302 HISTORICAL GEOLOGY. 



of purity ; 2. On degrees of bituminization ; 3. On the 

 relative proportion of fixed and volatile matter. 



1. Varieties depending- on Degrees of Purity. 



Coal consists of combustible and incombustible matter, 

 or ash. The combustible matter is organic, the ash min- 

 eral. Now, the relative proportion of these varies in 

 every degree. The purest coal may contain only 1 to 2 

 per cent, of ash ; but coal may contain 5 to 10 per cent., 

 20 to 30 per cent., 50 to 60 per cent., and so on to 90, 95, 

 99 per cent. ash. If a coal contains not more than 5 per 

 cent, ash, it is probably pure, i. e., its ash is wholly due 

 to ash of original vegetable matter ; but if it contains 

 more than 10 per cent., it is certainly impure, the excess 

 being due to mud deposited with the vegetable matter. 



2. Varieties depending on the Degrees of Bitu- 

 minization. Coal may be pure, and yet imperfectly 

 bituminized. Such are lignites, brown coal, and the like. 

 This depends mainly on age, the oldest coals being most 

 completely changed. 



3. Varieties depending on the Relative Propor- 

 tion of Fixed and Volatile Matter. In pure and per- 

 fect coal there are still varieties depending on the relative 

 amount of fixed carbon and volatile hydrocarbon, and it 

 is mainly this which produces the varieties of good coal, 

 and determines its various uses. If the coal contains only 

 5 to 10 per cent, volatile matter, it is called anthracite, 

 which is a hard, lustrous variety, breaking with a con- 

 ch oidal fracture, and burning with very little blaze, but 

 great heat. If it contains 15 to 20 per cent, of volatile 

 matter, it is called semi-bituminous, or steam-coal, because 

 of its excellence in rapid formation of steam. It burns 

 with a long blaze, but does not cake. If it contains 30 

 to 40 per cent., it is ordinary bituminous caking coal ; if 

 50 per cent., or more, highly bituminous, fat, or fusing 

 coal. In this series we might well put graphite, or plum- 

 bago, above anthracite ; for graphite consists of carbon 



