GEOLOGY OF THE SAN JOSE DISTRICT 261 



The hornblende crystals tend to be automorphic. Cross-sec- 

 tions are octagonal by the equal development of prism and pina- 

 coidal faces, rather than six-sided, as is usual where (lOo) is 

 suppressed. Terminal planes were not observed. The pleo- 

 chroism in certain instances is a = seal brown, b = yellowish- 

 green, c = light brown. Changing conditions in the composition 

 of the magma are indicated by the dark green borders which 

 usually surround the brown hornblendes. They are younger 

 than the augite. 



This mineral, which is much more abundant than the amphi- 

 bole, is pleochroic, between olive green and dull green. Its 

 surface is mottled by color changes in tones of green indicative 

 of differences in chemical composition. The central portions of 

 crystals are often tinged with violet. The short stubby prisms 

 show poorly developed forms. 



Apatite in six-sided prisms with perfect basal cleavage is 

 common. The crystals are .4 mm. or less in length. They 

 favor association with the dark silicates, as does also the mag- 

 netite. This mineral is in round grains .3 mm. across. Tita- 

 nite is important in the rock. Its crystals are often 1.2 mm. in 

 length. They are gray or nearly colorless. The Baril nephe- 

 lite syenite contains numerous small grains of pyrite dissemi- 

 nated through it. 



(^) Arroyo Grande Type. 



The variety of nephelite syenite here described occurs midway 

 between the Mesa Verde and Mt. Baril on the ridge overlooking 

 the valley of the Arroyo Grande to the south. Its field habit is 

 that of a very light gray rock in which the dark silicates make 

 up only a very small portion of the whole. In the hand speci- 

 men the nephehte is very prominent, contrasting strongly by 

 reason of its oily appearance with the kaolinized feldspar. With 

 the aid of a pocket lens twinning may be observed on the ortho- 

 clase. Some titanite may also be seen occurring close beside 

 the stray flecks of darker minerals. The texture is granitic 

 with no porphyritic developments, and the rock is medium- 

 grained. 



Microscopic Characters. — Under the microscope the essential 



