Coal and the coal mines

by Homer Greene

The anthracite coal beds were in the regions of greatest disturbance, and, together with the rock strata above and below them, assumed new positions, which were inclined at all angles to their old ones of horizontality. More than this, the heat and pressure of that period exerted upon these beds of coal, which up to this time had been bituminous in character, resulted in the expulsion of so large a portion of the volatile matter still remaining in them as to change their character from bituminous to anthracite. Although the strata, in the positions to which they have been forced, are at times broken and abrupt, yet as a rule they rise and fall in wave-like folds or ridges. These ridges are called anticlinals, because the strata slope in opposite directions from a common plane. The valleys between the ridges are called synclinals, because the strata slope from opposite directions toward a common plane. One result of this great force of compression exerted on the earth’s crust was to make rents in it across the lines of strata. These rents are called fissures. Sometimes the faces of a fissure are parallel and sometimes they inclose a wedge-shaped cavity. This cavity, whatever its shape, is usually filled either with igneous rock that has come up from the molten mass below, or with surface drift or broken rock fragments that have been deposited there from above. Where there is displacement as well as fracture, that is when the strata on one side of a fissure have been pushed up or have fallen below the corresponding strata on the other side, we have what is known as a fault. Sometimes the displacement seems to have been accomplished with little disturbance to the sides of the fissure; at other times we find, along the line of fracture, evidences of great destruction caused by the pushing up of strata in this way. A fault may reach a comparatively short distance, or it may traverse a country for miles. The vertical displacement may be only a few inches, or it may amount to hundreds or thousands of feet. In the bituminous coal regions, where the strata lie comparatively undisturbed, faults are but little known. In the anthracite districts they are common, but not great.

VERTICAL SECTION THROUGH SOUTHERN COAL FIELD.

VERTICAL SECTION THROUGH NORTHERN COAL FIELD.

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.

The hills and mountain ranges that were thrown up at the close of the Carboniferous age were many times higher and broader then than they are to-day. Heat and cold and the storms of a thousand centuries, working by disintegration and erosion, have worn away their substance, the valleys and low lands are filled with it, and the rivers are always carrying it down to the sea. The peaks and the crests have been the portions of the elevations that have suffered most. It is often as though the tops of the anticlinal folds had been sliced off for the purpose of filling the valleys with them to the level of the decapitated hills. A great part of the coal measures have thus wasted away; in some portions of the anthracite district by far the greater part, including many valuable coal seams.

When a fold or flexure of the earth’s crust has been decapitated in the manner mentioned, the exposed edge of any stratum of rock or coal is called its outcrop. The angle of inclination at which any stratum descends into the earth is called its dip. The direction of a horizontal line drawn along the face of a stratum of rock or coal is its strike. It is obvious that the strike must always be at right angles to the dip. That is, if the dip is downward toward the east or toward the west, the direction of the strike must be north and south. It is now apparent that if one begins at the outcrop of a coal seam and traces the course of the seam downward along the line of dip, his path will lie down the inclination for a longer or shorter distance, until the bottom of the synclinal valley is reached. This is known as the basin or swamp. Here the seam may be comparatively level for a short distance; more often it has a mild vertical curve, and starts up the dip on the other side of the valley, which inclination may be followed till the outcrop is reached. If now the decapitated portion of the fold could be replaced in its natural position, we could trace the same seam up to and over the anticlinal axis and down upon the other side. As it is, we must cross on the surface from the outcrop to the place where the corresponding seam enters the earth. In the southern and eastern anthracite coal districts of Pennsylvania decapitation of folds to a point below the coal measures is general; the coal seams dip into the earth with a very sharp pitch, and the coal basins are often very deep and very narrow, striking into the earth almost like a wedge. In the northern or Wyoming district decapitation is not so general, the angle of inclination of strata is mild, and the basins are wide and comparatively shallow. In the bituminous districts, where the disturbance to the earth’s crust has been slight, the coal beds lie very nearly as they were formed, the dip seldom exceeding an angle of five degrees with the horizon. The exposures here are due generally to the erosive action of water.