26 GRAND ALL— SAN FRANCISCO PENINSULA. [January 



its Index of refraction in one direction is nearly the same as that 

 of quartz, but in another position of rotation it shows a sHght 

 absorption along the cleavage lines which are parallel to its length. 

 Under cross nicols the colors are high, at least third order, and 

 somewhat irridescent. The last named mineral andalusite, is in 

 radial aggregates with a marked cleavage parallel to the length and 

 slight cross cleavage. It is nearly colorless in ordinary light and 

 with a relief higher than that of the quartz, but lower than the 

 titanite. In polarized light its colors are low, blue grey of the 

 first order with right angle extinction. 



It is worthy of note that though this schist is quite silicious, 

 neither garnets or lawsonite have been developed. If the rock had a 

 soda-bearing feldspar it may be that in the formation of the secon- 

 dary minerals, after the soda had been removed, the remainder of 

 ■the feldspar constituents formed this aluminium silicate. 



Quartz Glancophane Schist. — The other schist to be described 

 is a quartzose glancophane schist from about one mile northeast of 

 Lake Merced. There are three minerals present: quartz, glanco- 

 phane, and white mica. The rock shows its nature in the hand 

 specimen, appearing as a hard blue quartzite with a suggestion of 

 schistosity. 



The main mass of the rock is recrystallized cloudy quartz with 

 fine aggregate texture. 



Fine needles of blue glancophane appear developed with their 

 long axes parallel to the direction of schistosity. The length of 

 these needles is considerable in proportion to their width. Measure- 

 ments gave a maximum length observed as .18 mm. with an average 

 of .10 mm. The average thickness was about .01 mm. with a maxi- 

 mum of about .05 mm. This shows that these needles are mostly 

 thinner than ordinary rock section, so that pleochroism and interfer- 

 ence colors were, for the most part, much obscured. The pleo- 

 chroism of this glancophane was from colorless to violet to blue 

 with an absorption scheme of c > h > Qi. The extinction angle c : c' 

 is quite small, a maximum of 8° being observed. Ilmenite and 

 pyrite are present in very minute cubes. The cloudy appearance 

 of the quartz may be caused by the dissemination of these fine 

 particles of iron throughout. 



