ROCK FAULTS.
Figure 108 illustrates a fault frequently met in all coal-fields, and one common to the anthracite regions. It consists of a stratum of slate or rock rising from the bottom and ascending diagonally across the coal to the top, or vice versa. This is a troublesome character of fault, since it is difficult to determine whether it is a change of horizon or an " off-throw." When it comes in from the bottom and goes out at the top, the seam is below; but when it comes in from the top and goes out at the bottom, the seam is above. In the first case the miner digs down through the rock or slate to the coal, and in the latter he digs up. The dimensions of these faults are various, ranging from a few inches to several yards, but they are never very extensive ; thongh it frequently happens that hundreds of feet are driven through them in search of the missing seam, when it lies but a few yards off in a parallel course.
The causes of these faults are rather obscure, but they evidently resulted from some commotion, current, or force, which interrupted the regular course in the formation of coal, and deposited or conveyed an extra portion of sedimentary strata to such localities. An explanation of figure 109 may convey an impression of this process.
Figure 109 represents a frequent occurrence in the anthracite coal measures, and one which is peculiar to nearly all coal-seams. It consists of the enlargement of a slate parting, which may or may not belong naturally to the seam; but it generally springs from a natural parting, perhaps scarcely perceptible at first, but gradually increasing in thickness until the seam becomes parted beyond the point where it can be worked profitably as a single bed. In the anthracite regions these slate or rock partings grow from east to west: for instance, the Buck Mountain seam is thus divided by slate, which increases in thickness from east to west, until it forms two distinct beds; and the Mammoth is likewise divided into three large and separate seams in the same manner.
This, we think, is easily accounted for. Though the aggregate thickness of the sedimentary strata is greater in the east than in the west, the corre sponding thickness of the coal is much greater. The causes producing coal were more active and constant in the east than in the west : while the great Mammoth of 30 and 50 feet was in process of formation in the deep eastern basins, the growth or formation of coal was interrupted in the western portions, and sedimentary strata took its place for a season, or until the process could be restored. As a general rule, we find both the
seams of coal and the coal measures depreciating in a westward course, while the beds are frequently split; and one which may be very thick in the east may form two or three, which may be very thin, in the west. But there are also local eases of this form of division or splitting of the seams, originating, generally, from some band or stratum of slate which naturally exists in the bed. These slate bands occasionally enlarge and form a double bed from a single one; they also contract and form a single bed from a double one, if we simply trace them from localities; but if we follow the horizon of such double beds, we find them constantly changing and varying in their distance from each other. The cause can only be accounted for from the fact that the growth of coal is more limited in one locality than another, and that the sedimentary deposits are much thicker at one point than at another. As explained fully in Chapters III. and IV., we believe the sedimentary strata of the Paleozoic formations to have been derived chiefly from volcanic influences. The lava of volcanoes on being thrown in a molten state into the waters is instantly shivered to atoms, and either thrown into the air in the shape of ashes and sand, to be carried by winds to remote parts, or is disseminated through the waters, and carried by constantly changing currents to as constantly varying localities.
We represent in figure 110 three frequent forms of " rock faults," which, as far as our experience goes, are found in all coal-fields. They consist of narrow walls of rock, which cut the seam either perpendicularly or obliquely, and are from one foot to several yards in thickness. We find them extending from one seam to another through the measures, but generally only a short distance above or below the coal. They are like small trap dikes, but are invariably a coarse sandstone, having no appearance of igneous origin, and consequently must have been the result of sedimentary deposit, in the same manner and at the same time with the stratified sandstones of the coal measures. But how these isolated, narrow, and singular walls of rock were laid in long lines through the coal-seams cannot be satisfactorily explained.