THEIR ECONOMIC IMPORTANCE. 155 



Besides these varieties, founded chiefly on mineral charac- 

 ters, it is also customary to distinguish coals according to 

 the purposes for which they seem best suited, or to which 

 they are most frequently applied; hence we hear of household 

 coals, furnace coals, smithy coals, steam coals, gas coals, oil 

 coals, and similar distinctions. 



We have thus occurring in the crust of the earth not 

 only a great variety of coaly substances, but also coals of 

 different aspects and qualities occurring in the same geolo- 

 gical formation. The causes of these differences are, in 

 general, sufficiently obvious : age, and the amount of chemi- 

 cal change to which they have been subjected ; the amount 

 of earthy impurities commingled with them during their 

 aggregation and deposition ; the nature of the plants com- 

 posing the bulk of the mass ; the amount of decay which 

 the vegetable mass had undergone before it was finally 

 covered by other strata; and the porous or retentive nature 

 of the strata between which it is imbedded. All these and 

 other causes have tended to create the differences that now 

 exist among the different members of the Coal Family the 



of which clay, sand, and the like form the main proportion. The term 

 shade, on the other hand, refers to structure rather than to composition, 

 and is something that splits up or peels off in thin layers or laminae. Most 

 consolidated muds are characterised hy this quality of splitting or break- 

 ing up in thin leafy layers parallel to their bedding ; hence shales may be 

 regarded as consolidated muds, and may be distinguished as calcareous, 

 arenaceous, or bituminous according to their predominating ingredient. 

 Bituminous shales, therefore, have been mere vegetable muds their rich- 

 ness, like those of the coals, depending upon the amount of organic matter 

 and the conditions under which it was preserved. Some shales may be as 

 bituminous as some poor varieties of coal, but this does not entitle them 

 to be ranked as coals, any more than an excess of earthy matter in a hard 

 stony coal would entitle it to be called a shale. The terms refer to struc- 

 ture rather than to composition ; and though it is true that the shaly or 

 leafy structure is almost invariably characteristic of the earthier ingre- 

 dient, yet it must ever be borne in mind that both shales and coals are 

 mixed rocks, and that not unfrequently the one may pass into the other 

 by insensible gradations. 



