[907.] 



CRANDALL— SAN FRANCISCO PENINSULA. 



27 



White mica is present, intergrown with the quartz, and is notice- 

 able by the marked difference of rehef in one position of rotation. 

 In polarized light the colors are third order or higher. 



Small veins of secondary quartz appear in the section. These 

 show the same aggregate texture and wavy extinction as the other 

 quartz, but may be distinguished in ordinary light by their clear 

 and colorless appearance, as contrasted with the clouded appearance 

 of the quartz of the rock proper. Without exception the size of the 

 individuals of the aggregates is much larger than those of the other 

 quartz. The average size of the grains of quartz in the main rock 

 mass was about .02 mm. in diameter. 



Further careful search may reveal schists not mapped, but such 

 areas would not aft'ect the structure of the region as worked out. 

 The relations of the schists and other rocks will be considered 

 further on in the paper. 



III. Areal Distribution. 



In the areal distribution the geologic order of terranes followed 

 in the descriptive text is not adhered to. The distribution is best 



Fig. 9. 



Recent sand dunes near Golden Gate Park. These are derived from the 

 Pliocene and Pleistocene deposits. 



