514 GEOLOGY OF THE BLACK HILLS. 



These crystals are strongly dichroitic, changing from greenish-brown to 

 green, and especially so when cut about parallel to the base, as is shown by 

 the more distinct cleavage lines. All of the large crystals have very marked 

 fissures and lines, some of which, however, arise from lamination, but in 

 some the cleavage lines at the angle of 124° can be seen, these being the 

 most strongly dichroitic. They have inclusions of small sanidin crystals, 

 large grains of magnetite and some glass masses. Hornblende is also 

 present in very abundant small, prismatic, or blade-shaped crystals, thickly 

 distributed through the groundmass. Their terminations are quite rough, 

 forked and ragged, and not nearly so sharp as the similar crystals in [123]; 

 they are but faintly dichroitic. Grouped with two large hornblendes and 

 a mass of magnetite was observed a colorless mineral, irregular in shape, 

 and with quite a different structure from the sanidin. It polarizes faintly, 

 and is probably some zeolite or calcite, but its exact nature was not 

 determined. 



The groundmass is feldspathic, polarizing white with crossed nicols, 

 and appears to be an aggregation of small, needle-like feldspar crystals, in 

 which are imbedded the small hornblende prisms. This whole mass shows 

 a very plain fluid-like structure, as if it had flowed around the large sani- 

 dins in streams and become suddenly congealed, thus preserving the unmis- 

 takable signs of the former fluidity of the rock. This appearance is not so 

 marked as in [123], but sufficiently plain to be easily noticed under the 

 microscope. The silica was determined to be 60.77 per cent., and the 

 amount dissolved in acid was 15.33. This large percentage of soluble min- 

 erals may be due to the abundant hornblende and partly also to the pres- 

 ence of some zeolite or calcite. 



The rhyolite [166] from the north peak of Inyan Kara is of a gray 

 color, compact and homogeneous in structure, with a few lustrous sanidin 

 crystals visible to the naked eye. It resembles the rocks [147] and [152] 

 from the same locality. An examination of the thin section under the 

 microscope revealed its composition to be of small sanidin crystals, with 

 magnetite and dark-brown biotite needles, the latter being almost opaque. 

 Occasionally, a very large and clear sanidin is seen having the zonal struc- 

 ture mentioned before. The small crystals are of sharp outline and dis- 



