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University of California Publications in Geology [Vol. 13 



The feldspars range from oligoclase to anorthite. Under the micro- 

 scope they are seen to be almost invariably twinned according to the 

 albite law. Combinations of albite and carlsbad or albite and pericline 

 twinning are frequent. 



The hypersthene, to the unaided eye, has rather light colors, in- 

 eluding pale brown and pinkish tints. These colors would indicate 

 enstatite, but microscopic examination shows the mineral to have 

 always a negative sign. The pleochroism is buff to pink for the fast 

 ray, very pale green for the slow ray. In many of the thin sections 

 examined a mineral was seen having all the properties of hypersthene 

 except that it seemed to show oblique extinction. A more careful 

 examination showed that this was not oblique extinction, but sym- 

 metrical extinction, in which one of the two sets of cleavage lines was 

 poorly developed. There is no reason then to regard this mineral as 

 other than hypersthene. It is possible that "a pleoehroic augite pre- 

 cisely like the hypersthene, but with a distinct extinction angle," de- 

 scribed by Coleman 13 from the Sudbury norites, is of the same nature. 



The augite appears black in the hand specimen. Under the micro- 

 scope it shows a maximum extinction angle of 55°, is colorless to pale 

 buff, and frequently is filled with an opaque dust. 



The brown hornblende appears black to the unaided eye. Its micro- 

 scopic properties are as follows : Pleoehroic : o = colorless, faint green, 

 but generally pale buff; 6, c = dark shades of brown; absorption, 

 b>c>tt. The sign is negative, the extinction angle 28°, the maximum 

 birefringence .026. 



In hand specimens entirely unaffected by weathering, the olivine 

 appears colorless or very faint green, and is transparent. With in- 

 creasing weathering the olivine becomes yellowish and finally red. In 

 thin section the olivine is colorless or very faint green. Its sign is 

 negative. 



The green hornblende appears dark green megascopically. Under 

 the microscope it shows the following colors : a = greenish yellow or 

 pale buff ; h = dark brown-green, dark green or light brown-green ; 

 c = dark green to pale yellow green; absorption, 6>c>a. The maxi- 

 mum extinction angle is 30°, the sign negative. 



Besides these essential minerals there are certain minor constit- 

 uents. Apatite needles are found in almost all the rocks. Green spinel, 

 pleonaste, is frequently present, associated with the magnetite. No 

 picotite or chromite was noted. Every specimen of basic rock ex- 

 amined contained either pyrrhotite or magnetite or both. The biotite, 



is Coleman, A. P., Eept. Bur. Mines Out., vol. 14, pt. Ill, 1905, pp. 115-116. 



