LAWSON "I 



PalacheJ 



The Berkeley Hills. 



421 



a prevailing porphyritic habit, are referred to as the porphyritic 

 facies. Together they make up the greater part of the basal vol- 

 canic member of the Upper Berkeleyan. The porphyritic andesite 

 occurs typically along the upper part of the Frowning Ridge 

 escarpment and particularly on Grizzly Peak, and will be first 

 considered. It is, in the fresh condition, a compact massive rock 

 of black color, with pronounced porphyritic habit, the phenocrysts 

 being usually yellowish feldspars with occasional olivines. 



Under the microscope the porphyritic structure is strongly 

 marked, large phenocrysts of feldspar, augite, olivine, and mag- 

 netite occurring in a microlitic ground-mass. The phenocrysts 

 exhibit striking evidences of the effect of mechanical strains in 

 their very commonly fractured or fragmentary condition. Corro- 

 sion and absorption of the earlier formed crystals by the magma is 

 also evidenced. Nevertheless many of the phenocrysts are in 

 beautifully sharp idiomorphic forms, and most of them are clear 

 and unaltered. The feldspar, judging by the extinction angles, is 

 an acid labradorite or andesine. It offers excellent examples of 

 the inclusion of glass and augite, described on another page. The 

 augite is of a pale green color, sharply bounded and little altered. 

 Olivine is sparingly present in large, well-formed crystals, which 

 are mostly decomposed to a bright yellow serpentine. Magnetite 

 forms well-defined crystals of large size, and apatite of the pleo- 

 chroic variety is quite abundant. 



The ground-mass consists chiefly of feldspar microlites of 

 exceedingly small dimensions, with fewer augite granules, minute 

 crystals of magnetite, and a variable but usually small amount of 

 colorless glass. The microlites show a linear arrangement and are 

 tangentially grouped about the phenocrysts, giving evidence of a 

 fluidal structure, not visible in the mass of the rock. The red 

 color of weathered forms of this facies is due to the setting free of 

 hydrous iron oxide through the serpentinization of pyroxene and 

 olivine. The resulting iron oxide as well as the serpentine pene- 

 trates the rock in every direction along cracks and fissures, and 

 ultimately penetrates its whole mass. The feldspars decompose to 

 kaolin in the ordinary manner. Chlorite was not observed among 

 the decomposition products in any of the sections prepared, but in 



