FIG. 3.
FIG. 4.
FIG. 5.
Fig. 3 represents strata in a state called by geologists "conformable," which name is applied when the strata follow their natural or regular succession, whether this retains its horizontal position, or, as in fig. 4, assumes a position more or less vertical, which frequently happens from the subsidence of one part or the elevation of another; but they are sometimes found in the state represented in fig. 5, and in which they are said to be "unconformable." Such strata after having been tilted out of the horizontal have had other strata deposited upon them, which again may be more or less contorted from the same causes. The regularity of strata is often interrupted by what are called "faults" or "dykes," which have arisen from some part of the earth sinking down or another part being raised up, producing a fracture through all the strata and causing those on either side of this fault or fracture to occupy a situation not corresponding to those on the opposite, as in fig. 6, but yet to be found at a higher or lower elevation, and it is nearly always found that these strata are raised on that side to which the line of fracture inclines, as in the figure. These faults are often sources of great annoyance to the miner, who finds a sudden termination to the seam of coal or ore that he is working. The cracks are generally filled with some primitive rock, as basalt, rising from beneath while in a liquid state and filling up the interstices; it will often happen that the faults thus filled will have veins of the same rock branching out and not only filling up cracks but forcing its way between the various strata, thus interposing a stratum of basalt quite out of its proper position and altering, by the effects of the heat communicated, the character of the strata in juxtaposition; this is shown in fig. 7.
FIG. 6.