Page:The American Cyclopædia (1879) Volume I.djvu/593

ANTHRACITE 557 this anthracite gave 94*234 per cent, carbon. Anthracite also exists in Spain, Portugal, Ger- many, Austria, Norway, Persia, India, China, and South America ; and generally anthracite is found in connection with the altered or met- amorphic rocks, which accompany all great coal formations to a greater or less extent. Anthracite undoubtedly owes its existence to a superior heat or a comparatively high tem- perature during its formation. The hardest and most dense anthracite is always found where the coal has been subject to a high tem- perature ; but where the heat has been most intense, graphite rather than coal is found. In the New England field the outcrops of the coal beds frequently yield plumbago, which is col- lected and sold as " British lustre," and nests of pure graphite are found in the beds at con- siderable depth. An analogous condition is found in the pockets of carbonaceous coal which exist below A in the southern Pennsylvania field. The proportions of carbon are due to the vary- ing degrees of heat to which the coal or the elements forming anthracite have been subject- ed. This fact is fully illustrated in the Penn- sylvania anthracite beds, where the lowest contain the most carbon and the highest (in the measures) the most volatile matter. Where the coal is nearest to the igneous or plutonic rocks, whether granitic or metamorphic, whether in the deepest parts of the coal basins or on their edges, the conditions are the same, and all true coal fields are alike in these condi- tions. It is true, the Richmond (Va.) bitumi- nous coal field is formed in the crater of an extinct volcano ; but that field is a late creation of the Jurassic period, and was deposited when the earth and the rocks beneath it were com- paratively cool, and even there a trap dike is intruded evidently long after the completion of the coal field between the beds. The ef- fect of this heated and volcanic mass of rock has been to coke a coal bed 60 ft. beneath it, and burn one 10 ft. above it to a graphitic cinder. The general effect of trap intrusions seems to be the same in all cases, but the altered bitu- minous coal under such circumstances is rather a coke than an anthracite, which differs greatly in appearance, though the constituents are the same. Prof. H. D. Rogers explains the forma- tion of anthracite by supposing it to be the re- sult of altered bituminous coal metamorphosed by intense heat, and of course by heat induced subsequent to the formation of the bituminous beds; and he further explains the escape of the volatile portion of the latter as gas through cracks and openings caused by the plication of the anthracite strata. This plication follows closely the general type of the eastern palaeo- zoic rocks, which are intensely crushed and folded near the contact of their edges with the igneous or granitic rocks, and much less plica- ted and distorted in a western direction. This fact undoubtedly led to the above theory, which seems as natural as it is ingenious : but the facts do not sustain the theory. 1st. The upper beds and strata are more dislocated, distorted, and crushed than the lower beds, as plainly demon- strated by the plication of the strata on the apex of the leading anticlinals in the southern field. 2d. The measures are more plicated and crushed at the western extremity of this field, in the Dau- phin or south prong, than at the eastern extrem- ity ; yet the coal of the latter is a dense, hard anthracite, while that of the former is semi- bituminous. 3d. The heat must have been most intense during the early stages of coal formation. In view of these facts, it has re- cently been contended that true anthracite is not a metamorphosis of bituminous coal, but as much a normal creation as the bitumi- nous variety itself from a combination of its constituents under superior heat, however the original elements were produced. (See COAL.) The faults and irregularities of the anthra- cite beds and strata are the result of crust movements, and the plication of the distorted and crushed rocks indicates contraction, both lateral and perpendicular, as the cause. The effects of a combined lateral and perpendicular movement are simply those which are evident in the plication of the anthracite beds of the southern Pennsylvania fields, and their accom- panying shales ; but the crust movements have been slow and uniform, bending rather than breaking the strata, except in cases of sharp foliation of anticlinals or synclinals. Where the folding has been most abrupt the strata are inverted, and the coal is crushed and par- tially destroyed. The coal beds thus distorted are always subject to faults of the peculiar character described in the New England and New river coal fields, as well as those of Penn- sylvania. Such faults are more frequently met with in the upper than in the lower beds of the latter. A fault is rarely met with in the great white-ash beds B, D, and E, except where they are inverted or seriously dislocated by the plicating movements. The dislocations of Amer- ican coal beds are rarely vertical, and never to any great extent, as in the English fields, where this form of fault is peculiar. The near- est approach to this in the former is a "slip" which may slide one portion of a bed over the other, or remove it a few feet up or down. In the anthracite fields, however, faults are much more numerous than in the bituminous fields of England or the United States, but these are generally of the characteristic form peculiar to highly plicated strata before described. There are, however, other less frequent forms of fault, such as the occurrence of large areas of soft carbonaceous shale in place of the coal ; long ribbon-like streaks of rock or slate in the coaJ from the top of the bed, apparently to fill a crack in the same ; or the interposition of rock and slate between the strata of a bed, dividing it so as to render valueless sometimes one or both divisions. These faults do not affect the accompanying beds. The preceding are such as are strictly denominated faults in the Penn- sylvania anthracite fields ; but the ever- varying