,0 3 66 2.77 3.00 



Feb 2.44 2.30 3.40 



MnO 10 .09 



CaO 90 .94 .52 



BaO Trace 



MgO 1.39 .85 1.33 



Na.,0 6.45 5.28 4.62 



K.,6 2.99 .86 1.90 



P~0 5 88 .62 



SO. .08 



CO, 1.15 .38 



IYS. 03 



CI Trace 



H,0 (at 110° C.) 28 .06 .07 



Ignition 1.56 .96 .55 



100.37 99.96 100.21 



(1) Analyzed by C. P. Richmond in the laboratory at the University of California. 



(2) and (3) Analyzed by G. E. Colby in the laboratory at the University of California. 



10 U. S. Geol. Survey., San Francisco Folio, no. 193, p. 12, 1914. 



