62 



University of California Publications. [Geology 



in the ground mass. Flakes of hematite, and stains of the same, 

 are abundant. Two crystals of ilmenite, altering to leucoxene, 

 were observed. This rock contains 64.8% Si0 2 . A finer grained 

 rock from the same locality yielded 6.5.5% Si0 2 . Similar lava 

 extends from the hills on the east side of Santa Rosa Valley and 

 extends west of Penn 's Grove for about one mile. It rests uncon- 

 formably upon the yellow sandstone and gravel which, near the 

 headwaters of Petal uma Creek, some two miles northeast of 

 Penn's Grove, contain the distinctive upper San Pablo fossil, 

 Cyrena Calif 'ornica. These andesites are supposed by the writer 

 to be the same as those above described from Mark West Springs 

 and Mt. St. Helena. 



A specimen from one mile west of Penn's Grove station 

 showed the following characteristics. 



Macroscopically it is a heavy dark greenish gray rock of suffi- 

 ciently coarse texture to enable the eye to recognize the crystal- 

 linity of the ground mass. Occasional small, well shaped pheno- 

 crysts of feldspar were observed, and small yellow spots, due 

 apparently to the decomposition of the pyroxene, are numerous. 



Microscopically this rock is seen to be rather coarsely crystal- 

 line in texture. The phenocrysts are very abundant, consisting 

 of well shaped laths of feldspar and stocky prisms of augite, 

 embedded in a crystalline ground mass of small feldspar laths 

 and well shaped augite needles. 



The feldspar gave a maximum extinction angle on a section 

 cut perpendicular to (010) of 41°. This would indicate a feld- 

 spar somewhat more basic than Ab.> An 4 . The usual length of 

 the labradorite phenocrysts is about .38 mm. 



Augite is plentiful. While the needle-like crystals of the 

 ground mass are well shaped, the large phenocrysts are usually 

 badly corroded by the magma. The maximum length of these 

 phenocrysts is about .5 mm. Much magnetite occurs as small 

 cubes and octahedra, included principally in the augite. No 

 olivine was observed, and no glass could be certainly determined 

 in the ground mass. There is a distinct similarity between this 

 rock and the ones described from the vicinity of Mark West 

 Springs. 



Another specimen from the same locality showed the follow- 

 ing features : 



