522 GEOLOGY OF THE BLACK HILLS. 



netite can be seen, it having been altered to limonite. There are through 

 the slide a few veins of a partly fibrous mineral, having the appearace of 

 filling up a fissure; and in some parts of the section an indistinct fluid-like 

 structure was observed. 



The rock [187] from Grold Ledge, Warren Peak, is felsitic, compact 

 and porcelain-like in structure and appearance. On a fresh fracture it is 

 light-gray in color, mottled with pale-yellow spots of a resinous luster, and 

 some black spots. It weathers to an iron-red or brown, so that large parts 

 of the rock have the various intermediate shades from white, through flesh- 

 color, to dark-red. A few large, opaque white sanidin crystals can be seen, 

 which are exceedingly hard, as if altered to some siliceous mineral. The 

 yellowish spots, and also some of a bluish- white, milky color, resemble 

 chalcedony or opal, which minerals occur in trachytic rocks. In the thin 

 section, the felsitic groundmass remains cloudy and not at all transparent 

 and in it are seen dusty and cloudy sanidins, partially altered to a radiated, 

 brightly polarizing chalcedony or quartz mass. Some of the smaller, rect- 

 angular shaped ones are entirely changed to this chalcedony, which affords 

 a dark or colored cross in polarized light. This cross revolves and 

 changes color, following the motion of the prisms, and is due to the radi- 

 ated or fibrous structure of the chalcedony. The sanidin crystals are 

 quite cloudy and have lost all their crystalline characteristics except the 

 external form, which can still be recognized against the dark groundmass 

 by reason of their slight transparency. They do not polarize at all, remain- 

 ing dark between crossed nicols. Twinned crystals can be seen occasion- 

 ally, and in one case the alteration to chalcedony has begun at the twin 

 line. A few reddish spots and blotches of oxide of iron are scattered through 

 the rock sometimes surrounding a sanidin crystal, but no other mineral 

 constituents were observed. The rock is evidently trachytic in nature and 

 might be called a felsitic rhyolite, with chalcedony or opal as a result of 

 infiltration or secretion. The silica was determined to be 79.89 per cent, 

 much higher than any of the other Black Hills rhyolites. This rock, and 

 the following one [192], occurs, according to Mr. Jenney's report, p. 47, in 

 a "ledge of irregular shape, without any well defined Avails or boundaries, 

 and merging on all sides in the adjacent trachyte rocks." From the fact of 



