COAL, a mineralized form of carbon, con stituting one of the metamorphic rocks. It is found in seams or beds, often in a series sep arated by intervening strata of sedimentary rocks.
Many theories have been ad vanced as to the origin of coal. Thus coal beds have been attributed to the drying up of petroleum lakes on old land surfaces, to the separation of carbon by some fanciful chemi cal process from limestone and to accumula tion of seaweeds along old ocean beaches. These theories may be dismissed without dis cussion; they may help explain some particu lar instance, but are so unsupported by facts that they are of no wide application. . The generally accepted theory applies to practically all kinds of coal deposits and is, bnefly, as follows: When the woody material, cellulose, of the leaves and stems of plants, falls on the ground, it soon oxidizes or decays, and the oxygen, hydrogen, carbon. and nitrogen present pass into the air or soil as gases, the hydrogen and oxygen chiefly as water vapor, the carbon as carbon dioxide and the nitrogen as ammonia. Finally of the original material, say the trunk of a great tree, only the ash, composed chiefly of silica, alumina and iron oxide, is left. Thus it happens that the leaves, twigs and branches that have fallen for thousands of years in a forest are represented by a few inches of vegetable mold or humus, plant substance not yet oxidized to ash. If, however, the ground be covered by water, as in a swamp, air is partly excluded, and decay proceeds so slowly that vegetable or animal remains may be pre served for long periods of time. Still oxidation goes on; the dead plants gradually give up their hydrogen oxygen and carbon as water, marsh gas and carbon monoxide and dioxide, and change to a mass of partly decayed veg etable fibre or even to a black muck.
A damp climate and a land surface from which the rainfall runs off slowly, favor the formation of extensive swamps, though in a climate as damp as that of Ireland peat bogs climb hillsides, the mosses (S phagnum), the chief plant growth in such swamps, dragging up water by capillary action. On the plains of Alaska and Siberia, where the ground is permanently frozen, mosses cover the ground with a thick mat, and such swampy plains are called tundras. In a lake country can be found
areas which a little investigation shows were at no very remote date covered by shallow bodies of water, but are now swamps, the original lakes having been filled by the dead mosses, rushes and other aquatic plants. Along the seashore in a region of average rainfall, where the coast is of low relief and the rivers slug gish, sand-bars form by wave action off shore, and behind these bars are salt lagoons and marshes, changing, farther from the ocean, to bracicish, and finally fresh-water, swamps.
Fresh or brackish water and a fairly warm though not torrid clirnate are indicated by the fossils of plants and animals found in or near coal seams. From these facts and from the vast extent of some coal fields it is believed that coal beds represent old coastal swamps, possibly of the type of the Dismal Swamp in Virginia. Now if we suppose such a swamp covered coast to sink slowly, the encroaching ocean would cover the accumulated peat and muck with sand and silt until finally the swamp might be buried thousands of feet by sediments from the receding land surface. Instead of steadily sinking, however, the probabilities are that during the great coal-forming epochs the land alternately sank and rose through thou sands upon thousands of years, and thus one. swamp was buried over another, resulting in those alternating beds of coal, shale and sand stone characteristic of all coal fields.
The progressive diminution of hydrogen and oxygen compared with carbon is shown by the following table from Percy's 'Metal lurgy,' in which carbon is taken at the con stant amount of 100: The loss of water and of combined car bon, hydrogen, oxygen and nitrogen, in the change from peat to coal, caused a great loss of bulk. The pressure of overlying strata, or of those earth movements that warp and fold the rock formations, reduced the bulk still more. Thus it can happen that a coal seam one foot thick may represent what was 50 feet of peat in the ancient swamp, and it is fair to assume that an average seam of true coal is not one-fifth the thickness of the original peat beds.