Turner — Rocks and Minerals from California. 421 



Art. LI. — Notes on Rocks and Minerals from California; 

 by H. W. Turner. 



[Published by permission of the Director of the U. S. Geol. Survey.] 



1. A Peculiar Quartz-amphibole-Diorite. 



In the northwest portion of the Bidwell Bar Quadrangle on 

 the dividing ridge between Butte and Plumas Counties, is 

 a dike-like streak of a peculiar grayish green rock about 

 1*2 klm. in length. The exact locality is by the trail to 

 North Valley about 7*3 kilometers south of Table Mountain. 

 The surrounding rock of the district is a granodiorite (granite- 

 diorite) having a composition intermediate between typical 

 quartz-diorite and amphibole-granite. On account of the 

 decomposed condition of the surface rock the exact contact 

 between the supposed dike rock and the granodiorite was not 

 seen, so that its dike nature was not determined with certainty. 

 However, the chemical and mineral composition of the diorite 

 is so different from that of the granitoid rocks surrounding it 

 that its dike nature is regarded as probable. 



Two specimens (Nos. 817 and 780 S.N.) of this rock were 

 collected, about 400 meters apart. A microscopic investiga- 

 tion of thin sections of the two specimens shows them to be 

 very similar. The rock is medium-grained and of granolitic 

 texture. It is composed principally of amphibole, plagioclase 

 and quartz. The most abundant constituent, amphibole, is 

 green in color and occurs chiefly in the form of short needles 

 which show idiomorphic outlines in sections transverse to the 

 prism. The amphibole is clearly original, and is imbedded in 

 later feldspar and quartz, both of which are also clearly origi- 

 nal. An analysis of No. 817 by Dr. Hillebrand (see below) 

 shows an unusually high percentage of magnesia (11 "86 per 

 cent) for a diorite. Since the rock is composed chiefly of 

 quartz, plagioclase and amphibole, it is evident that the mag- 

 nesia is chiefly in the amphibole, and as much of the alumina 

 (12*9 per cent) must be in the feldspar it was surmised* that 

 the amphibole was rich in magnesia and low in alumina, pos- 

 sibly approximating actinolite in composition. The rather 

 high content of lime (7*74 per cent) was also suggestive of a 

 amphibole rich in lime, since the plagioclase appears to be ande- 

 sine, a feldspar containing only a medium amount of lime. 

 The amphibole was therefore separated from the rock by 

 means of the Thoulet solution. An analysis made by Mr. 

 William Valentine is given below. There is too much alumina 

 (8'13 per cent) for actinolite. The amphibole may be con- 

 sidered as belonging to the aluminous series. The large 

 extinction angle (23°) likewise places it in the aluminous series. 



* Seventeenth Annual Report, U. S. Geological Survey, Part T, pp. 57-1-575. 



