198 SMITH— PARAGENESIS OF MINERALS. [Octobers. 



Stones, glaucophane schists, in lawsonite gneisses and schists, and 

 in an altered quartz diorite, in which lawsonite and crossite are the 

 only secondary minerals, the feldspars being only slightly altered in 

 patches and the hornblende changed to crossite only around the 

 borders. 



Feldspars. — Orthoclase occurs rarely in the glaucophane rocks, 

 and only in the acid schists and gneisses, for what little potash was 

 in them has usually gone into the formation of muscovite. 



Albite is very abundant in both acid and basic schists and 

 gneisses, although it was almost never present in the original rocks. 

 In the albite gneisses of Berkeley and Angel Island it makes up a 

 large part of the rock, and in many others it occurs in veins and 

 scattered through the mass. Its genesis is due to the decomposition 

 of the plagioclase molecule, and to the crystallizing out of. the 

 excess of sodium aluminum silicate. It is not present in any cases 

 where members of the glaucophane group have not been formed, 

 which is sufficient proof that its presence indic'ates excess of soda. 



Oligoclase has been observed in a quartz glaucophane gneiss 

 near San Luis Obispo, but it was in small grains, and it was not 

 possible to determine whether the grains were original fragments. 



Labradorite, which is usually characteristic of igneous rocks, 

 has been observed in considerable cjuantities in veins in a glauco- 

 phane eclogite from the Junction School-house near Healdsburg. 

 It was determined by its extinction angles on the twinning planes, 

 the orientation of its interference figure, and the character of its 

 double refraction, all agreeing with labradorite. A qualitative test 

 showed the presence of both lime and soda. Its occurrence in 

 veins showed clearly that it is secondary. 



Zircon, etc. — Zircon has been observed in minute crystals in the 

 albite-crossite gneiss of Berkeley. Sillimanite occurs in the quartz- 

 glaucophane schists of Catalina Island, and in similar rocks near 

 San Luis Obispo. Cyanite, which is said to be a common companion 

 of glaucophane in the glaucophane schists of Europe, was found 

 in California only in a diopside-glaucophane eclogite on the San 

 Francisco Peninsula. The rock was a small piece of float, and its 

 original locality is unknown. 



Micas. — No micas are known in the original rocks from which 



