Coal and the coal mines

The process of growth, deposition, submergence, and burial, described in the preceding chapter, continued throughout the Carboniferous age. Each period of inundation and of the covering over of beds of vegetable deposit by sand and silt is marked by the layers of stratified rock that intervene between, and that overlie the separate seams of coal in the coal measures of to-day. The number of these coal seams indicates the number of periods during which the growth and decay of vegetation was uninterrupted. This number, in the anthracite coal regions, varies from ten to thirty or thereabouts, but in the bituminous regions it scarcely ever exceeds eight or ten. The thickness of the separate coal seams also varies greatly, ranging from a fraction of an inch up to sixty or seventy feet. Indeed, there are basins of small extent in the south of France and in India where the seam is two hundred feet thick. It is seldom, however, that workable seams of anthracite exceed twenty feet in thickness, and by far the largest number of them do not go above eight or ten, while the seams of bituminous coal do not even average these last figures in thickness. Neither is the entire thickness of a seam made up of pure coal. Bands of slate called “partings” usually run horizontally through a seam, dividing it into “benches.” These partings vary from a fraction of an inch to several feet in thickness, and make up from one fifth to one seventh of the entire seam.

The rock strata between the coal seams range from three feet to three hundred feet in thickness, and in exceptional cases go as high as five or six hundred feet. Perhaps a fair average would be from eighty to one hundred feet. These rock intervals are made up mostly of sandstones and shales. The combined average thickness of the coal seams of Pennsylvania varies from twenty-five feet at Pittsburgh in the western bituminous region to one hundred and twenty feet at Pottsville in the eastern anthracite district, and may be said to average about one fiftieth of the entire thickness of the coal measures, which is placed at 4,000 feet.

Some conception may be had of the enormous vegetable deposits of the Carboniferous era by recalling the fact that the resultant coal in each seam is only from one ninth to one sixteenth in bulk of the woody fibre from which it has been derived, the loss being mainly in oxygen and hydrogen. It is probable that the coal seams as well as the rock strata had attained a comparative degree of hardness before the close of the Carboniferous age. It was at the close of this age that those profound disturbances of the earth’s crust throughout eastern North America took place which have already been referred to. Hitherto, through the long ages of Paleozoic time, there had been comparative quiet. As cooling and contraction of the earth’s body were still going on, there were doubtless oscillations of surface and subsidence of strata in almost continuous progress. But these movements were very slow, amounting, perhaps, to not more than a foot in a century. Yet in Pennsylvania and Virginia the sinking of the crust up to the close of the Carboniferous age amounted to 35,000 or 40,000 feet. That the subsidence was quiet and unmarked by violent movement is attested by the regularity of strata, especially of the carboniferous measures, which alone show a sinking of 3,000 or 4,000 feet. Neither were the disturbances which followed violent, nor were the changes paroxysmal. Indeed, the probability is that they took place gradually through long periods of time. They were, nevertheless, productive of enormous results in the shape of hills, peaks, and mountain ranges. These movements in the earth’s crust were due, as always, to contractions in the earth’s body or reductions in its bulk. On the same principle by which the skin of an apple that has dried without decay is thrown into folds and wrinkles, the earth’s crust became corrugated. There is this difference, however: the crust, being hard and unyielding, has often been torn and broken in the process of change. Naturally these ridges in the earth’s surface have been lifted along the lines of least resistance, and these lines seem to have been, at the time of the Appalachian revolution, practically parallel to the line of the Atlantic coast, though long spurs were thrown out in other directions, isolated dome-shaped elevations were raised up, and bowl-shaped valleys were hollowed out among the hills.

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.

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.

The carboniferous measures are the highest and latest geological formation in the great coal fields of the United States. Therefore where the strata have not been disturbed by flexure the coal seams lie near the surface. This is generally the case in the bituminous districts, and it is also partially true in the northern anthracite coal field. Deep mining is necessary only in the middle and southern anthracite coal fields, where the folds are close and precipitous, and the deep and narrow basins formed by them have been filled with deposits of a later geologic age.

Some of the difficulties to be met and overcome in mining coal will by this time have been appreciated by the reader. But some of them only. The inequalities of roof and floor, the pitching seams, the folds and faults and fissures, all the accidents and irregularities of formation and of location, make up but a few of the problems which face the mining engineer. But the intellect and ingenuity of men have overcome most of the obstacles which Nature placed in the way of successful mining when she hardened the rocks above her coal beds, crowded the earth’s crust into folds, and lifted the mountain ranges into the air.

It will not be out of place at this time to make mention of those localities in which coal is found. Indeed, there are few countries on the globe in which there are not carboniferous deposits of greater or less extent. Great Britain, with Ireland, has about 12,000 square miles of them. In England alone there is an area of 8,139 square miles of workable coal beds. In continental Europe the coal fields are numerous, but the character of the deposit is inferior. Coal is found also in the Asiatic countries, in Australia, and in South America; and in Nova Scotia and New Brunswick there is an area of 18,000 square miles of coal measures. The combined areas of coal measures in the United States amount to about 185,000 square miles. The Appalachian or Alleghany region contains about 60,000 square miles, included in the States of Pennsylvania, Virginia, West Virginia, Maryland, Ohio, Kentucky, Tennessee, Georgia, and Alabama. The Illinois and Missouri region contains also about 60,000 square miles, and has areas not only in the States named, but also in Indiana, Iowa, Kentucky, Kansas, and Arkansas. Michigan has about 5,000 and Rhode Island about 500 square miles. There are also small areas in Utah and Texas, and in the far West there are workable coal fields in Colorado, Dakota, Indian Territory, Montana, New Mexico, Washington, Wyoming Territory, Oregon, and California. The entire coal area of the United States, with the exception of that in Rhode Island and a few outlying sections in Pennsylvania, contains coal of the bituminous variety only. Both the area and supply are therefore practically without limit. In the coal regions of Rhode Island the disturbances affecting the earth’s crust have been very violent. The motion, heat, and compression have been so great as to give the rocks associated with the coal measures a true metamorphic or crystalline structure, and to transform the coal itself into an extremely hard anthracite; in some places, indeed, it has been altered to graphite. The flexures of the coal formation are very abrupt and full of faults, and the coal itself is greatly broken and displaced. Its condition is such that it cannot be mined with great profit, and but little of it is now sent to market. The only areas of readily workable anthracite in the United States are therefore in Pennsylvania. These are all east of the Alleghany Mountains, and are located in four distinct regions. The first or Southern Coal Field extends from the Lehigh River at Mauch Chunk, southwest to within a few miles of the Susquehanna River, ending at this extremity in the form of a fish’s tail. It is seventy-five miles in length, averages somewhat less than two miles in breadth, and has an area of one hundred and forty square miles. It lies in Carbon, Schuylkill, and Dauphin counties. The second or Western Middle field, known also as the Mahanoy and Shamokin field, lies between the eastern headwaters of the Little Schuylkill River and the Susquehanna River. It has an area of about ninety square miles, and is situated in the counties of Schuylkill, Columbia, and Northumberland. It lies just north of the Southern field, and the two together are frequently spoken of as the Schuylkill Region. The Eastern Middle or Upper Lehigh field lies northeast of the first two fields, and is separated into nine distinct parallel canoe-shaped basins. These extend from the Lehigh River on the east to the Catawissa Creek on the west, and comprise an area of about forty miles. They are principally in Luzerne County, but extend also into Carbon, Schuylkill, and Columbia counties. The Northern or Wyoming field is a crescent-shaped basin about fifty miles long and from two to six miles broad, with an area of about two hundred square miles. Its westerly cusp is just north of the Eastern Middle field, and it extends from that point northeasterly through Luzerne and Lackawanna counties, just cutting into Wayne and Susquehanna counties with its northern cusp. It lies in the valleys of the Susquehanna and Lackawanna rivers, and in it are situated the mining towns of Plymouth, Wilkes Barre, Pittston, Scranton, and Carbondale. There is also a fifth district, known as the Loyalsock and Mehoopany coal field, lying in Sullivan and Wyoming counties. It is from twenty to twenty-five miles northwest of the Wyoming and Lackawanna field, its area is limited, and its coals are not true anthracite.

It will thus be seen that aside from this last field the anthracite coal area of Pennsylvania contains about four hundred and seventy square miles.