CHAPTER III. 



IRREGULARITIES ix COAL SEAMS. 



Coal seanis may have their regularity interfered with by 

 causes prior to their formation ; while they were being formed ; 

 or subsequent to their formation. The first case is illustrated 

 by irregularities in the original floor. The second by wash- 

 outs in the seam due to ancient creeks; also by partings and 

 bands due to floods and submergence. The third case is illus- 

 trated by rolls, which may thin and thicken the coal locally, 

 due to earth stresses; also by faults and dykes, or laccolites. 

 The throw of a fault may be insignificant, or may be over 

 two hundred feet, necessitating long stone-drifts, or even new 

 shafts, and a material alteration in the laying out of a mine. 

 In the case of intrusion by igneous rocks, the coal may be 

 coked or cindered over a considerable area. The difference be- 

 tween coked and cindered coal is that the former retains about 

 two or three per cent, of volatile hydrocarbons, while the 

 latter is not much better than an ash. Cindered coal, besides 

 being worthless, has a further disadvantage, inasmuch as it is 

 often harder to work than the dyke rock itself. This dyke 

 rock in the Southern coalfield, so frequently met with in some 

 of the collieries, is generally considered to be a dolerite, but 

 it is so altered, and is usually bleached by the coal, that it has 

 not yet been properly determined. 



In some districts coal seams are much disturbed by faults. 

 As seams are generally more horizontal than vertical, coal 

 miners look for the displaced portion overhead or underfoot, 

 and call it an up-throw or down-throw accordingly. Such 

 faults may be divided into two sorts, normal and reversed. The 

 former is shown in Fig. 15, where that portion of the seam on 

 the hanging wall side of the fault is lower down than that 

 on the footwall side. If the fault has a flat angle, it may 

 have the effect of causing a considerable area parallel to the 

 strike of the fault to be wanting in coal, as at (a). With a re- 

 versed fault, as shown in Fig. 16, the seani on the hanging 

 wall side of the fault is higher than that on the footwall side ; 

 the seam overlaps itself, so that if a borehole be sunk through 

 the seam near the fault, it would appear as if there were two 



