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Sources of Coal

Sources of CoalCoal is made from vegetable matter, soaked in water, buried in mud, and squeezed and dried into a brittle, black, stony substance.

It occurs in the form of broad beds or " seams," varying in thickness between a few inches and sixty feet; an average height would be about five feet. Some coal-seams extend over or rather beneath thousands of square miles, and yet are but remnants of the originals.

Coal usually rests upon a bed of hard clay, and is capped or overlaid by shale, limestone or sandstone. Most coal-beds have one or more flat layers or " bands " of shale or earthy matter running through them, thus dividing them up into two or more separate but variable layers or " benches," which benches are often composed of coal of different qualities and kinds. One may be a free-burning coal, another a gas coal, a third a coking coal.

The structure or texture of coals differ, but, generally speaking, it is laminated-full of thin, bright, brittle, black, wavy and forked streaks or patches; some short, others elongated, mixed or firmly embedded in a dull, tough, " bony " material.

In this dull stuff may sometimes be found millions of tiny, brown, flat discs (the seedcases of the coal-trees). Now all of these bright and dull, grayish and brownish filmy patches and variously shaped bodies are seen to lie flat in the coal or parallel to the earthy layers it contains.

We know that coal is decayed and decomposed remains of plants, because every test, whether made by chemists, botanists, microscopists or geologists, results in obtaining positive evidence, consisting in recognizing in it mineralized or carbonized portions of the original woody and cellular substances, corresponding in a general way with those of present-day vegetation; or in negative evidence, as pertaining to the chemical ingredients of coal, which closely resemble those of wood and of peaty materials.

Substances resembling coal have been produced artificially out of wood, in a variety of ways. But scientists are not yet agreed as to just what kind of vegetation coal is made of, nor in what manner these vast accumulations of plant remains were deposited or brought into place, and why a coal bed is so clearly separated from its clay "floor" and its rock "top."

While little leaves of ferns and other bits of charcoaly, woody fiber may often be seen upon the sooty surfaces of coal, the true nature or origin of the vegetation that went to form the bright, brittle coal, which constitutes about eighttenths of the seams, has not yet been satisfactorily determined. We may any of us observe how, in swampy places, dead and decaying leaves, bark, twigs, etc., blacken and gradually change into a pulpy, laminated mass, which seems only to need hardening and drying to convert it into something not unlike coal.

Millions of years ago, and long before the Allegheny Mountains were formed, the greater part of the earth's surface, since shaped into and called North America, was nearly flat; and to a great extent covered with shallow lakes and vast swamps, bordered and separated by dark, low-lying and well-wooded country. This age or period is known as the Coal or Carboniferous period, because, during it, more coalbeds were formed than in any previous or later geologic age.

The trees of that epoch possessed their fiber and tissue and reproductive appendages, more like those of our ferns, mosses, lichens and aquatic plants, than of existing timber trees and plants of lesser growth. In those days huge froglike and lizard-like creatures inhabited the swamps; and beetles, crabs, caterpillars, snails, etc., crawled about; while aloft flew dragon-flies, butterflies and bugs innumerable. The ponds and creeks held fish, shells, worms, bacteria, etc., and doubtless produced luxuriant and profuse crops of water-plants of many species.

The climate of the coal period was probably warm, moist and very equable ; anyhow, it was expressly fitted to produce, and admit of suitable storage for immense quantities (millions of millions of tons) of vegetable matters in a comparatively short time.

We learn from the stratified and laminated structure or make-up of coal, and from animal remains-fish scales and teeth and water shells-found embedded in it, that the seams were produced upon very gently sloping plains, the surfaces of which were a little below water. The vegetation, therefore, either grew upon the land and was carried by water and wind into the lakes, or it flourished where we find it, under water; or these conditions operated together, the one first in one area, the other in another, and then the process was reversed, and so on until the uppermost layer of leaves, etc., was deposited.

It is believed, however, that aquatic plants and not land plants furnished the most of the bright layers, and consequently that these occupy the exact places of growth. The dull material, which cements together these brittle, shining layers, probably for the most part owes its origin to the land plants, the decayed, decomposed, and finely broken-up condition of whose remains became washed by rain into creeks and ponds, and finally floated off and gently carried out to sea, where they were sifted and sorted and quietly sank to the bottom to mix with and become " food " for the prodigious undergrowth of subaqueous vegetation. In this way it is believed that all the more important phenomena of coal seams may be best explained.

Each layer of earthy or stony material in a coal-bed implies a pause in growth and deposition of vegetable materials; that is, mud or sand, instead of plant-substances, predominated over the coal-growing expanses, until the latter again crept over the scene to remain until it was time for another muddy incursion. It often happened that very shortly after the last vegetable layer of a coal-bed was in place, its surface had gained exposure above the water, for petrified tree-stumps with roots attached are frequently found standing erect in the rocks on top of the coal.

The fact that the "coal measures," as the strata of the Coal period are called, contain several beds of coal one above another at various distances, shows that the delta of deposition gradually went on subsiding as each stratum of mud, sand, lime, coal, etc., was added to its surface. Some coalfields have a pile of strata twelve thousand feet in thickness, in which over one hundred distinct beds of coal occur; so this means a subsidence of twelve thousand feet at least. But the fact that many of our coal-beds occur hundreds and even several thousands of feet above sea-level, proves that elevation followed depression.

And since earth and rocks above water-level are all the time being eaten into and gradually washed away by rain and streams, it is not hard to understand how it is that we have the edges or sides of beds of coal peeping out along the hill-slopes in hilly country, and not very far beneath the soil in flatter territory. The hard-coal beds have been folded and greatly distorted since they were formed, and probably since the vegetable masses, by heat and pressure, were converted into anthracite. The same fossil-plants occur in the hard coal and in its associated strata of slate, etc., as are met with in soft-coal regions.

Anthracite Coal picture
Bituminous Coal picture
Graphite Carbon picture