140 SILICA. 



associated with chalcedony. Much of what is so called ia 

 nothing but chalcedony ; but other specimens contain water, 

 and are allied to hydrophane. It contains also a little alum- 

 ina and adheres to the tongue. It was first brought from 

 the river Cach in Bucharia. 



Hyalite, Mutter's glass. A glassy transparent variety 

 occurring in small concretions and occasionally stalactitic. 

 It resembles somewhat a transparent gum arabic. Com- 

 position, silica 92*00, water 6*33, (Bucholz.) 



Menilite. A brown opaque variety, in compact reniform 

 masses, occasionally slaty. Composition, silica 85*5, water 

 11*0, (Klaproth.) It is found in slate at Menil Montant, 

 near Paris. 



Wood opal. This is an impure opal, of a gray, brown or 

 black color, having the structure of wood, and looking much 

 like common silicified wood. It is wood petrified with a 

 hydrated silica, (or opal,) instead of pure silica, and is dis 

 tinguished by its lightness and inferior hardness. Specific 

 gravity, 2. 



Opal jasper. Resembles jasper in appearance, and con- 

 tains a few per cent, of iron ; but it is not so hard owing to 

 the water it contains. 



Siliceous sinter has often the composition of opal, though 

 sometimes simply silica. The name is given to a loose 

 porous siliceous rock usually of a grayish color. It is de- 

 posited around the Geysers of Iceland in cellular or compaci 

 masses, sometimes in fibrous, stalactitic or cauliflower-like 

 shapes. Pearl sinter, or jiorzte occurs in volcanic tufa in 

 smooth and shining globular or botryoidal masses, having a 

 pearly luster. 



Tabasheer is a siliceous aggregation found in the joints of 

 the bamboo in India. It contains several per cent, of water, 

 **nd has nearly the appearance of hyalite. 



Dif. Infusibility before the blowpipe is the best character 

 for distinguishing opal from pitchstone, pearlstone, and other 

 species it resembles. The absence of anything like cleav- 

 age or crystalline structure is another characteristic. Its 

 inferior hardness separates it from quartz. 



Obs. Hyalite is the only variety of opal that has yet 

 been found in the United States. It occurs sparingly at the 



What is hyalite? wood opal ? siliceous sinter? tabasheer? How ifi 

 opa'l distinguished from pitchstone and quartz ? 



