216 SMITH— PARAGENESIS OF MINERALS. [Octobers. 



Sulphur Bank gneiss consists of quartz and orthoclase as a ground- 

 mass, with large prisms of glaucophane, small plates of biotite and 

 muscovite, small garnets, prisms of zoisite, and abundant titanite. 

 Ilmenite and zircon were also observed in small quantities. Some 

 plagioclase was also seen, probably albite, although it was not 

 definitely determined. 



An arkose slightly altered, which by further recrystallization 

 would have made a feldspathic gneiss similar to those described 

 above, has been described by Becker^ from the Coast Ranges. This 

 rock. No. 13, Sulphur Bank, Colusa County, is composed of quartz, 

 orthoclase, plagioclase (albite?). No glaucophane has been devel- 

 oped in this rock, and Becker does not state what the ferro-mag- 

 nesian mineral was, but it is presumably hornblende, since this 

 would be present in either a granitic or a dioritic arkose. A chem- 

 ical analysis of this rock by Melville is given by Becker along 

 with his description, and this is quoted below. 



Analysis of Arkose, No. 13, Sulphur Bank. 



SiO= 68.50 



ALO3 12.82 



FeoOs 129 



FeO 3-37 



MgO 2.27 



CaO 1.82 



Na.O 6.03 



K2O 1.26 



H,0 (above 100° C.) 2.11 



H2O (below 100° C.) 0.28 



TiOi 0.60 



P.O. 0.16 



MnO 0.02 



Total 100.47 



Albite gneisses are described from but two places in the glauco- 

 phane-bearing series. That described by Ransome^ from Angel 

 Island consists chiefly of albite with some needles of glaucophane, 

 plates of biotite, numerous very small crystals of garnet, and ir- 

 regular patches of titanite. Quartz is entirely lacking in this rock, 

 which is noteworthy, since the silica is so high. 



^Bull. Dcpt. Gcol. Univ. of California, Vol. I., p. 212. 

 2 Mon. XIIL, U. S. Geol. Survey, p. 92. 



