Hague and Iddings—Rocks of the Great Basin. 459° 
t It i 
Mixture. Hypersthene. Augite. 
SiO, 51°16 1°39 02 
1,0, 3°50 3°26 5°64 
EV), “73 73 73 
FeO 15°46 16°45 6°45 
MnO "56 56 56 
MgO 19°22 19°75 14°37 
JaO 8°84 7°31 22°60 
Ignition “42 “42 “42 
99°89 99°87 99°79 
_ Wherever the two minerals occur together, the hypersthene 
's seen to be of earlier crystallization than the augite, and, at 
the same time it undergoes decomposition much more readily, 
except typical olivine basalts and the most acidic rhyolites. It 
forms an essential ingredient in many of the hornblende-ande- 
sites, occurs sparingly in daciteand has been detected in some 
varieties of rhyolite, presenting almost as wide a range as 
augite. 
ily rsthene occurs as an essential ingredient in the rocks 
from Wishes described as augite-andesite by Mr. George F. 
Becker in his recent work on the Comstock Lode. nat 
Indeed it may be said that there is no pyroxene-andesite in 
the collection from this district in which hypersthene does not 
equal and in most cases surpass the augite in amount. It pre- 
* Geology of the Comstock Lode, p. 128. 
Am. Jour. Scr.—Tuirp Srerims, Vou. XXVII, No. 162.—Junr, 1884. 
31 . 
