W. Cross—Hypersthene-andesite. 139 
Art. XVI—Communications from the U. S. Geological Survey : 
Rocky Mountain Division. 
Ill. On Hypersthene-andesite ;* by Wutrman Cross. 
In the course of the investigation of some apparently normal 
augite-andesites, of the most typical variety, occurring at the 
Buffalo Peaks in South Park, Colorado, the writer found that 
a large part of the pyroxenic constituent possessed the crystal- 
line form and chemical constitution of hypersthene rather than of 
augite. The comparative study of similar andesites from this 
‘country and from well-known European localities has forced 
him to the conclusion that in very many, if not in all of them, 
augite is decidedly subordinate to a rhombic pyroxene, which 
is presumably hypersthene. As this conclusion, if proven to be 
correct, affects materially the current classification of andesitic 
rocks, the grounds upon which it is based will be concisely 
Stated. 
Hypersthene-andesite from Buffalo Peaks, Colorado. 
The rock in question occurs in mass, associated with a 
number of other andesites, prominent among which is a normal 
hornblende-andesite, and too in fragments imbedded in an 
important series of voleanic tufas, also of andesitic character. 
For farther information concerning ae rocks, the reader must 
be referred to the forthcoming “ Report upon the Geology and 
Mining Industry of Leadville, Culorado,” by S. F. Emmons. 
The rock with which we are now occupied is very compact, 
almost black in color, showing Se at a large number 
of small glassy feldspars, and a few dark, green grains. The 
ground-mass in which these crystals lie has a dull vitreous 
luster. When examined under the microscope in ordinary 
light the rock seems to be an augite-andesite of very typical 
composition and structure. Clear plagioclase crystals an 
pyroxene in small crystals and irregular grains, with magnetite 
and apatite, are the only mineral constituents to be recognized. 
These larger individuals lie in a ground-mass composed of deli- 
cate staves of plagioclase, light green microlites of ee 
and minute octahedra of magnetite, with a glass ba t 
them, which is usually a though seek Geis devitrified oy 
poke brownish globulite 
8. “Geological Fol Its publication sth dcaty bee ae she’ for sev eral 
0 
