MINERALOGY. 435 



Meissen, Saxony, and Hungary, etc., where it is used 

 for making stone walls, inclosing fields. Is a volcanic 

 product, like lava. H. = 5.5 to 6. G. = 2.1 to 2.3. 



Obsidian is volcanic glass. Vitreous, like common 

 glass, compact, often in globous crystals, translucent, but 

 smoky; seldom colorless; mostly black and gray; some- 

 times yellowish red or brown. Fracture conchoidal ; 

 very brittle. H. = 6 to 7.0. G. = 2.2 to 2.4. 

 Occurs only in volcanic regions, Island of Teneriffe, 

 Mexico, Peru, etc. Employed in making inferior kinds 

 of jewelry ; the ancient Mexicans formerly used it for 

 mirrors, knives, razors, and daggers. 



Bimstein Pumice Stone, is feldspath or feldspar in 

 constitution. Porous and sponge-like, evidently, like 

 obsidian, the scoria of a volcano. Luster vitreous and 

 silk-like ; translucent on the edges, mostly colorless, 

 often gray, dull yellow, seldom brown; is very rough 

 to the touch. H. = 6.0. G. (when broken or 

 pounded it is so light as to swim on water) = 2.19 to 

 2.20. Found everywhere in the neighborhood of the 

 volcanoes from which it has been ejected. It is used for 

 polishing mahogany, etc. ; powdered, for making filters. 



VII. Clay. Clay is distinguished not only by its 

 plastic and tenacious qualities, but also by its softness 

 and pliability. In a pure state it is white, nevertheless 

 it is often found variously colored, and forms an impor- 

 tant element in all the stratified deposits, whether of 

 soils or the primary rock formation. G. = 1.8 to 2.6. 

 The different species of clay rank thus, first, 



Pipe Clay is a pure, plastic clay, and if occasionally 

 found colored the result of iron ores among it it al- 

 ways becomes white by burning. Found in the United 



