The commotion causing those changes at length dies away, and quiet once more reigns. The finer particles of matter, held in solution by the waters, are thus precipitated, and form fire-clay, as the soil for another growth of Sigillaria; and thus the process goes on, and the coal-beds are formed.
Much has been said about the trunks of trees standing erect in coal-beds or in the coal measures, and many theories proposed to account for their existence. It- seems natural that the towering vines and gigantic calamites should stand on massive and comparatively solid bases ; and it would not be strange if those trunks should stand erect even when the foliage which they supported should be laid at their feet. Frequently, however, those massive forms are bent over and laid partially on their sides, with the stumps erect, and the top crushed between the strata. But the woody part of these trunks lying outside of the coal-beds are always silicious or calcareous, and do not form coal. Nor do the trunks of arborescent trees, or the terrestrial vegetation, found in the surrounding strata, form coal, though drifted in profusion into the coal measures from the higher grounds surrounding the coal-basins.
There are local phenomena in the anthracite coal-fields which would require volumes to describe, and much more to explain. One of those is the varying thickness of the larger veins in the numerous small basins and in some portions of the large basins.
For instance, the lower veins in the shallow basins of New Boston, Black Cret, &c., are larger than the same veins in the deeper basins of Wyoming and Schuylkill, which would seem to imply that they were formed under different circumstances,—in less depth of water, or more uniform action of the conditions and causes operating in this production. But these details will be considered more minutely in the description of those basins respectively, further on.
The causes which lead to the production of anthracite within the recog nized bounds of a great bituminous coal-field, cannot fail to be an interest ing subject, though our conclusions may be dogmatic.
Compared with the immense extent of the field in which the anthracites exist, their area is insignificant, but their comparative value, under present circumstances, is in inverse proportion. As a pure coal, containing a maximum percentage of carbon, the Pennsylvania anthracites are superior to any mineral fuel in existence. A pure specimen contains 95 per cent.
of carbon, and an average of the white-ash varieties will yield 90 per cent. It is, consequently, more dense and compact than any other kind of coal. A cubic yard will weigh about 2700 pounds.
One theory states that anthracite coal is a fresh-water formation, but does not specify the effects of fresh water in increasing the amount of carbon.
There is reason to credit the theory of fresh-water lakes, because there is evidence that our anthracite fields were detached formations, lower than the main Western basin, and, therefore, likely to contain fresh water; but the fact that the western ends of these lakes, or basins, contain semi-bitumi nous, and the eastern ends the purest of anthracites, seems to invalidate the theory in its application to the coal formation.
A second one is, that the bitumen has been driven from the coal by heat, the escape of the volatile matter being aided by the steep undulations of the strata and the frequent outcropping of the uptilted veins. This, of course, would be a sufficient reason, and would account fully for a dry, semi-bituminous coal. But it does not seem to meet all the conditions of a pure, hard anthracite. The fracture of all coals of a bituminous character is cubical, while the pure anthracite is conchoidal.
A coal once formed or created as bituminous will not lose its peculiar character, and no heat that can be applied will change its fracture without consuming it. We find a "natural coke" in the Richmond coal-field, and in other bituminous coal-fields, in the vicinity of trap dikes, where the bitumen has been expelled, leaving what should be anthracite according to the theory; but this coal has a cubical fracture and a dull, coke-looking appearance.
We have no doubt heat was the cause of dispelling the bitumen from our anthracite coals, but it was while the carbon was in a fluid state and before the coal was formed. The anthracite was formed in the earth as it now exists, and has not been materially altered by heat since i formation, though it has evidently changed its position, becoming more abrupt in its angles of dip by the continued subsidence or lateral contraction of the region.