524 



(iKOI.OCV OF THE BLACK HILLS, 



netite, and some very sparing quartz grains. The sanidins still retain their 

 nu-tangular outline, although they have become quite dusty and even 

 opacpie. With crossed nicols they polarize faintly white so as to be plainly 

 seen, a few giving })ale blue and yellow colors. One crystal, broken and 

 twinned with a second so as to resemble a geniculation, is quite interesting. 

 The fissure lines in these crystals polarize in bright colors, thus being very 

 distinct in the cloudy mass. A few brilliant grains were named quartz; 

 they are rounded and of irregular shape, containing some microlites and 

 round cavities, but may be sanidin, as some uncertainty as to their 

 nature stdl remains. The occurrence of biotite with a border of mag- 

 netite grains has been often mentioned, and in this section the border is seen, 

 but the interior is partly or entirely composed of transparent quartz. One 

 of these formations has an hexagonal shape, but they are generally pris- 

 matic. The biotite in only a few cases retains its greenish color in the 

 small fragments, around which the magnetite border is quite conspicuous. 

 Magnetite is scattered quite uniformly through the groundmass in minute 

 grains, with some large and sharply defined masses. 



The groundmass is made up of minute and almost feathery sanidin 

 crystals, which, although very indistinct, are faintly white with crossed 

 nicols, showing also very sparing traces of a fluid-like structure. The rather 

 low percentage of silica, 61.98, gave rise to the uncertainty as to the 

 quartz grains in the rock. Amount dissolved in acid is 9.93 per cent. 



The rhyolite [214] from Terry Peak (1) is fine-grained, with small, 

 white feldspar crystals. The rock is quite full of pores, which contain 

 hydrated oxide of iron, but is otherwise tolerably homogeneous. In the 

 section under the microscope, it is seen to consist of large and cloudy 

 crystals of sanidin, with large grains and masses of magnetite, brown 

 biotite and abundant quartz grains in a groundmass of very small and 

 indistinct sanidin sections. The large crystals of sanidin are in every way 

 analogous to those in [213], being cloudy and opaque while still retaining 

 a distinct outline. Many of the crystals are banded The quartz is in 

 clear grains, having brilliant colors and microlitic inclusions. The biotite 

 is mostly so decomposed that it is very brown and opaque, with, however, 

 a good crystalline form. • It is very abundant through the rock, much more 



1 I 



