496 GEOLOGY OF THE BLACK HILLS. 



the .sanidin. Tlio plaj^ioclase is quite fresh and has distinct lines of twin 

 himelhition, its crystals being both prismatic and rectangular in shape. It 

 is also occasionally grown upon the extremity of a sanidin in such a manner 

 as to form one large crystal together; an occurrence noted by Zirkel as 

 quite frequent. Some of the sanidin, but oftener the plagioclase crystals, 

 have a dark border or rim entirely around the edge, as if from decomposi- 

 tion. 



The quartz is not as distinct and easy to determine as the feldspars, 

 but is present in comparatively large, roughly shaped hexagonal crystals, 

 quite transparent and containing an abundance of microlites sharply 

 defined in the clear mass. These masses do not polarize very brilliantly 

 between crossed nicols, except upon turning the slide. It is also in irregular 

 masses and small grains, all having the microlites, the smaller grains giving 

 brighter colors in polarized light. 



Hornblende is very sparingly present in small bright-green crystals. 

 Some brown films of oxide of iron were also noted. 



The groundmass consists of an aggregation of fine microlites, interlaced 

 and piercing the innumerable small grains of sanidin and quartz, so that 

 with a high power the needles may almost be separately distinguished. 



The sanidin-trachyte [113] from the ridge east of Terry Peak is quite 

 different from the preceding. It is of a decided greenish color, with yellow- 

 ish-brown ferruginous veins through it. In the section, the sanidin is in 

 long, coluumar and almost needle-shaped crystals, which make up the 

 greater part of the rock, but which are not very pellucid. Some are rectangu- 

 lar and twinned, but all are quite dusty in appearance. The brown masses 

 may be remains of a mica, as they polarize somewhat and are quite trans- 

 parent The crystalline form of the sanidin is quite distinct in a few of the 

 larger crystals, and around the only large group of sanidins a fluid-like 

 structure in the arrangement of the needle-shaped crystals and microlites 

 is very apparent, being also seen in other parts of the section. The silica 

 is lower than in [109], being 58.33 per cent. The groundmass is composed 

 of grayish, miiuite microlites, in which are imbedded the slender feldspar 

 crystals mentioned above. 



The rhyolite [120] from the hill southeast of Terry Peak has a flesh- 



