COAL AND COAL-OIL.
Hydro-carbons, petroleum, or naphtha, are the condensed results of car bonated hydrogen gas,—either the direct results of volcanic heat, or pro duced by the action of internal heat on the carbonated rocks.
They were never surface formations, because their lightness would prevent precipitation; consequently, if formed on the surface, they would still exist on the surface, either as oil or solids, and could not, therefore, form our present subterranean deposits of petroleum.
As before stated, oils escaping to the surface or formed on the surface and exposed to water or air soon form solids, bitumen, coal, &c. We must, therefore, conclude that the gases fbrming our present supply of petroleum, or naphtha, are subsequent productions, formed since the deposit of the strata in which they exist, and produced by the action of internal heat, or the heat caused by pressure, on the carbon of the rocks. Those gases, confined and condensed, form a combination with the hydrogen of water, and the result is a hydro-carbon, or coal-oil.
The constant production of those gases in the deep recesses of the earth, from whence there is no adequate means of escape, keeps them in a high state of tension,—like steam in a boiler ; and they therefore, avail themselves of every crack or crevice which offers a means of exit. On arriving near the surface, the heavier portion of those gases again forms oil if arrested by water, with which, however, it does not mix, but floats on it.
At the base of Mount Vesuvius the vapors of carbon, escaping through the sea, form naphtha, which is seen floating on the water in great quan tities.
The lowest stratum in which oil is found under our Western coal-fields lies very near the great or Auroral limestone, and is, therefore, a much and a much deeper formation than coal.
In the East, the Auroral limestone is 25,000 feet below the coal, and per haps 10,000 feet below the rocks in which oil is known to exist. But so rapidly does the strata thin towards the west, the probability is that 3000 to 10,000 feet would be the maximum thickness in the Western coalfields, from the conglomerate to the Auroral or Matinal limestone ; while the interval contains the great Carboniferous limestone, and a world of thin limestone and bituminous strata, from the "old red" to the "Medina Sandstone." The lime-rocks must, under heat, give off carbonated gas ; and there is every reason to believe the production of carbonated hydrogen gas, and consequently hydro-carbon, or coal-oil, must have been greater before the formation of coal than since, because the heat which appears to produce these gases was greater before than after the formation of the coal measures.
If so, and we cannot doubt it, the flow of gas and oil into the great sea or basin now holding our coal must have been immense, and the formation of coal in connection with the magnificent vegetation of •that period was the result. Such, we think, was an absolute condition or result of the natural processes of that era. Since the flow of oil into the waters, after the escape of its more volatile parts, would result in sedimentary bitumen, and moderate heat would only facilitate the process, as now exemplified in cur petroleum refineries, and in which we find the solids are by no means an impure, earthy residuum, but the richest portions of the constituents of oil, we may therefore trace our coal-beds to the gas direct, without the mediation of vegetable carbon. But the fact that vegetation existed at the time in such great profusion, and in close connection with our coal-beds, and that the vegetable oils expelled by pressure and heat must have been in contact with the rock oils, indicates their combination in the production, of coal. Nor can we overlook the fact that the air contained more carbon, in all probability, than even the luxuriant vegetation of that era could absorb ; consequently, carbonic acid would be formed; but whether it would unite with the hydro-carbons to form coal, or with the metallic bases to form lime, is a scientific question that we cannot determine. It is known, however, that carbonic acid, solidified, forms a snow-white substance, which has none of the properties of coal, but intimately con nects it with lime.t In the beginning of this chapter we presented the theory of coal vege tation, not precisely as at present in vogue among geologists, but such as will conform to a natural process, and which can be explained or elucidated without the aid of earthquakes, convulsions of nature, or prodigious phe nomena.
If we have expressed ourselves clearly, it will be found that no conflict exists in the two theories of coal formation here presented, viz. : that of vegetation and that of the condensation of naphtha, but, on the contrary, the one is an auxiliary to the other, and clears up some of the most doubtful mysteries in the practical solution of the question. It enables us to shorten our Carboniferous period some million of years, and give Nature the credit of a rapid worker and a wonderful chemist, instead of being sloth fUl, mutable, complex, and