PHYSIOGRAPHY. 109 



Pitchstone. 



duce a very remarkable composition. Vesicular : the cav- 

 ities often elongated in one direction, parallel, and in such 

 number, that the mass appears fibrous, and of a pearly 

 lustre. 



1. The varieties of Pitchstone have generally been treated of under 

 four species : viz. Obsidian, Pitchstone, Pearlstone, and Punn'ce. Obsi- 

 dian possesses the most perfect conchoidal fracture, and high degrees of 

 a pure vitreous lustre. A variety of Obsidian which is transparent, is 

 called Marekanite. When the high perfection of the conchoidal frac- 

 ture disappears, and we meet with an uneven or splintery fracture, its 

 lustre at the same time diminishing and passing into resinous, we are 

 presented with the variety Pitchstone. Pitchstone often contains those 

 faces of distinct concretion, which arise from composition. When these 

 are numerous, variously curved, and containing but little matter be- 

 tween them, a transition into Pearlstone is formed ; the peculiarity of 

 which depends upon the roundish masses into which it separates, and 

 which generally allow themselves to be resolved into thin films, not unfre- 

 quently including a grain of Obsidian. The Obsidian itself is often vesi- 

 cular, 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 color disappears, and there is pearly 

 or silky lustre in one direction. Thus Pumice is generally formed. 



2. Before the blow-pipe, these varieties melt with more or less facility, 

 according to the fusibility of their ingredients, into a vesicular glass, or 

 they yield an enamel. 



3. Analysis. 

 By DESCOTILS. By BEUTHIER. By KLAPROTH. 



Silica 



Alumina . 



Potash . -. > 



Soda . . > 



Ox. iron and mang. 2 00 



Lime 



Magnesia 



Water . 



VOL.11. 10 



