254 MINERAL STRUCTURE OF TRACHYTE. 



crystals of felspar are often decomposed, and the rock is sometimes 

 brecciated. North of Waggon Canon the quartz crystals in the rock 

 are very dark, and rarely visible to the eye. But at Shoshone Peak 

 the quartz grains are frequently from one-eighth to one-quarter inch 

 in diameter, and here the ground mass is sometimes so coarse that 

 the crystals may be distinguished by the eye. Dacites are also well 

 seen in the Virginia range, and there the large quartz grains contain 

 fluid inclusions, and frequently stand out prominently on the smooth 

 weathered surface. The dacite breccia, which breaks through the 

 purple dacite of this range, contains a good deal of dark-brown mag- 

 nesian mica, as well as more free quartz than the earlier rock. Be- 

 longing to the close of the eruption in this region is an apple-green 

 dacite, which is richest of all in quartz, and the grains are frequently 

 double six-sided pyramids. 



In the hills about American City, Washoe, the ground mass of 

 the dacite is brownish or greenish grey, and contains striated felspar 

 and quartz grains as large as peas. Sometimes the structure of the 

 ground mass is rhyolitic, but the rock is distinguished from rhyolite 

 by its plagioclase and hornblende. The ground mass may be minutely 

 spherulitic. Fluid inclusions occur in the felspar crystals, but are 

 absent from the quartz, and sometimes the felspar is speckled with 

 calcite. The hornblende crystals contain yellowish-green viridite, 

 rhombohedral calcite, brownish-red oxide of iron, and yellowish-green 

 epidote, which almost replace the hornblende. The circumstance that, 

 on analysis, the potash is usually equal to the soda, suggests the 

 presence of sanidine in the ground mass. 



Trachyte. 



Trachytes are felsite-like rocks, which abound in sanidine. They 

 are regarded as the volcanic equivalents of syenite, and as corre- 

 sponding to the old quartzless porphyries. The trachytes of some 

 parts of the Elkhead range, and other districts in North America, 

 have quartz as a normal constituent ; and thus, on the one hand, 

 trachyte closely approximates towards rhyolite just as the trachytic 

 rocks in "Washoe approximate towards andesite, owing to the amount 

 of their plagioclase. 



While sanidine is always the principal constituent of trachyte, it 

 is associated with plagioclase, and hornblende, biotite, or augite ; 

 or all of these minerals may be among its constituents, but they 

 always occur in relatively small quantities. Magnetite, titanite, and 

 apatite are usually associated with them, but are unimportant as con- 

 stituents of the rock. 



Sanidine commonly occurs in crystals, which show a tabular or 

 ledge-like section. The regular outer border may be absent, and 

 the entire rock occasionally may be granular. Sanidine crystals are 

 usually Carlsbad twins, but sometimes simple prisms. Baveno twins 

 are mentioned by Rosenbusch from the dark trachyte of Toreggia in 

 the Euganian Hills, and from the Puy de Dome. Sanidine is often in 



