THEORIES OF COAL FORMATION.
It is a fixed law of nature that matter cannot become exhausted or de preciated in weight, though it may change from one substance to another. Carbon, being one of the simple substances of the globe, was diffused through all matter composing it, existing in the vapors of the atmosphere surrounding it, and in the solid portions of the earth ; but during the early periods of the earth's existence, when the internal heat held the gaseous substances in vapor, the earth must have been shrouded by carbon and oxygen, as .fixed air or carbonic acid; and the manner or condition in which it has been condensed and solidified is admirable and wonderful, evincing an economy in the processes of nature that we must consider as provi dential and with a view to subsequent results.
It has been preserved as lime, coal, bitumen, and oil, as we have noticed in the foregoing pages. But how, or in what form and manner, is the sub ject of the present thesis. It has been inferred that all carbon returns to the earth in the shape of vegetation; and we admit that as a fact now, when our atmosphere contains only one-thousandth part of carbonic acid ; but when the atmosphere was surcharged with heated vapors, and carbonic acid, being the heaviest, shrouded the earth, the early vegetation, however great, could not have absorbed it, and it returned, as we have seen, in con nection with the salts of calcium and magnesia, forming lime.
It is evident, however, that a large amount of carbon was taken up by the early vegetation of the coal era, which we cannot lose sight of, and which must be accounted for. As we before noticed, this vegetation did not appear to form coal in a direct manner, but the carbon it contained was distilled or expelled, by pressure and heat, in the shape of oil, which must have been a carburetted hydrogen ; and this would form coal.
Those processes would form coal rapidly and in great bulk. The aque ous vegetation and oils of vegetation and gas combined, or either one of them, would produce the same result more slowly. But both of these processes must have been carried on in water. In fact, the evidence is overwhelming that our coal-fields were formed in basins of water,—lakes, seas, or oceans ; and every practical theory of the production of coal re quires this condition, to reconcile the coincidences and detail which meet the eye of the miner.
The disposition and formation of the rocky strata forming the coal measures are the same in all cases and the result of the same causes.
This theory seems to prevail since the large development of our carbon oils, or petroleum, which is formed from subterranean gases, resulting from the action of heat and water on the carbon of the rocks,—principally limestones and bituminous shales. It exists in various forms. The naphtha of Persia is lighter than our petroleum, and is constituted of car bon 82.20, hydrogen 14.80; and our lighter or lower oils are of much the same consistency. The heavy or lubricating oil is denser and of a much thicker consistency. A third variety is still less fluid, and is known as maltha, mineral pitch, sea-wax, &c. It is of the consistency of tar, but is sometimes found in a pure state, resembling tallow, paraffine, &e. A fourth
is called "elastic bitumen," and is nearly the same substance chemically as caoutchouc, which contains carbon 90, hydrogen 10. A fifth variety is known as compact bitumen, or asphaltum. It contains carbon, hydrogen, and oxygen in various proportions, according to locality and purity. It is found extensively diffused over the earth, and is apparently the result of solidified naphtha or coal-oil, since it is generally found when in large bodies near volcanic localities, as in Judea and Trinidad, but is rarely found among the older or primitive rocks. It exists stratified or in veins among all the rocks, from the great or auroral limestone up.
There is a singular connection between asphalt and mineral coal and asphalt and lime. In its purest state it has the appearance of the most beautiful coal, and contains the constituents of coal, but with a larger pro portion of bitumen than our best cannel. Asphaltic rock contains from 80 to 90 per cent. of carbonate of lime, and from 10 to 20 of bitumen. Pure limestone is devoid of bitumen, as pure coal is devoid of the carbon ate of lime or the salts of calcium and magnesium ; and both may have been formed in the same manner, but under different temperatures, and in combination with a greater or less amount of carbon and earthy substances.
The sixth variety of naphtha is cannel coal, which is simply solidified petroleum combined with vegetable matter. The eighth is our common bituminous coal. The ninth is pure anthracite coal, which is simply changed by heat to the purest mineral carbon except the diamond. Per haps we may add the tenth variety as a limestone; but we do not wish to extend these speculations beyond a brief notice, since volumes might, and will, be written to prove or disprove them. Our purpose is simply to pre sent facts in evidence of what has been advanced, or in support of the theory of the condensation of carbon oil as the primary and chief cause of the formation of coal-oils, bituminous matter, and our extensive beds of mineral coal.
The vapors of carbon escaping and mixed with the oxygen of the air form carbonic acid. This, solidified and deprived of its oxygen and in combination with the minerals calcium and magnesium, forms lime. Pure carbon condensed forms the diamond.* The gases or vapors of carbon arising through the pores of the heated earth, or direct from the bases of volcanoes in water, form carbonated hydrogen gas. Hydro-carbon oils and asphaltum, in connection with vegetation or the oils of vegetation, form bituminous coal, and anthracite under higher degrees of temperature.
The gases which arise from the action of internal heat on the carbonated rocks also produce carbonated hydrogen gas and coal-oils through the same process, but such oils are more highly charged with bitumen than the oils resulting direct from the vapors of combustion.
Thus in the economy of Nature there is no great complexity of operation. The carbon of combustion or the products of the heated earth are arrested by both air and water, and condensed to both liquids and solids, and, in connection with the carbons taken up by vegetation, form mineral coal.