238 PHYSIOGRAPHY. [chap. 



directions than in others. Thus, it breaks easily along the 

 planes of bedding, which of course run parallel to the 

 general stratification of the coal-measures. The top and 

 bottom layers thus broken, commonly present dull black 

 almost sooty surfaces, and easily soil the fingers when 

 handled. But the mass of coal also splits with ease in 

 certain directions which run vertically across the stratifi- 

 cation, and the broken surfaces thus produced are generally 

 bright and smooth, and do not soil the fingers : the direction 

 along which these joints run is often known as the "face" 

 of the coal. Then there is a third set of planes at right 



Fig. 68. — Cuboidal block of coal split along natural planes. 



angles to both the other sets, and less perfect, so that the 

 fracture here is more irregular ; this direction is sometimes 

 called the " end " of the coal. On the whole, then, there 

 are three directions, perpendicular to one another, in which 

 the coal may be divided ; and it thus yields blocks more or 

 less regular in shape, and roughly resembling cubes or dies, 

 such as that shown in Fig. 68. 



The dull black substance running along the planes of 

 stratification or bedding of a piece of coal is sometimes 

 called, from its resemblance to charcoal, mineral charcoal, 

 and it is likewise known as mother of coal. This substance 



