GEOLOGY OF THE SAN JOSE DISTRICT 277 



The magnetite is usually surrounded by narrow rims of 

 titanite, which may be secondary. The larger pieces of mag- 

 netite .4 mm. in diameter are to be taken as phenocrysts. 

 Those of the second generation average .05 mm. They are in 

 definite square sections of octahedra. Apatite needles are rare 

 but titanite in dull grayish-yellow grains is often seen near the 

 dark minerals. No pyrite was observed. 



3. Basalt 



Occurrence. — A lava flow of basalt may be traced on the east- 

 ern side of the San Carlos Mountains, extending back toward the 

 range, along the course of the Arroyo Grande for four or five 

 miles. The sheet is scoriaceous and vesicular on its upper sur- 

 face, and appears to have issued from a fissure below the crags, 

 at this point on the east of the Baril Range. The flow is per- 

 haps half a mile in width. No volcanic cone was found on trac- 

 ing it to its source. 



Macroscopic Appearance. — In the hand specimen the rock is 

 aphanitic and fine-grained. In color it is bluish-black. It 

 breaks with a smooth splintery fracture. Blow holes show over 

 its surface. The vesicular cavities are filled with calcite. 



Microscopic Characters. — The rock is porphyritic with pheno- 

 crysts of feldspar and pyroxene. The augite was not observed 

 in a second generation in the ground-mass, where lath-shaped 

 feldspars, biotite and magnitite are the principal constituents. 



The plagioclase phenocrysts are small, not above .4 mm. in 

 diameter. The mineral occurs in poorly bounded thick tablets 

 which are heavily charged with magnetite. The only inclusions 

 are minute prisms of pyroxene. Wavy extinctions from center 

 to margin are characteristic, but definite clearly marked zones 

 of varying chemical composition are not observed. Twins on 

 the albite law are common, but no other forms of twinning 

 occur. Symmetrical extinction angles point toward labrador- 

 ite. There is frequently a dense crowding together of magne- 

 tite grains for half a millimeter around a core of feldspar. Often 

 the crystals of plagioclase have been acted upon by the magma 

 until a mosaic of feldspar grains has resulted. Large, very 



