COAL is a mineral of vegetable origin, ranging in color from dark brown to black, and consisting chiefly of carbon, hydrogen, oxygen and nitrogen. When distilled coal yields coal gas and coal tar, the latter the principal source of benzine and numerous other products.
Coal is found in seams of varying thickness, which are imbedded between the rock formations of three of the five main periods in the earth's geological history, namely the second, third and fourth periods, called the palæozoic, the mesozoic and the tertiary periods, and particularly in that sub-division of the palæozoic period which is called the carboniferous period or the "Coal Measures."
The geological period represented by the rocks of the Coal Measures was one of the most remarkable of which there is any record in the geological history of our planet.
Enormous areas of the earth's surface were covered with dense forests, which grew in lagoons and marshy regions. The whole land must have been very flat in those days, and little above the level of the sea, something similar to the everglades of Florida or to the great deltas of tropical rivers, in which locations wide expanses are occupied by sluggish rivers and lagoons of fresh water, where the mangrove grows along the shore and the bottom is covered with water-logged, decaying vegetation. The climate of those times was warm, equable and moist and the atmosphere was probably rich in carbon dioxide. The growth of vegetation must have been rapid, and dark evergreen plants and ferns lent a somber aspect to the scenery. As yet there were no flowers, no birds, and none of the higher four-legged animals. How long ago this was, nobody knows with exactness. Some believe it was hundreds of thousands of years ago, others say millions.
Prolonged but very slow sinking was in progress, and when for years the vegetation had densely clothed the soil, it was carried down below the water and covered over with mud and sand. Then, by some upheaval or gradual silting up of the sea bottom, a land surface was once more formed; luxuriant vegetation again sprang up, in course of time decayed, sank and became overlaid with silt and sand as before.
The vegetable layers thus deposited, subject to the heat of the earth and of decomposition, and to the pressure of accumulating masses of stratified matter, were gradually mineralized into the brown or black rock which we now call coal.
In other cases great masses of decaying vegetation drifted out into lakes or estuaries, and were there gradually submerged, covered with debris, and similarly converted into fuel for coming ages.
The changes which have taken place in converting the decaying vegetation into coal are partly chemical, partly structural.
The oxygen, hydrogen and nitrogen of the woody fibre of vegetation tend to be expelled in the form of marsh gas and carbonic acid gas, while the carbon increases in proportion as the process advances, till in anthracite coal carbon forms nearly the whole of the resultant mass. The vegetable structure disappears—peat, lignite, bituminous and anthracite forming a series more and more removed from wood in composition and character.
The Coal Measures consist chiefly of beds of sandstone, shales and ironstones, throughout which the seams of coal, varying in number from a few to more than a hundred, are evenly distributed.
The fossils most common in the coal-bearing strata are plant remains, the stems and leaves of many kinds of ferns, fossil bark of giant trees. Land and fresh-water shells are also found, along with remains of fish, insects and air-breathing amphibians (animals living both in water and on land) .
The thickness of the coal seams varies considerably, from less than an inch up to 40 feet, or even over 80 feet as in one case in Wyoming, but when they are very thick they consist, as a rule, of a number of beds, separated by partings of shale or other rocks.
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