1915- No. 4- REPORT ON ROCK-SPECIMENS. II 



shown: magnetite, apatite, zircon, biotite, muscovite, plagioclase (oligoclase), 

 orthoclase and quartz. The oligoclase is in great excess over the ortho- 

 clase, which is partly solidified simultaneously with quartz. Biotite is 

 the prevailing muscovite. 



According to the descriptions quoted the granites from Florence, Mass., 

 and from Rocklin, Cal. are very closely allied with the plagioclase-granite 

 from Mt. Betty. 



The above mentioned specimen of a pure potash graiiitc is more 

 coarse-grained and shows microcline as the chief felspar. The content 

 of quartz is less than in the ordinary type; the mineral is shown 

 as irregular grains in microcline and interstital between the lelspars. 

 Plagioclase (oligoclase) occurs very sparingly either in groups of small 

 grains between the microcline-crystalls, or small grains bordering the sec- 

 tions of microcline. Sometimes oligoclase is replaced by pretty myrmekite 

 (= vermicular quartz). Small crystals of apatite are not uncommon in and 

 between the felspars. Of biotite only very few small scales are seen. 



Gneiss. The gneissic rocks from Mt. Betty are dark gray, fine-grained, 

 — sometimes very fine-grained — rocks with well-marked schistosity. On 

 the cleavage-planes scales of biotite and muscovite occur. 



The microscopic examination has shown that the gneiss-specimens can 

 be referred to two different groups : 



a) gneiss with granitic mineral composition. 



b) gneiss with tonalitic mineral composition. 



The mineral constituents of the granite-gneiss are: microcline, oligo- 

 clase, quartz, biotite, muscovite, zircon, apatite and magnetite. 



Microcline is the prominent felspar. The sections show poicilitic inter- 

 growth with small scales of biotite and small rounded grains of quartz. 

 Along the borders mvrmekite is often developed. ' 



Oligoclase occurs very sparingly. Only small grains are seen, rather 

 irregularly distributed. The composition is AbsAn^. 



Quartz is abundant. The larger grains are crushed and show undulatory 

 extinction. 



Biotite is ver}- abundant, arranged in parallel bands. The asorption 

 II ß and y is strong (dark reddish-brown). The biotite is partly altered 

 into chlorite. 



Muscovite: is present in smaller amount than the biotite (about ^0). 



The presence of zircon is shown by numerous pleochroic halos in the 

 biotite. 



Apatite and niagnetitc are very sparingly present. 



