ORDER vir. EMPYRODOX QUARTZ. 339 



pure vitreous lustre. Agreeably to the degrees of trans- 

 parency, it is divided info transparent and translucent Obsi- 

 dian, in which the paler and darker shades of tolour like- 

 wise correspond to the degrees of transparency. Transpa- 

 rent Obsidian is sometimes called Marekanite. If the high 

 perfection of the conchoidal fracture is lost, and we meet 

 with uneven or coarse splintery fracture, the lustre at 

 the same time diminishing and passing into resinous, the 

 passage is formed from Obsidian to Pitchstone. Pitchstone 

 is faintly translucent, generally only on the edges. It 

 contains often those faces which are usually called the 

 faces of distinct concretion, and which no doubt arise 

 from composition. If there are many of them, variously 

 curved, and containing but little matter between them, a 

 transition into Pcarlstone is formed, the distinguishing mark 

 of which consists of those roundish masses into which it 

 may be separated, and that generally allow themselves to 

 be resolved into thin films, not unfrequently including a 

 grain of Obsidian. The Obsidian itself is often vesicular, 

 the cavities being small, and keeping a constant direction. 

 If there are a great many of them of larger sizes, the whole 

 mass becomes apparently very light, the original colour 

 disappears, and there is pearly or silky lustre in one direc- 

 tion. Thus Pumice is generally formed ; but there are 

 also transitions from Pearlstone into it. The subspecies 

 of Pumice are the glassy, common, and porphyritic Pumice. 

 The glassy kind still betrays its connexion with Obsidian 

 by its small and imperfectly conchoidal cross-fracture. This 

 is no longer the case in the second kind ; but the whole 

 mass seems to consist of a delicate tissue of glassy fibres ; 

 the third kind con tarns imbedded crystals and grains of 

 prismatic Feld-spar, and hence assumes a porphyritic ap. 

 pearance. All these transitions may be easily observed in 

 nature in almost every collection made on a somewhat 

 larger scale, if the artifice has not been used to remove 

 those varieties by which they are produced. 



2. According to an analysis of Obsidian by DESCOTILS, 

 one of Pitchstone, one of Pearlstone, and one of Pumice 

 by KLAPROTH, the empyrodox Quartz consists of 



