,907.] CRANDALL— SAN FRANCISCO PENINSULA. 11 



apart in a large amount of cementing material. This cementing 

 material seems partly silicious and partly ferruginous but at times 

 it is calcareous. Most of the grains appear to be quartz, but when 

 optical figures were attempted, on various sections, biaxial figures 

 were obtained showing a portion of them to be feldspars instead of 

 quartz. Plagioclase is present in all the slides and is noticeable 

 by its twinning. In one slide a large angular rock fragment was 

 observed which was composed of quartz and plagioclase. The ex- 

 tinction angle of this plagioclase gave about 18°, placing the feld- 

 spars with the acid labradorites or near the oligoclase. Dr. Becker 

 in his monograph on the coast quicksilver deposits, remarks on the 

 uniformity of this arkose throughout the state. He says the typi- 

 cal, feldspar is oligoclase with occasionally a more basic one, quartz 

 and orthoclase being in about equal amounts. 



One point of interest in connection with this sandstone is the 

 presence of feldspars. As the experiments of M. Daubree show 

 that pulverized feldspars change to kaolin very rapidly in the water, 

 the conclusion is that these rocks are shallow water deposits. 



The presence of the orthoclase and oligoclase with quartz, in 

 the slides, taken with field relations between this sandstone and 

 the Montara granite leads to the conclusion that this granite may 

 be part of this land mass, though it is not necessarily true that 

 this sandstone is all formed from the Montara granite, but from a 

 granite that possibly underlies the whole Coast Range and of which 

 this is a part. 



Small fragments of brown mica are often present, and little, 

 long flakes of black shale that appear blue black in ordinary light, 

 but which are not affected by polarized light. 



Quartz appears throughout the rock in small veins, traversing it 

 in all directions. These veins are newer than the mass of the rock, 

 but they were formed before the dynamic movements took place, for 

 they give the same wavy and banded extinction that all the quartzes 

 and feldspars do in the main rock mass. The secondary quartz 

 shows fine aggregate texture with each little crystal of the aggre- 

 gate having its own position of extinction. Secondary quartz of 

 this nature is seen in nearly all of the rocks of the Franciscan series. 



Calcite, another secondary mineral, is present and is later than 



