296 Turner— Bock-forming Biotites and Amjphiholes. 



probably feldspar. The impurities, However, form such a 

 minute portion of the material that the analysis may be con- 

 sidered as sufficiently correct for use in calculating the con- 

 stituents of the granitic rock of which it forms a part. The 

 mica is quite black in- color, as seen with a hand lens. Under 

 the microscope it is a darker brown than the biotite of the 

 biotite-granite. This biotite is characteristic of the granodio- 

 rite and quartz-diorite series, and of the quartz-monzonite. It 

 contains more magnesia than any other of the biotites analyzed 

 but nearly the same as amphibole 2652, also from quartz-mon- 

 zonite. Dr. Hillebrand examined specimen 1751 and the 

 biotite from it for vanadium. He found that the rock con- 

 tained *012 per cent of V 2 3 and the biotite *066 per cent. 

 Quartz-monzonite 1751 is composed of plagioclase>quartz> 

 orthoclase>biotite>amphibole. There are present as acces- 

 sories titanite, apatite, and iron ore. 



Biotite No. 1743 S.N. is from a pyroxenic gneiss from the 

 south bank of the North Mokelumne River, about 1 klm. up 

 stream from the mouth of Bear River, in the Big Trees quad- 

 rangle. The material analyzed was found, on microscopic 

 examination, to contain occasional, grains of amphibole, but 

 these probably do not sensibly affect the result. This biotite is 

 reddish-brown in color and quite similar to the biotite so often 

 found in contact metamorphic rocks. Dr. Hillebrand subse- 

 quently tested the material for vanadium and found gneiss 

 1743 contained '08 per cent Y 2 3 , and the biotite from it *127 

 per cent. Gneiss 1743 is composed of plagioclase, reddish- 

 brown biotite, greenish-brown amphibole, quartz, pyroxene, 

 titanite, magnetite, apatite, and pyrrhotite, the relative abund- 

 ance being approximately indicated by the order in which the 

 constituents are enumerated. 



Biotite No. 2652 is from a quartz-monzonite from a block by 

 the Tioga road southeast of Mt. Hoffmann in the Mt. Dana 

 quadrangle. The amphibole 2652 analyzed is from the same 

 block, and both were separated as well as analyzed by Dr. 

 Hillebrand. The block above referred to was far richer in 

 amphibole and biotite than is usually the case with the quartz- 

 monzonite of the region. There is thus a possibility that these 

 minerals may differ somewhat in composition from the same 

 minerals where they occur evenly distributed through the 

 magma. An examination of the powder analyzed shows on 

 one slide a very few foreign particles, namely, apatite and 

 amphibole. The apatite should of course be deducted from the 

 analysis as well as the water below 105° C. Dr. Hillebrand 

 found that the mica contains no rare earth, zinc, or metals of 

 the H 2 S group. The specific gravity of biotite 2652 at 21° C. 

 is 3*05. Quartz-monzonite 2652 was not analyzed, but for 



