328 DESCRIPTIONS OF MINERALS. 



low flame, and lias hence been used as candles — whence the 

 name. It affords when heated a large amount of mineral 



oil. and may be used for its production. The volatile in- 

 gredients sometimes amount to 50 or 00 per cent. It is 

 often made into inkstands, snuff-boxes, and other similar 

 articles. 



4. Brown Coal usually has a brownish-black color, and 

 contains 15 to 20 per cent, of oxygen, but much resembles 

 in appearance bituminous coal. The term brown coal is ap- 

 plied generally to any coal more recent in origin than the 

 era of the great coal beds of the world. The name lignite 

 has sometimes the same general application, though without 

 strict propriety. Lignite is the part of brown coal which 

 has the woody structure still apparent. 



Jet resembles cannel coal, but is harder, of a deeper black 

 color, and has a much higher lustre. It receives a brilliant 

 polish, and is set in jewelry. It is the Gagates of Diosco- 

 rides and Pliny, a name derived from the river Gagas, in 

 Syria, near the mouth of which it was found, and the origin 

 of the term jet now in use. 



Native Coke resembles somewhat artificial coke, but is 

 more compact, and some varieties of it afford a consider- 

 able amount of bitumen. It occurs at the Edgehill mines 

 near Richmond, Virginia, according to Genth, who attri- 

 butes its origin to the action of a trap eruption on bitumi- 

 nous coal. 



It is now well established that mineral coal is mainly of 

 vegetable origin, and that the accumulations out of which 

 the coal beds were made were very similar in character, 

 though not in kinds of plants, to the peat beds of the pres- 

 ent day. Peat is vegetation which has undergone, in part, 

 the change to coal; and in some cases it has become brown 

 coal. The conditions of change are somewhat different from 

 those of the beds of good coal, since, in the case of the peat, 

 the air has access, while in that of the coal the air was more 

 or less excluded by overlying strata : and the more perfect 

 the exclusion, other things equal, the better the coal. As 

 the composition of mineral coal is closely related to that of 

 mineral oils, the explanation of the origin of the latter, given 

 on page 323, suffices to illustrate also the origin of the 

 former. With a less complete exclusion of the air, oxygen- 

 ated hydrocarbon compounds, like coal, would be a natural 

 result. 



