The second seam in ascending order is a regular seam of bituminous coal, ranging from 5 to 10 feet in thickness, but is very much injured by the numerous small bands of bone and slate which are mixed through the coal, and which it is almost impossible to separate by the means ordinarily in use at those mines. The coal, however, is good, and might be separated from impurities by proper care and mechanical arrangement.
The third seam is much better in character than the former, and contains much less impurity. Its usual size is 4 feet. The fourth seam is also a good workable coal, ranging from 5 to 7 feet thick, and produces an excellent article of fuel. It is, however, streaked with bone and slate, but not to the same extent as the lower seam, and perhaps not to a greater extent than many of our work able beds in the older and more regular coal-fields.
The fifth seam is a bed of natural coke, ranging from 5 to 6 feet in thickness. The seam originally appears to have been less mixed with impurities than any of the underlying beds, since the coke is singularly pure and ex cellent. In appearance it is very much like the artificial coke formed from a rich caking bituminous coal; and in character it is a true carbon, containing the same constitu encies as an artificial coke after being exposed some time to the weather, or common red-ash coal of the anthra cite mines. It has less lustre than newly-broken artificial coke, and its fracture is more like that of bituminous coal,— though it sometimes tends to the conchoidal or irregular. There are two or three varieties or grades of this natural coke in the same seam, and often within short distances. The most valuable is denoted as " soft or free-burning coke," and the less valuable as " hard or popping coke," from its tendency to fly and "spit," like hemlock or spruce wood, on a fire. The soft coke burns beautifully, and much like good red ash anthracite ; but the " hard coke" ignites with difficulty, and burns slowly unless under a strong draft. The gas or moisture which it contains does not readily escape on heat being applied, owing to its hardness and density, but, on expanding under the heat, bursts its cells and scatters pieces of the coal around, often with great violence.
About 60 feet over this bed of natural coke lies a "whin rock," having all the appearance of trap, or basalt, and is of excessive hardness. The strata in its vicinity have a reddish appearance, as if altered by heat; while the next seam above it, and only divided by a few feet of fire-clay, is a perfect cinder, and entirely valueless. The seam containing the cinder appears to have been about four feet in thickness, but is now much mixed with fire-clay and iron and sulphur balls. The whole appears to have been changed by intense heat, and the carbon of the coal entirely expelled, leaving the cinder in irregular masses, mixed with the clay, sulphur, and iron balls.
There is no evidence externally of volcanic eruption ; but there can be no doubt of the fact that this "whin rock" is a formation of subsequent era, and, instead of being ejected at right angles through the measures, it found a more easy exit between them, and under the bed of coal which it has so singularly changed.
We are not sure that this bed of whin rock extends unbroken through the east basins on the north side, but it is found on the south side of the river, north of the anticlinal before mentioned. It has only been found in the basins of the east side, in the north end of the field, and is not found at Dover, on the west side.* It is developed extensively at the Carbon Hill mines, on the east side of the basin, but has not been found on the west side. In the deeper portions of the basins the coke depreciates in thickness, and the lower part of the seam is a semi-anthracite, while the coke is "soft," but excellent. There are two beds of thin iron-stone over the " cinder" bed ; but they are poor and unreliable, and not of any commercial value.
The measures of the north end of the field are made up chiefly of slates and shales; but several massive rocks of coarse quartzose sandstone are stratified through them. The material filling the basins of this coal-field evidently came from the surrounding country,—chiefly from the higher grounds up the rivers,—and are not the result of volcanic eruptions, as are the measures of the great Appalachian fields generally.