LAWS' 

 l'Al.A< 



*] The Berkeley Hills. 423 



sented by lavas of considerable volume and extent, but in its most 

 typical occurrence is found immediately beneath the lavas last 

 described on the front of the Frowning Ridge escarpment. It also 

 forms a large part of the Ruin Peak Ridge, on the northeast side of 

 Wildcat Creek. It is generally characterized by a lamellar jointage, 

 which has been described on a previous page. In hand specimens 

 it is a dark gray to reddish brown rock of uneven fracture and 

 rather coarse porphyritic habit. It is locally vesicular or amyg- 

 daloidal. In thin section it presents a distinctly holocrystalline 

 porphyritic texture. The porphyritic constituents are feldspar, 

 augite, hypersthene, and magnetite, imbedded in a ground-mass of 

 feldspar microlites, augite granules, and grains of magnetite. The 

 plagioclase feldspar, judging from its extinction angles, is a basic 

 labradorite or bytowni.te. It abounds in characteristic glass and 

 augite inclusions, and shows little evidence of magmatic corrosion 

 or mechanical fracture. The forms of the crystals are not so well 

 defined as were those of the phenocrysts of the porphyritic facies. 

 Orthoclase feldspar is probably present in occasional phenocrysts. 

 The phenocrysts of augite and hypersthene are imperfectly idio- 

 morphic. Augite is very much the more abundant of the two 

 minerals. The feldspar microlites of the ground-mass are generally 

 twinned and extinguished at angles which indicate that they have 

 the composition of labradorite. They show an imperfect linear 

 grouping suggestive of flow structure, but its expression is hin- 

 dered by the abundance of the phenocrysts. Yellow ocherous 

 serpentine, and a little calcite and chlorite, are the only secondary 

 minerals present in the typical phases of the rock. The calcite is 

 in some cases confined to distinct veins sometimes half an inch in 

 thickness, in which it is crystallized with more or less quartz. 



The flow 01 klinker breccias which are associated with these 

 andesite lavas are as usual more decomposed and altered than the 

 massive lava. The fragments, being often thoroughly vesicular, 

 appear brightly colored, due to the oxidation of the contained iron. 

 The thin sections of these breccias are much obscured by the 

 abundance of iron oxide disseminated through the rock, but the 

 coarser-textured masses seem to have the structure and general 

 microscopic characters of the holocrystalline variety of andesite 



