1907-1 



CRANDALL— SAN FRANCISCO PENINSULA. 



19 



polarized light its colors are high,, being upper second in general. 

 Groups of these olivine grains situated together would often extin- 

 guish simultaneously, showing that the original crystals of this min- 

 eral were quite large. 



Magnetite in black patches and chromitc in brownish black 

 masses were present, scattered through the serpentine and as inclu- 

 sions in the diallage. 



The pyroxenes showed marked wavy extinction, due to the 

 amount of pressure and movement to which they have been sub- 

 jected. 



The serpentine appears yellowish and greenish in ordinary light 

 and surrounds the grains of olivine and in lattice work forms 

 through the pyroxenes. It seems stained with iron to a consider- 

 able extent. In polarized light it is blue grey with a felted appear- 

 ance and compensatory extinction. 



Slickensided Fades. — The slickensided facies is similar to this 

 except that none of the original mmerals remain, the serpentine 

 derived from the pyroxenes being the same as that derived from 

 the olivine. 



Analyses of Serpentine. 



The fresh facies of the perido.tite from the serpentine mass of 

 the Presidio gave sections that showed the presence of no serpen- 

 tine at all. There were two main minerals present with slight 

 amounts of the same accessory minerals, as seen at the Potrero. 

 Diallage and enstatite, the same as described from the Potrero, are 

 present, the last named mineral in subordinate quantities. Xo oli- 



