1907.] 



CRANDALL- SAN FRANCISCO PENINSULA. 13 



morphism of the tuff is plainly seen in the presence of a dike of 

 diabase which appears to have a width of at least one hundred feet. 

 Further south on Railroad avenue the presence of much broken 

 and contorted jaspers determine the proper horizon of the tuffs. 



The specimens for this work were taken from the new Southern 

 Pacific railway tunnel through the hill on Railroad avenue. 



In a slide the main part of the rock is a dark green mass that 

 does not react to either ordinary or polarized light. Tests with 

 hydrochloric acid show that it is mainly amorphous silica stained 

 with iron. Set in this groundmass are clear irregular shaped 

 crystals some of which are quartz and some are feldspars. Plagio- 

 clases showing banded twinning and wavy extinction are present 

 and the measurement of extinction angles, though not entirely accu- 

 rate, gave about 18°. Rock fragments were seen in one slide and 

 these were micro-perthite, intergrowth of orthoclase and albite. This 

 with the presence of the untwinned feldspars led to the conclusion 

 that the twinned feldspars belong to the albite group, although the 

 extinction angle is rather high, due perhaps to inaccuracy in meas- 

 uring a feldspar with wavy extinction. No large amount of ferro- 

 magnesian minerals is present, small amounts of a brown mica 

 being the only ones observed. 



The quartz of each grain present has been recrystallized into 

 minute aggregate texture, the average size of the new grains being 

 about .02 mm. in diameter. Small patches of the same dark blue 

 shale are present that were noted in the sandstone, as are also 

 secondary quartz and calcite veins. 



This tuff that has just been described is from the Silver Terrace 

 hills ; those from Baker's beach region show the presence of two 

 other minerals, chlorite or a chlorite-like mineral and irregular 

 masses of pyrite. 



The chlorite is present in long fibrous seams and irregular 

 patches. It is strongly pleochroic from dark green to yellowish 

 green. In one slide this mineral appears along a seam in the rock 

 in the form of fibrous spherulites ranged on both sides of the seam, 

 and growing toward the center. Under cross nicols it shows anom- 

 alous colors, Berlin blue, yellow and brown, with a wavy extinction 

 that is approximately parallel. 



