Coal and the coal mines

by Homer Greene

The animal life of the Carboniferous era was confined almost entirely to the water. The dry land had not yet begun to produce in abundance the higher forms of living things. There were spiders there, however, and scorpions, and centipedes, and even cockroaches. There were also land snails, beetles, locusts, and mayflies. Reptiles, with clumsy feet and dragging tails, prowled about on the wet sands of the shore, leaving footprints that were never effaced by time or the elements, and are found to-day in the layers of the rocks, almost as perfect as when they were formed, millions of years ago. But the waters teemed with animal life. On the bottom of the shallow seas lay shells and corals in such abundance and variety that from the deposits of their remains great beds of limestone have been formed. Broken into minute fragments by the action of the waves and washed up by the sea during periods of submergence, they were spread over the beds of carboniferous deposits, and became the rock strata through which the drills and shafts of to-day are sunk to reach the veins of mineral coal.

Fishes were numerous. Some of them, belonging to species allied to the modern shark, were of great size, with huge fin spines fully eighteen inches in length. These spines have been found as fossils, as have also the scales, teeth, and bones. Complete skeletons of smaller fishes of the ganoid order were preserved in the rock as it hardened, and now form fossil specimens which are unequaled in beauty and perfection.

Besides the fishes, there were the swimming reptiles; amphibian monsters, allied to the ichthyosaurs and plesiosaurs which were so abundant during the Reptilian age that followed. These animals are known as enaliosaurs. They attained great size, being from twenty-five to fifty feet in length; they had air-breathing apparatus, and propelled themselves through the water with paddles like the paddles of whales. Their enormous jaws were lined with rows of sharp, pointed teeth, and their food was fish, shell-fish, and any other kind of animal life that came within their reach. They devoured even their own species. Living mostly in the open seas or fresh-water lagoons, they sometimes chased their prey far up the rivers, and sometimes basked in the sunshine on the sands of the shore. Frightful in aspect, fierce, and voracious, they were the terror and the tyrants of the seas.

Such were the animals, such were the plants, that lived and died, that flourished and decayed, in the age when coal was being formed and fashioned and hidden away in the crust of the earth. That the fauna and flora of to-day have few prototypes among them should be little cause for regret. There was, indeed, hardly a feature in the landscape of the coal era that would have had a familiar look to an inhabitant of the world in its present age. In place of the hills and valleys as we have them now, there were great plains sloping imperceptibly to the borders of the sea. There were vast marshes, shallow fresh-water lakes, and broad and sluggish rivers. Save by isolated peaks the Rocky Mountains had not yet been uplifted from the face of the deep, and the great West of to-day was a waste of waters. In the wide forests no bird’s song was ever heard, no flashing of a wing was ever seen, no serpent trailed its length upon the ground, no wild beast searched the woods for prey. The spider spun his web in silence from the dew-wet twigs, the locust hopped drowsily from leaf to leaf, the mayfly floated lightly in the heavy air, the slow-paced snail left his damp track on the surfaces of the rocks, and the beetles, lifting the hard coverings from their gauzy wings, flew aimlessly from place to place. In seas and lakes and swampy pools strange fishes swam, up from the salt waters odd reptiles crawled to sun themselves upon the sandy shore or make their way through the dense jungles of the swamps, and out where the ocean waves were dashing, fierce monsters of the sea darted on their prey, or churned the water into foam in savage fights with each other.

But in all the world there were no flowers. Stems grew to be trunks, branches were sent out, leaves formed and fell, the land was robed and wrapped in the richest, most luxuriant foliage, yet the few buds that tried to blossom were scentless and hidden, and earth was still void of the beauty and the fragrance of the flowers.

CHAPTER IV.
HOW THE COAL BEDS LIE.

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.