DIRT OR SOFT FAULTS.
There is not much difference between the slate fault and the dirt fault. They are both organic defects, and not the results of any subsequent action. The dirt faults are generally more extensive than the slate faults, and are almost exclusively confined in the anthracite regions to the red-ash seams, and differ from the crushed faults—resulting from the forces of lateral contraction—in containing no available coal. These faults generally "come in" on the top, and gradually increase until all or most of the coal disappears, and go out on top in the same manner, with an enlargement of the coal as the fault disappears.
The character of this fault is, however, variable, and its forms are changeable. In place of the coal we find a soft, carbonaceous dirt, of a plumbageous or graphitic appearance, mixed with slate or Shelly coal,— always one or the other. Some faults appear as if their carbon had escaped, as we find displayed in the outcrop of coal-seams,—the "bloom" or black dirt of which, though a part of the bed or seam, contains no coal, until confined and covered by a considerable body of overlying strata, when the coal dirt changes to coal by a gradual hardening. In other "dirt faults,"
where the slaty principle predominates, we would assign a different cause, much the as that mentioned in connection with the preceding slate fault, but with this difference;—in the slate faults the formation or growth of coal is interrupted to the extent of the fault; but in the dirt fault the growth is only checked and partial, and mixed with impurities, caused by currents, eddies, or some commotion of the waters in which it was formed.
With these remarks, we must close for the present this short chapter on faults. The subject demands more elaborate treatment, and would form an interesting subject for a volume ; but time and space admonish brevity, and we can only promise to return to the subject on some other occasion, if opportunity offer.