184 



University of California. 



LVol. 3. 



same as in the main mass. Some few grains of highly refractive 

 sphene are also seen. 



RHYOLITE. 



On the top of the hill above the quarry were found small 

 scattered boulders of disintegration of a rock totally different 

 from any seen in place in the vicinity. The occurrence was lim- 

 ited entirely to a few scattered pieces of the rock, which appear 

 to be remnants of a lava which once mantled a portion of the 

 region. There is no connection between these fragments and the 

 plutouie rocks above described. To the unaided eye the rock is 

 seen to be composed of phenocrysts of clear quartz, a clear 

 feldspar, cloudy feldspar, and biotite, in a traehy tic- appearing 

 ground mass of a yellowish gray color. Under the microscope 

 the phenocrysts, in order of relative abundance, are oligoclase, 

 sanidine, quartz, and biotite. 



The oligoclase, recognized by its small angles of extinction on 

 on the albite lamellae, makes up about 15 per cent, of the rock. It 

 is in well formed crystals averaging 3 mm. in size, which some- 

 times show a zonal structure. The sanidine occurs in larger 

 phenocrysts sometimes 6 mm. in size. It is very glassy, and 

 exhibits a rather poor cleavage, which distinguishes it from the 

 quartz. It makes up about 10 per cent, of the rock. The quartz 

 occurs rather sparingly in more or less rounded grains, averaging 

 2 mm. in diameter. It is free from inclusions, and makes up 

 about 5 per cent, of the whole. 



The ground mass is too finely crystalline to be resolved under 

 the microscope into its constituents. It appears almost purely 

 feldspathic, however. Chemical analysis, to determine the silica 

 content, was relied upon to settle the classification of the rock. 

 As given in the table following, the percentage of silica is 

 74.12. It is therefore a rhyolite. 



CHEMICAL CHARACTERISTICS. 



Following is a table containing complete and partial analyses 

 of the rocks described in these notes, with a number of similar 

 ones for purposes of comparison: 



