PETEOGRAPHY— RHYOLITE. 523 



the evident chalcedony druses in [192], it may be inferred that the brightly 

 polarizing mineral in [187] is of the same nature. 



The specimen [192], also from Gold Ledge, Warren Peaks, has an 

 exceedingly compact, jaspery or felsitic, reddish-brown groundmass, in 

 which are imbedded large, whitish sanidin crystals, quite lustrous on their 

 cleavage faces. There can also be seen small, dark colored druses, which 

 appear to be lined with a bluish-white mineral of botryoidal form like 

 chalcedony. The sanidin crystals have tolerably sharp outlines, being 

 quite conspicuous in the red mass, and, by the difference in luster between 

 two parts of the same crystal, it is noticed that some are twinned. The 

 groundmass resembles hornstone or jasper in its compactness, and has the 

 cavities of chalcedony very abundantly scattered through it. 



In the section under the microscope, the sanidin is conspicuous by 

 reason of its translucency and behavior in polarized light. The cleavage 

 lines are very dislinct, and the only noticeable inclusions are small, round 

 particles of red oxide of iron. The groundmass is of a whitish color, trans- 

 parent in places, but quite filled with small, red, ferruginous granules, which 

 occasion the color of the rock. The cavities mentioned are quite numerous 

 and are filled with chalcedony, its usual formation in layers conforming to 

 the shape of the cavity being plainly seen. In the interior of the druses, 

 between the layer of chalcedony, is some transparent, crystalline quartz, as 

 is often seen in geodes. The feldspar is more like sanidin than ordinary 

 orthoclase, having fissures and cracks and being also quite transparent, 

 while the groundmass has also a rhyolitic appearance the same as that of 

 [187], but with the addition of particles of oxide of iron. The rock is a 

 variety of rhyolite, in the cavities of which the quartz or chalcedony has 

 been secreted at the same time as the iron granules, greatly changing its 

 appearance. 



The sani din-trachyte [213] from White wood Creek, has a ground- 

 mass of a bluish-gray color, in which are numerous opaque, white feldspar 

 crystals. The rock is quite porous and full of holes, left empty by the 

 weathering of the sanidin crystals. Under the microscope, the section 

 shows a fine groundmass consisting chiefly of minute sanidins, in which are 

 a few. quite large and well defined but cloudy sanidin crystals, with mag- 



