512 GEOLOGY OF THE BLACK HILLS. 



groundmass exhibits the peculiar half-fibrous and half-crystalline appear- 

 ance of the rhyolites, as if it consisted of microlitic needles of sanidin 

 between the larger but still indistinct crystals, which only betray their 

 form by being of a faint whitish color with crossed nicols. 



The sanidin-trachyte [157] from the hill west of Inyan Kara has the 

 most characteristic appearance of a normal trachyte of any of the rocks 

 examined. Its color is light grayish-white, with occasional reddish and 

 brownish spots and cavities, which have the shape of biotite crystals, the 

 latter having disappeared and left only a little magnetite. At first sight 

 these cavities seem to be black needles of mica, but a closer examination 

 shows that they are empty holes. An occasional glassy sanidin may also 

 be seen. The microscopical examination of the thin section showed some 

 very beautiful and interesting sanidin crystals in a crystalline groundmass 

 of feldspar, with magnetite and a little biotite. It resembles [149], but 

 has no hornblende. The amount of silica is 01.88 per cent. 



The sanidin crystals are quite transparent, but are colored in irregular 

 blotches and spots in polarized light, owing to their composite character. 

 They include colorless needles of apatite, considerable magnetite and 

 biotite, the needles being sometimes grouped in a sort of star around a 

 grain of magnetite. The larger and more perfect crystals are quite rect- 

 angular in shape, and have the usual zonal structure of sanidin. The inte- 

 rior portion of the crystal has a very perfect form, being terminated Avith 

 the base and hemi orthodome, while the exterior, rectangular form is made 

 by an additional growth of feldspar in a different optical position, or of a 

 different nature. The result is that the interior, well formed crystal polarizes 

 a violet color, while the exterior shell or coating is deep blue, making a 

 distinct contrast. In other cases it is yellow and blue. There were also 

 observed some very nice twin crystals, both from interpenetration and juxta- 

 position. The most interesting occurrence is that of a large sanidin crystal, 

 long and rectangular in shape, which has been broken by a fragment of 

 feldspar so as to cause the larger piece to be pushed out of place. The 

 former continuity of this crystal is shown by the fact that the two pieces 

 polarize a fine deep blue at the same time, while the irregular fragment is 

 yellow. By revolving the analyzer or slide the colors change — the crystal 



