SILTCA. 241 



smooth and shining globular, botryoidal masses, having a 

 pearl)' lustre. 



Float Stone. A variety of opal having a porous and fibrous 

 texture, and hence so light that it will float on water. It 

 occurs in concretionary or tuberose masses, which often 

 have a nucleus of quartz. 



Tripolite, or Infusorial Earth. A white or grayish-white 

 earth, made mainly of siliceous secretions of microscopic 

 plants called Diatoms. It forms beds of considerable extent, 

 and often occurs beneath peat. It is used as a polishing 

 powder ; also to mix with nitroglycerine and make dynamite ; 

 and, owing to its poor conduction of heat, it is applied as a 

 protection to steam boilers and pipes. 



Tabaslieer is a siliceous aggregation found in the joints of 

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

 and has nearly the appearance of hyalite. 



Diff. Infusibility before the blowpipe is the best charac- 

 ter for distinguishing opal from pitchstone, pearlstone, and 

 other species it resembles. The absence of anything like 

 cleavage or crystalline structure is another characteristic. 

 Its inferior hardness and specific gravity separates it from 

 quartz. 



Obs. Hyalite occurs sparingly at the Phillips ore bed, 

 Putnam County, N. Y., and in Burke and Scriven counties, 

 Georgia. In Washington County, Ga., good fire opal is 

 obtained. The Suanna Spring in Georgia affords small 

 quantities of siliceous sinter. Tri polite occurs in Maine, 

 New Hampshire, Nevada, California, and elsewhere. 



Tridymite. Pure silica, like quartz and opal, with very nearly the 

 hardness and specific gravity of opal, but occurring in tabular hexag- 

 onal prisms, which are twins under the triclinic system. If not crys- 

 tallized opal, it is a third state of Si0 2 . It occurs in trachytic and 

 some other volcanic rocks. Asmanite is from a meteorite, and may be 

 the same as tridymite. 



JenzscMte. Silica, Si0 9 , in, it is supposed, a fourth state, it resem- 

 bling opal in aspect and in solubility in alkaline solutions, but having 

 the specific gravity of quartz, or 2 6. From Hiittenberg in Carinthia, 

 resembling a white cachalong ; from near Weissig ; Regensberg ; and 

 in Brazil. 



Melanophlogite. Colorless cubes consisting of silica, with a little 

 sulphuric trioxide and water. On sulphur from Girgenti, Sicily. 



