504 GEOLOGY OF THE BLACK HILLS. 



of the small crystals, but one large mass had a green, columnar microlite 

 and a few acicular ones. Some of the neph elite has become cloudy and 

 thick, probably from decomposition. In the rectangles are seen the same 

 kind of inclusions as in the hexagons, near the center in both cases, which 

 proves quite conclusively that the two forms of sections belong to the same 

 mineral, the crystals of which are short, stout, hexagonal prisms with the 

 basal pinacoid. The hexagons are sections cut parallel to the base ; and, 

 the optic axis being coincident with the vertical axis, they are consequently 

 simply refracting and dark between crossed nicols. On the other hand, the 

 rectangular sections are of a bluish-white color when the nicols are crossed, 

 so that in the dark field they are quite conspicuous. In a mass of horn- 

 blende are several of the two forms of sections lying close together which 

 show this behavior perfectly, the hexagons remaining dark, while the rec- 

 tangles become bright in striking contrast. In the illustration (Plate I, Fig 

 3) they are drawn when both are light, in order to show the outlines dis- 

 tinctly. There are also a few square nephelite sections, which polarize 

 quite brilliantly. 



The hornblende is in irregularly shaped green masses, crystalline and 

 dichroitic, but very thickly crowded with transparent nephelite crystals It 

 has the appearance of being large crystals much broken and separated, so 

 as to resemble broad bands through the rock. It is also quite abundant in 

 fine needles through the groundmass. 



Although nosite is a frequent constituent of phonolite, it was only after 

 much consideration that it was determined to be present in this rock. The 

 two particularly large and distinct sections, one shown in Plate I, Fig. 4, 

 were at first named sanidin, but a closer examination revealed the fact that 

 the fresher parts of the crystals were simply refracting, remaining dark with 

 crossed nicols, which could not be possible if they were sanidin. Again, 

 the peculiar dusty appearance, parts of the crystal being lighter that others, 

 and a, dark border around the edge are all characteristic of nosite. The 

 two sections mentioned are five-sided, different in this respect from the 

 usual forms of sanidin. They are not very transparent, except in places 

 which have been altered to a doubly refracting mineral, probably natrolite, 

 as is often the case. These altered parts are faintly bright with crossed 



