We think the fact above set forth conclusive, and needs no demonstration, because the heat must have been greater at, or before, the time coal was formed, than since; and we need scarcely state that the volatile or bitumi nous matter would escape more readily when in a fluid than in a solid state; when unconfined rather than when sealed in the rocky strata of the earth.
We find the change from anthracite to bituminous gradual, and locally the point of change is imperceptible, while the gradation is general from east to west. In the vicinity of the volcanic regions, at the east, we have the pure anthracite; while at the western end of the same basin we find semi-bituminous, or soft anthracite. At Broad Top and in the Sulli van county detached basins—lying between the anthracite and the bitumi nous fields—the coal is in a transition state, containing from 80 to 85 per cent. of carbon, and, consequently, a very small amount of bitumen. At Blossburg, Ralston, and Cumberland we find a "steam coal" with an increasing amount of bitumen, or from 75 to 85 per cent. of carbon. Farther west, the amount of bitumen increases rapidly, ranging from 15 to 50 per cent. In the Kanawha region, in Kentucky and some portions of Ohio, the bitumen preponderates, while the carbon exists in minimum quantities. As a coal, the cannel contains the least, while the anthracite
contains the largest amount of carbon.
The carbon forming anthracite came direct from its volcanic source, and was not affected by carbonic acid or lime, or by hydrogen to any great extent, as the coals of the West are. The gases or oils forming the bituminous coals must have been produced by internal heat, as in the case of anthracite; but these gases and oils arose through or from the great limestones and bituminous shales, and were, consequently, changed thereby.
We might extend this chapter to an indefinite length in explanation of those great chemical processes of Nature, and in giving many other theories of our coal formations; but, while aiming to be practical, we are in danger of giving more speculation and theory than fact.
In justice, however, to Prof. H. D. Rogers, whose eminent position and laborious researches among our coal-fields entitle his opinions to respect and regard, we give his theory of their formations, which will be found in the Appendix. It covers the ground of the terrestrial vegetation, peat-bog, and drift theories, or combines them all.