GEOLOGY OF THE SAN JOSE DISTRICT 273 



rounded by a glassy zone of later growth. Inclusions of apa- 

 tite, in very slender needles are common, and the whole sub- 

 stance of a labradorite crystal is often clouded with microscopic 

 irregularly disseminated inclusions, probably pyroxene. Again 

 iron-bearing solutions have stained the feldspar along cleavage 

 cracks. 



Augite, which is the most important of the dark silicates, is 

 rather more abundant than hornblende. It is found in simple 

 light green crystals bounded in the prism zone by the faces 

 m(iio), a(ioo) and ^(oio), which favor association with the 

 hornblende and magnetite. The crystals show^ an unusually 

 good cleavage parallel to a(ioo). Zonal structures are rarely 

 observed. The extinction angle is 42°. 



The hornblende is in very long slender prisms which are 

 either rounded at the ends or terminated by two flattish faces. 

 Occasionally they are twinned. They are of a dull brownish- 

 green color and absorb the light strongly. Pleochroism is 

 confined to shades of green. 



Titanite, which appears sporadically in large irregular crys- 

 tals, is the most notable of the accessory minerals present. 

 The customary cleavage parallel to /(no) is not well developed. 

 Acutely rhombic sections are not common. 



Magnetite, which but rarely shows crystal boundaries, is 

 abundant in this andesite. Apatite is common among the older 

 inclusions. 



The ground-mass is a fine-grained aggregate of minute lath- 

 shaped and irregular feldspars, microscopic magnetite grains and 

 minute augite prisms of a second generation. The extinction 

 angles on the feldspar microlites are low and point to oligoclase. 

 Rude flow lines may at times be noted. 



2. Dacite 



{a) Florencia Type. — The rock here described exhibits every 

 gradation towards the less siliceous andesite in the laccolithic 

 mass. It is found in the hills immediately south of San Jose. 



Macroscopic Description. — This type is very fine-grained. In 

 the hand specimen the rock is of a pale brown color. It is 



