458 Hague and Iddings—Rocks of the Great Basin. 
the line of Pacific Coast voleanoes, but that all the pyroxene- 
andesites examined by them from along this belt' may be 
referred to. hypersthene-andesite. ; 
ver the wide-area of the Great Basin hypersthene is found 
in nearly all varieties of voleanic rocks. Its microscopic char- 
acters are very constant and quite similar to those given for 
this mineral in the andesites of the volcanoes of California, 
Oregon and Washington Territory. It is of a light brown 
color in thin sections and with few exceptions strongly pleo 
chroic, being green parallel to the ¢ axis and_yellowish- 
brown at right angles to it. The strong pleochroism, generally 
gray or yellow color between crossed nicols and the constant 
parallelism of its extinction with the direction of the ¢ axis dis- 
tinguish it from the accompanying augite. The hypersthene 
varies somewhat in the strength of its pleochroism, which, as 
- shown by analysis, probably corresponds to a variation in its 
chemical composition. Its determination rests upon a micro- 
scopic study of the thin sections, together with optical invest 
ations upon isolated crystals showing their orthorhombic 
all the hornblende anda little pyroxene, and the other made 
of a mixture of hypersthene and augite. By repeated 
the purest hypersthene patents indicated that considerably less 
|to augite. An analysis by Mr. S. L. 
Penfield, of the Sheffield Scientific School, is given in column L. 
given a calculated theoretical composition of the hypersthene 
and augite based upon Mr. Penfield’s analysis of the mixture — 
of the two minerals. » 
- This Journal, Sept., 1883. 
