154 COAL AND COAL-FORMATIONS. 



in situ) being associated with sedimentary strata without 

 its being mingled more or less with the earthy impurities 

 of these sediments. These impurities, according to their 

 amount, must necessarily confer on different coals different 

 structures, different aspects, arid different qualities. Be- 

 sides, varieties will also arise from the conditions of the 

 vegetable mass 4 itself, according as it may have been im- 

 bedded while fresh or been long exposed to atmospheric 

 decay, according as it may have been suddenly covered up 

 or long exposed to maceration and comminution in water, 

 and notably also according to the nature of the plants 

 composing the mass. These varieties, according to their 

 structure, texture, and qualities, are generally known as 

 coking-coal, which is soft and tender in the mass, like that 

 of Newcastle, and swells and cakes together in burning ; 

 splint or slate coal, which is hard and slaty in texture, 

 like most Scotch coals, and burns free and open cannel or 

 parrot coal, which is compact and jet-like in texture, spirts 

 and crackles when thrown suddenly on the fire, but when 

 ignited burns with a clear candle-like flame, and from its 

 composition is chiefly used in gas-manufacture ; and coarse, 

 foliated, or cubic coal, which is more or less soft, breaks up 

 into large square blocks, and contains in general a large per- 

 centage of earthy impurities. Between these varieties there 

 is, of course, every gradation coals so pure as to leave only 

 one or two per cent of ash, others so mixed as to yield from 

 ten to thirty per cent, and many so impure as to be unfit 

 for fuel, and so to pass into shales more or less bituminous.* 



* As bituminous shales are now so extensively mined for the distillation 

 of paraffin, it may be of use to advert to some distinctions that subsist be- 

 tween them and the coals properly so called. A coal, though often contain- 

 ing a considerable amount of earthy impurity, consists chiefly of vegetable 

 matter, or, in other words, carbon is its prevailing ingredient. Where 

 the earthy or mineral ingredient greatly exceeds the organic, it become's 

 unfitted for combustion, and is regarded merely as a carbonaceous stone, 



