i9o6] SMITH— PARAGENESIS OF MINERALS. 233 



The albite-crossite gneiss of North Berkeley may have been 

 formed from a syenitic arkose, as also the albite glaucophane gneiss 

 of Angel Island. The lawsonite gneiss from near Redwood may 

 have been a dioritic arkose, for its chemical composition is almost 

 exactly that of a quartz diorite. The albite gneiss and the lawsonite 

 gneiss both resemble somewhat in composition the altered arkose 

 described by Becker/ which is evidently made up of fragmental 

 material from a diorite or a granodiorite. 



Sandstones usually contain varying proportions of clay, partlv 

 decomposed feldspars, and ferro-magnesian minerals. The feld- 

 spars were in part orthoclase, for we sometimes find this mineral in 

 siliceous glaucophane schists. But more often we find evidence of 

 the former presence of a potash feldspar in the secondary muscovite, 

 which is almost never absent in this group. The silica has simply 

 recrystallized as quartz, whether it came from original sand grains, 

 or from the organic silica of the chert. Plagioclases are rarely pres- 

 ent in the recrystallized product, but evidence of their former pres- 

 ence is seen in the soda that went into the glaucophane, and the lime 

 and alumina that went into the garnets. Titanite is always present 

 in the recrystallized siliceous rocks, but its origin is somewhat doubt- 

 ful. The titanium probably came from titaniferous hornblendes, 

 which on recrystallization took up soda to form carinthine or glauco- 

 phane and set free titanium. No pyroxenes of any kind are found in 

 the recrystallized acid rocks. No epidote has been found in the 

 siliceous rocks of the Coast Ranges, the lime of the plagioclase hav- 

 ing invariably gone into the formation of garnet. 



The reactions of the clay shales are essentially the same as those 

 of the sandstones, except that there is usually more impurity in the 

 shales, and consequently a greater variety of minerals. Glaucophane, 

 garnet and white mica are the chief products, with subordinate 

 quartz, from sand grains. " No cyanite nor andalusite has been ob- 

 served in any of the aluminous shales, though they would be ex- 

 pected, from the preponderance of silicate of alumina, and sillimanite 

 occurs very rarely. 



^ Mon. XIII., U. S. Geol. Survey, p. 92. 



