1907] CRANDALL— SAN FRANCISCO PENINSULA. 25 



lei to c'. In ordinary light it is colorless and non-pleochroic, the 

 relief is fairly high, with a slight cleavage in the direction of elon- 

 gation. In this section the colors are upper first and second order, 

 but in basal sections the colors and relief are not so high. The 

 lawsonite is idiomorphic with regard to the hornblende and glau- 

 cophane. 



Pyrite and ilmenite occur in irregular masses as inclusions in 

 the other minerals. Secondary quartz veins traverse the sections. 



Considering the minerals now present and the texture of the rock 

 it seems probable that it is an altered form of hornblende diabase, 

 the glaucophane having developed from hornblende and the law- 

 sonite having been formed by hydration of basic feldspars. 



From this same locality (a mile east of Baker beach) is a light 

 blue fissile schist that contains two minerals, a blue soda-bearing 

 amphibole, undetermined, and minute amounts of lawsonite scat- 

 tered in the interstices between the amphibole fibres. 



Acid Glaucophane Schist. — The more acid schist has the follow- 

 ing minerals : glaucophane, a groundmass that is quartz or a mix- 

 ture of quartz and feldspars, large cubes of pyrite, chlorite, titanite, 

 white mica and a mineral determined as andalusite. The glauco- 

 phane is present in large elongated crystals with indefinite end 

 boundaries. The pleochroism is very strong, the section being cut 

 parallel to the direction of schistosity while the blue to violet pleo- 

 chroism is most common. The quartz or feldspars, without twin- 

 ning, make up the large part of the rock. This is most likely all 

 quartz, being of a fibrous nature, and the fibres are often much 

 twisted and distorted. 



The cubes of pyrite are perfect and have affected the deposi- 

 tion or the recrystallization of the quartz, so that the quartz has 

 formed in fibres, so arranged that they appear attached to the pyrite 

 cubes, with the long axes at right angles to the faces of the cubes. 



Chlorite occurs in small yellowish green patches scattered ir- 

 regularly throughout the quartz, showing anomalous colors under 

 cross nicols. 



The titanite is found mainly as small irregular inclusions in 

 the glaucophane, some also being seen in the quartz. 



There is some white mica with the quartz. In ordinary light 



