W. Cross— Hypersthene-andesite. 141 
so situated that the ortho-axis falls in the plane of the section. 
In all of the sections prepared from the various specimens of 
uffalo Peaks ‘“‘augite-andesite,” a majority and usually a large 
majority of the prismatic sections of pyroxene gave extinction 
parallel to the vertical axis. 
Believing that the optical behavior indicated the presence of 
rhombic pyroxene in this andesite, and bearing in mind the 
results obtained by Fouqué in his researches on the Santorin 
lavas, an attempt was made to confirm the microscopical deter- 
mination by the isolation and analysis of the questionable 
mineral. The method of procedure adopted was the same 
which was used by Fouqué.* A specimen of rock was chosen 
in which according to the microscopical diagnosis the rhombic 
yroxene predominated but slightly over the other, which will 
be called augite for convenience, though much of it differs 
optically from that mineral, as has been described. After 
being suitably crushed, the rock-powder was treated with strong 
ydrofluoric acid until ‘all but the iron-bearing minerals, pyrox- 
ene and magnetite, were dissolve e latter mineral and 
-such crystals of pyroxene as contained inclusions of it were 
then extracted with a magnet. The microscopical exam- 
ination of the residue showed more clearly than ever that two 
distinet minerais were Pe resent, the one markedly pleochroic 
was found to consist almost slang of the pleochroic mineral 
with but a very small amount of the ene This residue was 
then subjected to a quantitative analys 
In this manner the apparently cy Lek mineral was isolated 
from two different rocks from Buffalo Peaks, and the operation 
was repeated for one of them. Both the isolation and the 
t y 
Fouqué for the Eerie of a Santorin lava; VI hypers- 
thene from Labrador 
‘“Santorin, et ses éruptions,” Paris, 1879, p. 190, 
*F. Fo 
Se gyn : ses éruptions, p. 
tJ.D. Dana. System of Mineralogy, p. 210. 
