352 



RULIFF S. HOLWAY 



is strongly pleochroic as follows: a, light greenish-yellow; b, ohve- 

 green; C, greenish-blue. The orientation is the same as in glauco- 

 phane the b axis corresponding to the axis of symmetry and the 



extinction angle of C 

 large for hornblende. 



on c' is nearly 25°. This angle is unusually 

 Hintze/ however, gives some still greater. 

 His table of extinction angles 

 for hornblende varies from 

 16° to 27° 30', the percent- 

 age of AI2O3 varying at the 

 same time from 1.89 to 

 20.73. ^ black hornblende, 

 pargasite, has an extinction 

 angle, C on c' , of 24° 30' 

 the AI2O3 varying from 

 11.92 to 13.75 per cent. A 

 light green pargasite is given 

 with a slightly higher ex- 

 tinction angle, and also a 

 higher percentage of alu- 

 mina. The^pleochroism is 

 not given in the same table, 

 but a few pages later he gives the following for a light pargasite: 

 a, g eenish-yellow; b, emerald-green; C, greenish-blue. It should 

 be noted that pargasite is sometimes blue and appears to be usually 

 soda-bearing, as the analyses quoted by Hintze show. 



Blasdale^ describes a hornblende from the vicinity of Berkeley 

 with a pleochroism of C, bluish-green; b, yellowish-green; a, 

 lighter green. The extinction angle is given as 14° 34', and the 

 percentage of soda 2.45 and of alumina 2.05-3.45. The glaucophane 

 in the Calaveras hornblende eclogite is along the borders of the horn- 

 blende crystals, and is apparently of a later growth. A similar 

 instance of such a change is described by Cross,^ who found blue 

 hornblende upon the ends of a brown hornblende occurring near 

 Silver Cliff, Colo. 



1 Handbuch der Mineralogie, Vol. II, p. 1188. 



2 " Contributions to Mineralogy," Bulletin of the Department of Geology, University 

 of California, Vol. II, p. 328. 



3 "Some Secondary Minerals of the Amphibole and Pyroxene Groups," American 

 Jotirnal of Science, Vol. XXXIX (1890), p. 359. 



Fig. 5. — Hornblende eclogite, Calaveras 

 X 24. I = garnet, 2= quartz, 3= hornblende 

 4 =rutile, 5 = glaucophane. 



