24 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



Titanite is present and appears somewhat similar to the spinel 

 in ordinary light except that it is very faintly pleochroic from color- 

 less to yellowish brown. Under cross nicols it may be distinguished 

 from the spinel by its high interference colors. In this rock much 

 of the hornblende has altered to chlorite, the latter being present 

 as pseudomorphs after the former. It is pleochroic from yellow 

 to green and hardly distinguishable from the hornblende in ordinary 

 light. Under cross nicols, the hornblende shows second order colors, 

 while the chlorite is greenish or blue with varying extinction. This 

 is more clearly exemplified in the rock of the north side of Visita- 

 tion Valley than in this one. 



VII. Schists. — The schists are all of a blue amphibole (glauco- 

 phane) variety. There are various grades from a schistose sand- 

 stone to a fine glaucophane rock. 



Typical of this group is a schist from one mile east of Baker 

 beach along the Strait. It shows nothing but fibrous masses of 

 pleochroic blue amphibole, without any other minerals. The hand 

 specimen is soft, blue, talcose looking rock, that might easily be 

 mistaken for serpentine, but there was no serpentine present in the 

 slides. This rock may be an altered clay shale. 



Petrography of the Schists. 



Basic Glaucophane Schist. — From the beach along the Strait a 

 mile east of Baker beach come two other schists, one basic the 

 other acid. The basic schist is a very dark rock and does not show 

 its schistose nature. The slide contained the following minerals : 

 hornblende, glaucophane pseudomorphed after hornblende, lawson- 

 ite, pyrite, and ilmenite. The hornblende was present only in rem- 

 nants, the main part of the crystals being glaucophane. The pleo- 

 chroism of the hornblende was from yellowish green to dark green 

 but much mantled by the strong blue and violet of the glaucophane. 



The elongation of the glaucophane was parallel to c'. The pleo- 

 chroism was from colorless to violet, to blue, with an absorption 

 scheme c > b > cr. The extinction of the glaucophane varied only 

 a few degrees from parallel while that of the hornblende in the same 

 crystal was about 13°. 



Lawsonite is present in rectangular prisms with elongation paral- 



