Page:The American Cyclopædia (1879) Volume IV.djvu/754

ties of bituminous coal is C. 81.2, H. 5.5, O. 12.05; showing still the same proportion in the diminution of the oxygen and the increase of carbon. The chemical action is therefore constantly the same, and is recognized in the whole process; that is, the slow combustion of the woody matter by the action of the oxygen which it contains, or contained originally. Chemistry has not perfectly explained the process, or obtained similar results and products by its experiments. Prof. Dana says that the changes attending the ultimate decomposition of woody matter into coal depend: 1, on the affinity of the carbon for oxygen, making carbonic acid; 2, on that of hydrogen for oxygen, producing water; 3, on that of carbon for hydrogen, making carbo-hydrogen gas or oil; and 4, on the tendency of the carbon and the hydrogen under certain proportions to form with a portion of oxygen the staple compounds included in the term coal. In anthracite the amount of carbon is still increased, while that of hydrogen and oxygen has become proportionally less, and the volatile matter is reduced to a minimum. Hence pure anthracite is debituminized and burns without any flame. The anthracite of Pennsylvania becomes harder and more free from gas in proportion to the distance of the basins eastward from the Allegheny mountains, where its beds are folded in more numerous and sharper flexures. It has been supposed that its debituminization had taken place from some cause connected with the uplifting of the mountains. The first supposition was that the coal had been reduced to anthracite by heat. This opinion has been contradicted by another hypothesis which ascribes the transformation to great compression of the mineral coal by the upheaval of the mountains between whose sides the basins were slowly pressed, and thus slowly forced into numerous folds, and perhaps to a considerable amount of caloric produced by mechanical agency, movement, compression, &c. Many facts seem to contradict this last hypothesis, and support the opinion that the original heat of the earth has contributed to the metamorphism of the coal, as it has to that of the rocks. The problem is however complex, and cannot be discussed in a few words. The facts have to be recorded, and the conclusions may become evident in time. In Pennsylvania the debituminization decreases in proportion to the distance eastward from the mountains. At Trevorton, in Zerbe's gap, the coal is semi-anthracite; it has 84 to 86 per cent. of carbon, 7.50 of inflammable gas, and 2.50 of water. Though this basin is far distant from the mountains, the undulations of its beds are nearly as sharp as those near Pottsville and Tamaqua, being inclined at an angle of 50° to 60°. As the thickness of the strata is great, the pressure seems to have been equal to that nearer the mountains. In the Rhode Island basin the anthracite is still harder and more debituminized. Here the undulations are repeated, very numerous, and short, but not sharp, resembling the waves of the sea, and the strata are not thick; but the anthracite is in close proximity to the primitive rocks, and the shales over and under the coal show by their color and density the evident traces of metamorphism. There is here a peculiar phenomenon marking the influence of heat; it is the liquefaction of the shale and the effects of it on the vegetable remains, particularly the ferns. Their branches are generally elongated in one direction and contracted in the other side, as though drawn to one direction by the flexure of the shales in a state of semi-fusion. The plants too bear upon their surface a kind of intumescence, seemingly produced by heat. At Trevorton the shales over the coal are more or less marked by small round holes varied in size, filled with a pulverulent bituminous matter which looks as if formed by a kind of ebullition, or rather by gas forcing its way from the anthracite and stopped and enclosed within the shale. In Arkansas the Spadra coal is semi-anthracite. The strata wherein it is interlaid are nearly horizontal, their dip scarcely marked by an angle of 2°. It is also at a distance of 30 m. from the mountains. It has about the same composition as the Trevorton coal, 88.75 per cent. of carbon, with 7.7 of volatile matter. The rocks all around in the country bear traces of metamorphism, and the change by heat becomes more and more evident in advancing toward the Hot Springs, a volcanic region, away from the mountains. The same phenomenon is still more evident, and its cause more appreciable, in the tertiary lignite basins of the Rocky mountains. At Golden, Colorado, the thick lignite beds, 12 to 16 ft., are thrown up to the perpendicular by compression, in close proximity to the base of the uplifted granitic mountains, and between them and thick deposits of lava. This coal is soft, bears no trace of metamorphism, and even crumbles from the contact of the atmosphere. In New Mexico the strata are horizontal, but split by thin dikes of basalt, along which the coal shale is changed by heat and nearly as hard as silex. The nature of the coal in contact with these dikes has been recorded from a locality further south, near the valley of the Gallisteo, where the Placiere coal at one exposure of the bank is true lignite, while at another exposure, and in contact with an enormous dike of basalt, it has been changed into true anthracite, having 89 per cent. of carbon and only 3.18 of volatile matter, while at a distance from the dike the amount of carbon is only 58 per cent. The dip of the strata even in coming closely in contact with the dikes varies between 10° and 14° only. These facts are evident proofs of the debituminization of the coal and its change to anthracite by the action of heat. In this we have at the same time an insight into the chemical changes causing the modification of vegetable matter and its transformation into coal. For the action of heat does not deprive the coal of any part