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University of California. 



[Vol. 3. 



a tendency to idiomorphism but for the most part its outlines 

 are allotriomorphic. There may be occasionally observed a few 

 irregular cracks traversing the feldspar and these are filled with 

 a secondary colorless mineral having a very strong double refrac- 

 tion, probably a carbonate. 



The hornblende is of a prevailing olive-green color in trans- 

 mitted light, and its pleoehorism in tints of olive-green, and 

 yellowish green, while distinct is not so intensely marked as 

 is commonly the case in green hornblendes Its maximum 

 observed extinction angle is about 16? This hornblende has all 

 the characteristics of an original pyrogenous constituent of the 

 rock, there being no indication of its secondary derivation from 

 pyroxene. 



Black, opaque iron ore occurs in sparing amounts in all the 

 slides and occasionally it shows the cleavages characteristic of 

 ilmenite. It is often enclosed in the hornblende. Olivine is not 

 found in all of the specimens studied, but in those sections where 

 it occurs it is perfectly fresh, though traversed by characteristic 

 cracks. It generally has more or less rounded or elliptical oirtlines 

 and is usually surrounded by an envelope of secondary green 

 hornblende which is interpreted as a reaction rim. 



Hyperstheue, like the olivine, is variable and is usually found 

 in those sections containing olivine. It occurs partly intergrown 

 and interlocked with the original hornblende and partly in 

 allotriomorphic grains in the midst of the feldspar. It is 

 perfectly fresh and presents the characteristic optical characters 

 of the mineral. 



Titanite in sparing amount is found in a few of the slides. 

 Neither augite nor apatite were detected in any of the slides. 



Various Local Fades. — This coarse grained hornblende-gabbro 

 passes locally into a much finer textured and darker variety in 

 which hornblende and feldspar are about equally represented and 

 in which black iron ore is more abundant in small granules 

 evenly distributed through the thin section. Neither olivine nor 

 hyperstheue could be detected in this variety. The normal facies 

 of the rock also passes locally to a somewhat finer grained rock 

 which would be more properly designated an olivine-norite. 

 This facies consists chiefly of basic plagioclase, olivine and 



