THE PRIMROSE.
The Primrose is a large and productive bed, generally regular and reliable in character, and is mined with economy. It ranges from 9 to 16 feet in thickness, and is generally classed with the white-ash beds, though it is more properly a pink-ash. The bottom bench is a white-ash coal, and the upper benches red-ash coal. In other regions, how ever, it produces all white-ash coal, and is considered as among the white-ash veins, and particularly so in the Mahanoy region and in the Scranton district, where this vein is very large and productive. It lies from 300 to 400 feet above the Mammoth. We consider the Primrose as the counterpart of the Pittsburg seam, since it occupies a corresponding position in the horizon of the coal measures, and is gene rally identical with the Pittsburg in character, position, and associations.
The Primrose is the most consistent bed in the anthracite coal-fields, and is, in all probability, equally consistent in the bituminous regions. Its variations are not so great as the Mammoth, and it appears to have been formed with less interruption, since it is rare to find more than one slate parting in it, and frequently there is none, the divisions being merely partings of bone or coarse coal; while the Mammoth is frequently divided by massive slates or sandstones from 2 to 40 feet in thickness, and is often divided into four distinct veins,—that is, the upper or "seven-feet vein," the upper "Twin," the lower "Twin," and the "cross-cut,"—and these, in all probability, form the chief veins, independent of the Buck Mountain, or B, in the bituminous regions beneath the barren measures.
All the lower beds, including the Mammoth, are subject to sudden and excessive expansions and contractions. The Mammoth is found at one place over 60 feet thick, in regular and uniform strata,—for instance, at New Boston; and at another it is found depreciated to two thin plates of less than 6 feet respective thickness, or 12 feet, which is the minimum thickness, near Shamokin. These sizes may be regarded as the minimum and maximum changes, and between them this great bed vibrates from point to point; but its most persistent size is between the extremes of 20 and 35 feet, which is its best condition.
The Primrose, or G, is less changeable. Its size or diameter is usually —almost uniformly-10 feet, though it varies from 9 to 16 feet when in measures not confused and contracted by faults; but in faulty ground this bed, like all others, is liable to extreme fluctuation.
It is worthy of note, and we may perhaps appropriately state the fact here, that all beds of coal formed in extremely deep basins are smaller than when formed in moderately deep basins. We think this rule will hold good the world over. But they are still thinner and much more unreliable when formed in extremely shallow basins than when formed in the extremely deep basins; and this fact is a very strong argument against the theory which makes our coal-beds the productions of arborescent or marsh and bog vegetation.
For fear of misconception, it may be necessary here to remark that many of our present deep beds were not the primary formations of deep basins. They have been subsequently depressed, as is evident from the actual thickness of the strata at right angles and the absence of the upper veins, which are conclusive evidences of their original depth and of their subse quent changes.
The deep basins of the Mahanoy, with their sharp angles and narrow troughs, were not formed in their present position, but have since been contracted and depressed; and we may say the same thing of the Lehigh formations generally. But the central portions of the Shamokin and Pottsville basins must have been deep originally, and some of the wide basins of the Western bituminous coal-fields must also have been extremely deep, as were the formations of the Great Northern coal field of England and the Arcadian coal-fields of the British Provinces.
This subject requires more elaboration and proof to make it intelligible. This, however, is not the place to discuss it; but we briefly mention the fact here, as we find it abundantly illustrated by a hundred concurring evidences.