BLACK HILLS GEOLOGY. 269 



Biotlte. — This mineral is present in the more trachytoid varie- 

 ties of phonoHte as occasional dark flakes and may also be ob- 

 served in the tinguaites. One variety of trachytoid phonolite 

 deserves special mention. It occurs as a sill in the Cambrian 

 shales about half a mile below Maurice in Spearfish Canon. 

 The rock is light gray, has a greasy lustre, and can be seen to 

 contain innumerable flakes of biotite. Some of them attain a 

 diameter of three-eighths of an inch. The microscope shows 

 the rock to consist of an almost granular aggregate of twinned 

 feldspars amongst which an occasional nepheline can be de- 

 tected. Through this mass are scattered shredded crystals of 

 aegirine, the usual larger crystals of aegirine-augite and numer- 

 ous irregular crystals of biotite. 



The biotite is in deep red-brown flakes, and is very pleochroic. 

 Around it is grouped in a thick felt-like mass, often equal in 

 diameter to one-half the width of biotite, a maze of aegirine 

 microlites. Mixed in with these are longer, shredded aegirines, 

 and an occasional terminated crystal. Besides the biotite and 

 also surrounded by a coating of irregularly piled microlites are 

 masses and hexagonal cross-sections of an isotropic mineral. It 

 is filled with minute dusty inclusions of unknown character, and 

 with crystals of aegiHne, and is perforated through and through 

 by long needles of the same mineral. It is absolutely isotropic, 

 showing not the slightest indication of cross twinning and when 

 viewed between crossed nicols letting no light through, except 

 where doubly refracting inclusions are present. It is generally 

 hexagonal in outline, but occurs also in irregular blotches. It 

 is identical with the isotropic mineral discussed under leucite. 



Magnetite. — Magnetite occurs but sparingly in the phonolite- 

 trachyte series. It is, however, seen in the phonolites from 

 Maurice and Ragged Top, in considerable quantities. In the 

 latter rock it occurs in irregular masses, with characteristic color 

 and lustre. 



Titanite. — Titanite occurs, as an almost invariable accessory 

 in nearly all of the rocks of this series. It is generally in long, 

 tabular, lath shaped crystals and exhibits the usual high relief 

 and cleavage. It is often included in the aegirine-augite, and 

 feldspar phenocrysts. The crystals are frequently twinned. 



