Page:The Journal of geology (IA journalofgeology11893univers).pdf/254

J. P. Lesley,^[1] in 1879, offered some interesting suggestions. If the anthracite be metamorphosed bituminous coal, the change might be caused by exposure to comparatively high temperature at a great depth below the surface. As the temperature increases one degree Fahrenheit for each fifty feet, more or less, of descent, the coal under cover of a great thickness of rock could not fail to be deprived of its volatile matter. He compares the composition of coal from the highest available bed in western Pennsylvania with that from the lowest bed in the same region, and finds less volatile in that from the lower bed. As all of the Paleozoic rocks thicken eastwardly, there must have been a much greater pile of Coal Measures in the anthracite region than in the bituminous areas, though erosion has removed the proof. Necessarily then the coals of the anthracite region should show less volatile than do those of the bituminous area, where the pile of rocks was less thick.

Professor Lesley suggests also that if one desire to explain the origin of the anthracite by oxidation in preference to metamorphism, the conditions afford basis for such explanation, since in the anthracite region the rocks are not only broken and shattered by the folding, but they are made up largely of sand and gravel, so that the conditions are such as to favor percolation of water, evaporation, and consequently oxidation; whereas, in the undisturbed bituminous areas, clayey beds are in large proportion and lute down the buried coals so as to prevent percolation and the rest.

There is no possible room for doubt that bituminous coal can be converted into anthracite by heat. The Galisteo, Elk Mountain and other localities within the United States, the Hesse Cassel and New Zealand areas in foreign lands, prove beyond dispute that, under proper conditions, contact with molten rocks suffices for the conversion. But no question of such conversion is at issue here, for in Pennsylvania no dikes occur near enough to the anthracite areas, or large enough even if near enough, to