22 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



The texture is typically ophitic, the feldspars are long and lath- 

 shaped and idiomorphic with regard to the other minerals. Twin- 

 ning is common and measurements of extinction angles in sections 

 of equal illumination gave 44°, placing them between labradorite 

 and anorthite. The feldspar shows wavy extinction in all cases. . 



Ilmenite is present in black irregular patches surrounded by the 

 derivative mineral leucoxene. The magnetite shows borders of thin 

 red translucent hematite. These both occur as inclusions in the 

 other minerals, feldspars and pyroxenes. 



Hornblende Diabase. — The hornblende facies of the hornblende 

 diabase does not show any hypersthene or malacolite, but only horn- 

 blende, idiomorphic feldspars and the accessory minerals magnetite 

 and ilmenite. Since we saw in the previous description that the 

 hornblende was derived from the hypersthene and malacolite, then 

 in this rock we may consider all the hornblende as secondary, 

 making the rock a true epidiorite. 



At a certain outcrop on the top of the Potrero hills, just east 

 of the end of Twenty-fourth and near Rhode Island streets, schis- 

 tose structure is developed so that the hand specimen shows it quite 

 plainly. In a section this schistosity is not quite as noticeable, but 

 the results of pressure are evident. There is a new mineral, some 

 blue soda-bearing amphibole, developing in long, slender, fibrous 

 needles around the edges of the hornblende. This mineral is con- 

 sidered to be glaucophane, but owing to the minute size of the 

 crystals no optical tests were attempted. In a section of horn- 

 blende that is pleochroic from yellow to green, the glaucophane is 

 pleochroic from colorless to blue. 



Intruded in the tufifs of Silver Terrace is a dike of hornblende 

 diabase or epidiorite. The texture of this rock is ophitic and the 

 constituent minerals are hornblende and labradorite feldspars, with 

 the accessory minerals, ilmenite and magnetite, as described in the 

 hornblende diabase from the Potrero. Three secondary minerals 

 are present, quartz and intergrowth of epidote and clino-zoisite. 

 The quartz is in fine seams with the usual aggregate texture. The 

 epidote and clino-zoisite occur in long fibrous or columnar aggre- 

 gates. In the seams the columnar texture is most common. The 

 crystals are small and colorless but not clear and in ordinary light 



