. THE NONMETALLIC MINERALS. 215 



deeper and found cryolite all the way. Johnstrup, as quoted by Dana, 1 

 describes the cryolite as: 



Limited to the granite; he distinguishes a central and a peripheral part; the former 

 has an extent of 500 feet in length and 1,000 feet in breadth and consists of cryolite 

 chiefly, with quartz, siderite, galena, sphalerite, pyrite, chalcopyrite, and wolframite 

 irregularly scattered through it. The peripheral portion forms a zone about the cen- 

 tral mass of cryolite; the chief minerals are quartz, feldspar, and ivigtite, also fluor- 

 ite, cassiterite, molybdenite, arsenopyrite, columbite. Its inner limit is rather sharply 

 denned, though there intervenes a breccia-like portion consisting of the minerals of 

 the outer zone enclosed in cryolite; beyond this it passes into the surrounding granite 

 without distinct boundary. 



Cryolite in limited quantity occurs at the southern base of Pike's 

 Peak, in Colorado, and north and west of St. Peter's Dome (Specimen 

 No. 48220, U.S.N.M.). It is found in vein-like masses of quartz and 

 microcline embedded in granite. 



Uses. Until within a few years the material has been utilized only 

 in the manufacture of soda, and sodium and aluminum salts, and to a 

 small extent in the manufacture of glass and porcelain ware. It is also 

 used in the electrotytic processes of extracting aluminum from its ores, 

 as now practiced. 



The principal works utilizing the Greenland cryolite in chemical 

 manufacture are, at time of writing, those of the Pennsylvania Salt 

 Manufacturing Company at Natrona, Pennsylvania (see series of 

 crude and manufactured products Nos. 6332T to 63334, TJ.S.N.M). 



IV. OXIDES. 

 1. SILICA. 



QUARTZ. The mineral quartz, easily recognized by its insolubility 

 in acids, glassy appearance (Specimen No. 67985, U.S.N.M.), lack of 

 cleavage, and hardness, which is such that it readily scratches glass, 

 is one of the most common and widely disseminated of minerals. 

 Chemically it is pure silica, of the formula SiO 2 . It crystallizes in 

 the hexagonal system with beautiful terminations, and is one of the 

 most attractive of minerals for the amateur collector (Specimen No. 

 61768, U.S.N.M.). The common form is, however, massive, occurring 

 in veins in the older crystalline rocks (Specimen No. 55244, U.S.N.M.). 

 Common sand is usually composed mainly of quartzose grains which, 

 owing to their hardness and resistance to atmospheric chemical agen- 

 cies, have withstood disintegration to the very last. 



The terms rose, milky (Specimen No. 62381, U.S.N.M.), and smoky 

 (Specimen No. 67986, TJ.S.N.M.) are applied to quartzes which differ 

 from the ordinary type only in tint, as indicated. Chalcedony is the name 

 given to a somewhat hornlike, translucent or transparent form of silica 

 occurring only as a secondary constituent in veins, or isolated con- 

 cretionary masses, and in cavities in other rocks. Agate is a banded 



System of Mineralogy, 1892, p. 167. 



