Besides the great folds into which the earth’s crust was crowded, there are usually smaller folds corrugating the slopes of the greater ones, sometimes running parallel with them, oftener stretching across them at various angles. A marked instance of this formation is found in the Wyoming coal basin, the general coal bed of which is in the shape of a canoe, about fifty miles long, from two to six miles broad, and with a maximum depth of perhaps one thousand feet. Running diagonally across this basin, in practically parallel lines from one extremity to the other, is a series of gentle anticlinals, dividing the basin into some thirty smaller synclinal valleys or sub-basins. The irregularities produced by folds, fissures, faults, and partings are not the only ones with which the miner has to deal. So far we have supposed the coal seams to have been laid down in horizontal layers of uniform thickness, with smooth and regular under and upper surfaces. This is true only in a large sense. As a matter of fact each separate seam varies greatly in thickness, and its roof and floor are often broken and irregular. The beds of clay on which the deposits were laid were pushed up unevenly by the exuberant growth of vegetation from them. The action of waves and ocean currents made hollows in them, and laid down ridges and mounds of sand on them, around and over which the decaying vegetation rose and hardened. The same forces, together with the action of running streams, made channels and hollows in the upper surfaces of these beds of incipient coal, which cavities became filled by sand and gravel, and this also hardened into rock. These irregularities are found by the miner of to-day in the floor and roof of the coal seam, and are called _rolls_, _horses_, or _horse-backs_. When the coal seam thins out so rapidly that the floor and roof come nearly together, this state of things is called a _pinch_, or _squeeze_, though the latter term is more properly applied to the settling of the roof rock after the coal has been mined out. The inequalities of a coal seam that have now been mentioned, although perhaps but a small portion of those that are daily met with in the process of mining, are neverthless characteristic of the whole.