1907. 1 



CRANDALL— SAN FRANCISCO PENINSULA. 21 



hornblende diorite. The hand specimens are all more or less alike 

 — hard green rock of close texture, only occasionally varying to a 

 coarse grained rock in which the plagioclase and ferro-magnesian 

 minerals are easily distinguishable with a hand lens. These will 

 be described in the order given. 



Petrography of the Igneous Intrusives. 



Hypersthene Diabase. — The hypersthene diabase is the freshest 

 rock of the series, and has the following constituents : hypersthene, 

 malacolite, labradorite, hornblende, magnetite and ilmenite. The 

 orthorhombic pyroxene, hypersthene, is present in small quantities. 

 It is reddish, with a high relief, the pleochroism being fairly strong 

 from red to greenish. The ray a is red and the ray c green. This 

 occurs in large irregular prisms without any definite boundaries. 



The hypersthene is characterized by parallel extinction. Around 

 the border may be seen the growth of fibrous hornblende as in the 

 malacolite. Some of the crystals showed a dark appearance due to 

 the presence of small black rods of an unknown mineral. Besides 

 the hypersthene are large irregular shaped crystals of a colorless 

 augite, probably malacolite. In ordinary light it is clear and color- 

 less with a medium relief. Under cross nicols the interference 

 colors are about orange of the first order and the extinction angle 

 is 45°. Twinning was observed. 



The interesting thing in connection with the hypersthene and 

 the malacolite is the derivation of secondary hornblende. Crystals 

 of this colorless augite show the growth of pleochroic hornblende 

 all about the border, noticeable in ordinary light, but in polarized 

 light the hornblende has much higher colors. The cleavage of the 

 hornblende is parallel to that of the augite but is more sharply 

 marked. In a crystal with secondary hornblende of this character 

 the extinction angle of the hornblende was 11° while that of the 

 augite was 43°. 



The hornblende is in crystals that do not show their derivation. 

 It is in irregular allotriomorphic patches with a greater length in 

 the direction of the cleavage. The pleochroism is from yellow to 

 yellowish green to dark green, corresponding with the axes, c, b, a, 

 with an absorption scheme lt>Ii>c. The extinction angle, c : c'= 17°. 



