500 GEOLOGY OF THE BLACK HILLS. 



very brilliantly, bat are irregular in form and have inclusions and irruptions 

 of the groundmass, as well as black specks and some rectangular crystals 

 of an undetermined mineral. Magnetite occurs in small grains, but very 

 sparingly, and mica and hornblende are present only in a very few crystals. 

 The groundmass is colorless and quite transparent, polarizing with crossed 

 nicols in white irregularly shaped grains in a dark ground, this latter being 

 the isotropic portion. There are also some needle-shaped, white microlites. 

 The absence of any defined crystalline structure, and the partially glassy 

 character of the groundmass is remarkable and different from the other rocks. 



The rock [127] from the same locality has more the characteristic 

 appearance of rhyolite. It is very compact indeed and slightly banded in 

 color, which is a light-gray, inclining to greenish. The banding resembles 

 faint traces of a fluid-like structure, although this was not observed in the 

 section. Scattered through the rock are macroscopical crystals, colorless, 

 but very brilliant; part of these are sanidin and some are quartz, the dis- 

 tinction being made by the presence or lack of cleavage, sometimes, how- 

 ever, difficult to observe. A few dark crystals of mica are also to be seen. 



In the section, it consists of large, clear quartzes, with numerous small, 

 rectangular sanidin crystals in a fine-grained groundmass, with biotite and 

 some hydrated oxide of iron films. The silica present is 73.22 per cent., 

 this large amount being due to the quartz. 



The sanidin occurs occasionally in large rectangular crystals, but it is 

 mostly in small ones, quite uniformly scattered through the groundmass. 

 They do not polarize very brilliantly, and in some cases contain microlites 

 and glass inclusions. They may be distinguished from quartz by being 

 less transparent and more uniform in shape. The quartz is exceptionally 

 fine in this section, some of the crystals being twinned so that one crystal 

 with crossed nicols will be brilliant, while the other remains dark, making 

 the line of twinning very marked. There are two such examples in the 

 slide, one of which is shown in Plate I, Fig. 1. It contains a few cavities 

 and microlites, but not to any marked extent. There are also small grains 

 and crystals through the groundmass. 



The biotite is in its characteristic, yellowish-brown prismatic crystals, 

 showing very plainly the lines of lamination. They are strongly dichroitic, 



