W. Cross—Hypersthene-andesite. 143 
Results of the comparative study of the so-called “ Augite- 
andesites,” 
The limited space of this article does not allow a full dis- 
cussion of the grounds upon which the conclusion reached by 
the writer is based. They are, however, stated at length in the 
forthcoming Bulletin. 
A careful study of thin-sections of all the “ augite ”-andesites 
of the normal type referred to above which are at the command 
of the writer, some 31 in number, and representing especially 
the rocks of Hungary and the Western United States, shows 
that a rhombic pyroxene is more abundant than augite in each 
and every one of them. This statement rests chiefly upon the 
numerical ratio found to exist between those prismatic sections 
of pyroxene, giving extinction parallel to the vertical axis, and 
those with oblique extinction. In every case examined the 
some rocks no distinctly monoclinic pyroxene could be found. 
Such evidence cannot be ignored simply because there is a pos- 
sible position for a monoclinic prism in which it cannot be dis- 
tinguished from a rhombic by the test of extinction. he fre- 
quency with which such a position is liable to occur in massive 
rocks may be fairly ascertained by testing the extinction of 
hornblende in diorite, or of augite in diabase, in the manner 
above indicated. 
It may be thought strange that the true character of the 
pyroxene, in so many well-known rocks, has so long escaped 
detection. A single instance will here be cited, however, 
which will serve to partially explain the fact. The single 
hypersthene-andesite mentioned by Rosenbusch, in his standard 
work, “Die mikroskopische Physiographie der massigen Ges- 
teine,” p. 480, has been annihilated by Dr. E. Hussak with the 
derselben steht, und die Ausldschungsschiefe zu tiber 
* Verhandl. d. k. k. geol. Reichsanstalt. 1878, p. 338. 
