MUSEUM OF COMPARATIVE ZOOLOGY. 231 
specific gravity in most determinations was 2.621, but went as high as 
2.623. A sufficient quantity of this feldspar was carefully selected, 
freed from visible impurities, and analyzed in the laboratory of the 
United States Geological Survey at Washington by Dr. W. F. Hille- 
brand, with the following result :— 
SiO, 62.31 
Al,O; (containing a very little iron) 22.63 
CaO 63 
SrO 57 
BaO 17 
K,O 4.79 
Na,O 7.68 
H,0 at 100° C. 16 
H,0 above 100° C. 72 
100.26 
The optical characters show the triclinic nature of this feldspar, and 
an apparent homogeneity, even with high powers, excludes its reference 
directly to a microscopic mixture of microcline and albite (microcline- 
microperthite of Broégger) ; it appears to belong in the anorthoclase 
group of Rosenbusch (soda-microcline of Brogger). The per cent of 
strontia and baryta is unusual, and only comparable to the baryta and 
strontia sanidin from the nephelinite from Meiches, analyzed by Knop.? 
The smaller feldspars occur in long lathe forms, and are more decom- 
posed than the others ; in decomposing they become opaque and fibrous. 
The augite crystals are similar to those of the fine-grained rock, and 
have the same zgirine border. Independent acicular crystals of egirine, 
and sometimes of acmite, also occur. The angular spaces between these 
minerals are occupied generally by a feebly polarizing substance, which 
gelatinizes with acid, and is evidently nepheline. In decomposing, it 
breaks up into strongly polarizing fibrous zeolitic aggregates. Sodalite 
is rare in the coarse rock, except in the apophyses, or near the contacts 
of the sheets, where it occurs in small crystals between the feldspars. 
The coarse rock under these conditions assumes the acmite-trachyte 
character of the dikes and smaller sheets. 
The following analyses of these rocks are presented here, but the 
discussion of their relations to the other alkaline rocks of the Crazy 
Mountains is deferred to the monograph in preparation. Nos. 65, 131, 
and 297, represent the Acmite-trachyte type, and No. 145 the Eleolite- 
1 N. J. Min., 1865, p. 688. 
