I 



510 



GEOLOGY OF THE BLACK UILLS. 



ite. Ill a few instsinces the plane of section is })arallel to the base, when 

 it is not at all dichroitic. It is not so plentiful as the hornblende, which is 

 distin«i;uished from it by its fine green color in almost every case. There 

 are a few large plagioclase crystals, which have quite sharp bands of color, 

 but they are so much grown together with the sanidin that it is difficult to 

 discern any dividing lines between the two. From the preceding descrip- 

 tion, it is readily understood how abundant the magnetite is through the 

 whole rock, giving at almost every point its bright luster. Long, colorless 

 apatite needles in considerable abundance can also be recognized without 

 difficulty in the groundmass. 



The groundmass is coarsely crystalline and composed of irregularly 

 shaped sanidin crystals which polarize in bright colors, mixed with horn- 

 blende, biotite, and magnetite. There does not appear to be any isotropic 

 glass mass between the above ingredients. The percentage of silica was 

 found to be 60.51, which, with the observations already noted, determines 

 this rock to be undoubtedly a normal sanidin-trachyte. 



The sanidin-trachyte [150] from a locality five miles northeast of 

 Inyan Kara has a much duller and more weathered look than [149], being 

 also less crystalline in its structure. The color is dark-gray, mottled with 

 greenish spots, Avhich have disappeared on exposed surfaces, leaving empt}' 

 cavities. 



In the section, it is observed to consist of sanidin and hornblende in a 

 somewhat fibrous groundmass with but very little magnetite. The sanidin 

 crystals are very much confused with the surrounding mass and ragged in 

 theu" outlines, but occasionally there are some twins. In the simple crys- 

 tals, cloudy streaks resemble the dusty-looking parts of the groundmass. 

 From the variation of color in the large masses of sanidin it is quite easy 

 to make out their polysynthetic nature, being built up of a number of small 

 crystals. The hornblende is in fragments and small, sharply defined crys- 

 tals, quite dichroitic and sometimes showing the lines of cleavage. 



The groundmass consists of cloudy or dusty masses and crystals of 

 sanidin, which only faintly betray their outlines with crossed nicols The 

 rock is similar to [124]. 



The sanidin-trachyte [151] from the north peak of Inyan Kai'a 



