108 PHYSIOGRAPHY. 



Pitchblende Pitchstone. 



3. It chiefly occurs in silver veins, and is accompanied by various ores 

 of silver and lead, and often intimately mixed with Yellow Copper Py- 

 rites and Galena. 



4. Its chief localities are Johanngeorgenstadt, Marienberg, Annaberg, 

 and Schneeberg in Saxony ; and Joachimsthal and Fribus in Bohemia. 

 In Cornwall, it has been found in the tin mines of Tincroft and Tolcarn, 

 near Redruth. 



5. It is used in painting upon porcelain, yielding a fine orange color 

 in the enamelling fire, and a black one, in that in which the porcelain it- 

 self is baked. 



PITCHSTONE. Empyrodox Quartz. MOHS. 



Regular forms unknown. Grains. 



Cleavage none. Fracture conchoidal, sometimes highly 

 perfect, sometimes less distinct. Surface, the larger grains 

 uneven and rough, the smaller ones smooth. 



Lustre vitreous and resinous. Color black, brown, red, 

 yellow, green, grey, white : none of them bright. There 

 occurs a distinct velvet-black. Streak white. Faintly 

 transparent . . . translucent on the edges. 



Hardness = 6-0 . . . 7-0. Sp. gr. == 2-395, Obsidian 

 from Iceland ; =2-212, Pitchstone from Meissen. 



Compound Varieties. Massive : composition granular, 

 strongly connected, so as to be scarcely recognizable; 

 fracture more or less perfectly conchoidal, uneven and 

 splintery. The whole mass is often traversed with sepa- 

 rating faces, which may be considered as rudiments of the 

 faces of lamellar composition : often the composition is 

 granular, thick or thin, and generally bent ; the faces ol 

 composition being smooth, and possessing pearly lustre. 

 Small grains of Obsidian are often enveloped in a number 

 of successive thin coats ; several of these are again sur- 

 rounded by other coats, and soon several times, which pro- 



