CHANGE OF HORIZON.
Figure 105 represents another class of faults found in all coal-fields, but to a greater extent in beds which have been forced from their normal condition than where the original conditions exist. For instance, a lower bed may be moved from its horizon by contraction or volcanic causes, and form an uneven horizon of strata; the consequence is that the over lying strata would conform, in a measure, to its moulding influences; or the deposited sedimentary strata may be swept away by currents and shifted from point to point, thus forming their arenaceous beds much thicker at one locality than another, even in coal-fields which have not been changed from their normal conditions, such as our Western coal-fields generally.
The tendency in faults of this character is to depreciate the coal in cha racter and extent; but they do not generally exert their influence to any great extent on the overlying seams.
As represented in the engraving, the deposits of slates are thicker on the lower than on the upper portions of the coal, and, consequently, the natural tendency would be to return to the uniform horizon of formation, which can only be changed, as before stated, by violence or some irregular process.
Figure 106 is another form of fault, arising from the causes explained in connection with the preceding figure. It is an expiring anticlinal, which may have been of much greater extent at a greater depth. Faults of this character are known generally as "horsebacks," and "troubles." They are, like the foregoing, frequent in all coal formations, and, we pre sume, result from the same causes. Sometimes they are extensive, and tunnels are required to be cut through rock hundreds of feet to connect the seams ; and it is a difficult matter to tell whether the faults are simply changes of horizon, or horsebacks, and proof is necessary, by following the seam, before the permanent workings can be continued. Horsebacks, however, are not so troublesome to the miner as an extensive change of horizon : in the former case a tunnel will connect the plane of the coal, but in the latter inclines must be used in flat seams ; in pitching -seams a simple change of the course of the "gangway" or heading will obviate the difficulty. If an up-throw, the course changes towards the top of the seam ; if a down-throw, it changes towards the bottom.
In the nearly horizontal coal-fields of the West, these rolls or horsebacks are frequent, but generally more gentle in angle, though more extensive in scope, than the one represented. They exist also in an inverted position, or they form bellies, or " swellies," in the coal-strata, instead of horse backs or saddles. These faults—if faults they may be called—or irregu larities are more troublesome to the miners than the former, because in the first the coal can be obtained without trouble in contending with water, but in the latter it cannot, since a basin instead of a saddle is formed.
Figure 107 belongs to the class of irregularities illustrated in 105 and 106, but is the result of quite a different process, and is peculiar to granitic or volcanic formations. In the Richmond coal-field an anticlinal axis is known as a trouble ; but there these axes are local and occupy short lines, or are anticlinal cones instead of anticlinal ridges.
The illustration we give in figure 107 is from the Richmond (Virginia) coal-field, where such formations are frequent, as shown in the sections accompanying our description of that field. There is not much evi dence of contraction or subsequent crust-movement in the Richmond coal measures, but the unequal and irregular granite base upon which those measures were deposited influences the strata to a great extent. Though the basins are frequently over a thousand feet deep, the granite peaks pro ject through the measures, and often appear at the surface. Generally, however, they are of less elevation; but their frequent occurrence seriously interferes with mining operations. We have known several instances in which deep and costly shafts have been sunk on the crown or apex of one of those "troubles," and which were, consequently, unavailable. It is a very difficult matter for inexperienced men, or, in fact, miners generally, to locate properly in this field on unexplored lands. But an anticlinal always shows its dip at some distance from its axis; and the prudent will seek such evidence before risking years of labor and large outlays of money.
We find the coal in those deep Richmond basins increasing in thickness towards the synclinal axis, or both the sedimentary deposits of arenaceous and argillaceous strata and the coal are thicker in the basins than on the dips, while on the troubles or anticlinals there is scarcely a sign of coal, and the measures are correspondingly thin. The inference, therefore, is that the present is their original and normal condition, and that the coal was deposited as it now remains.
It is natural to suppose that sedimentary deposits should tend to the deepest parts of a basin ; and here we find it exemplified. Were the pre sent form of the anthracite basins their original or normal condition, we might expect to find nearly all our coal in the deep basins ; but, as such is not the case, we find the best and most productive seams in moderate or medium basins, rather than in the deepest.