OTHER LOCALITIES.
Bitumen and petroleum like those of Trinidad and Cuba are also found near the city of Maracaybo in Venezuela, at Murinda in New Granada, and in numerous volcanic localities in Mexico and California; but the consistency and purity of the resulting bitumen de pend on the character and quantity of earthy impurities with which it is associated.
A description was given to the War Department in 1844 of a small lake of petroleum which exists in Texas. This lake is about one hundred miles from Houston. It is reported to be filled with bitumen which, during winter, is hard. In the summer, petroleum boils up continually, which by the evaporation of its hydrogen becomes bitumen, and hard, black, and brilliant.
All the localities here described are in volcanic regions, and the oils originate in fissures leading from smouldering subterranean fires, or from volcanoes direct, and, con sequently, cannot result from organic remains or fossils of animals or plants, since none ever existed in the plutonic rocks. Those hydro-carbons must, therefore, result from the vapors of smouldering subterranean fires, volcanic heat, or the chemical action of latent heat under pressure on the carbonated rocks.
It is a well-known fact that all rocks contain more or less carbon ; and we do not speculate in assuming the original constituents of the earth, when in its liquid con dition, to hold a large amount of carbon, as the chief cause of its caloric. As the carbon escaped in vapor, the liquid and fiery ball began to form its rocky crust,—at first slowly and by the condensation of its surface, in which but little carbon and no bitumen could exist, since carbon, hydrogen, and oxygen were held as vapor by the heat, and could not condense in the forms in which it now exists in the carbonated or bituminous rocks, slates, and shales.
But when the crust of the earth grew thick, and the igneous rocks were covered by the metamorphic, the radiating heat had diminished so that the vapors of carbon began to return to the earth in the shape of carbonic acid, in limestones, &c. During later periods, the vapor of carbon which issued, and still issue, from the internal heat of the earth did not escape entirely in vapor, but became condensed while arising through the thickening crust, in connection with hydrogen, and formed hydro-carbon oil, which, in localities having the requisite conditions, formed coal; in others, bituminous shale, asphaltum, bitumen, &c. But in localities not prepared to hold it in basin-shape, the oil was carried away by tides or waves, became too minutely distributed and mixed with earthy impurities to produce distinct masses or beds of pure bitumen or coal.
After the formation of coal in our great basins, the production of those subterranean gases diminished, and those produced became condensed before reaching the surface in the form of oil; while the continual contraction of the rocky crust of the earth by con densation on cooling closed its pores and fissures, and sealed much of the oil and gas in its deep cavities. In the coal-fields the strata grew continually, until the basins in which they formed were filled to their brims, mud and clays extended in immense horizons from edge to edge, while stratum upon stratum of sand and shale and coal added thickness to thickness, and formed a sealed and impenetrable cover to the gases still arising from the lower and still heated rocks. These gases accumulated, and, by their great tension, forced themselves into and between the strata wherever a lodgement could be found, and escaped to the surface in the form of gas or oil through every fissure or crack presenting the means of escape.
When we examine the solidity of our rocky strata, even in the most disturbed and dislocated localities, it seems strange that gas or oil should find a way to the but when we consider the tension in which these subterranean gases must exist, we cease to wonder. The constant generation of gas, which may exert several thousand pounds' pressure to the square inch, under a surface of many thousands of square miles, would lift the whole bodily, unless vents were found through which it could escape. When large fissures exist, through which petroleum and gas escape to the surface, they invariably become filled with solid bitumen by the evaporation of its lighter and more volatile portions. Thus, in West Virginia we find a vein of bitumen or asphaltum existing in a fissure which extends across the strata of the sedimentary rocks in which it exists; while in Cuba and many other parts of the world, as before stated, we find the same resulting bitumen in fissures, lakes, &c.
It may be possible that both gas and oil are still forming to a limited extent, but it is scarcely probable. The carbon and bitumen of the rocks, which resulted from a former excess of carbon, may now yield it again to the chemical action going on in the earth under pressure and contact with water. (?) Water could not penetrate deep into the earth when the rocks were in a heated or warm condition, but would evaporate in steam. On this part of our subject we do not propose to theorize, since there is no natural process on which to form a thesis with any certainty, or which can be demonstrated by existing facts.