MUSEUM OF COMPAllATIVE 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 ana\j'zed in the laboratory of the 

 United States Geological Survey at Washington by Dr. W. F. Hille- 



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 all)ite (microcline- 

 microperthite of Brogger) ; it appears to belong in the anorthoclase 

 group of Rosenbusch (soda-microcline of Brogger). The per cent of 

 strontia and baryta is \inusual, 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 opaqne and fibrous. 

 The augite crystals are similar to those of the fine-grained rock, and 

 have the same segirine border. Independent acicular crystals of segirine, 

 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 tlie 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 Elseolite- 



1 N. J. Min., 1865, p. 688. 



