That petroleum forms bitumen by evaporation and sediment, requires no proof here ; it is a fixed and commonly accepted fact; and that this oil was the product of this era and existed in profusion, the immense bituminous deposits testify: therefore, it is natural that the resulting bitumen should unite with the vegetation to form coal, since both must have resulted in the same basins and could not escape contact. It is not only the most natural process that can be suggested, but it is the only one that can be reconciled to all the conditions of our coal formations.
The interruptions to the continuous growth or formation of coal are frequent even in a single bed, and it is rare to find more than 5 or 6 feet of a solid bench that is not intercalated with slate or some evidence of a check or interruption to the continuous accumulation of the bed. In figure 98 we find one bed of 12 feet, and in other beds illustrated we have given nearly equal benches; but these are streaked with horizontal divisions or layers, giving evidence of depression and change,—perhaps marking the periods of subsidence beneath the surface of the water.
There are seven principal interruptions or divisions in the Mammoth at New Boston, marking the eras of complete submergence, if not precipita tion, since the strata of slate indicate an interruption to the formation of coal, and a deposit of argillaceous or arepaceous matter. In this respect, the MammOth is made up of seven distinct seams; but these dividing slates are not only liable to change, to increase or decrease, in the same basin, and at short intervals, but they do not appear in the same character in other basins.
From evidences accumulated from various sources, and the facts de veloped by our examination of the anthracite and bituminous coal forma tions in this country especially, we conclude: 1. That basins of 1000 feet are the best and most favorable condition for the growth or formation of coal, and that the largest beds are formed at a depth of from 200 to 500 feet from the surface.
2. That the coal measures were originally formed in gently undulating basins filled with water, and that their normal condition has been changed in all cases of high angles and inverted dips by contraction of the earth's crust, and the consequent elevation and depression of the axes.
3. That coal was formed in water by the accumulation of carbon and bitumen, from an excessive growth of vegetation and the condensation resulting from the evaporation of carbon oils, which were produced in profusion in the same basins in which the magnificent vegetation of the coal era flourished; and, consequently the contact and result could not be avoided.
4. The extraordinary growth or formation of coal in the anthracite regions is due to an excess of naphtha or petroleum, which resulted from superior heat due to volcanic influences ; and the nature of the coal is also due to the same causes.
We have thus tersely stated propositions which we believe may be called facts, and which we think will stand the critical investigations of the prac tical. Merely scientific inductions have had no influence in forming these
conclusions; and we do not think that any but the practical, who are familiar with existing facts, can properly appreciate a subject so abstruse and obscure.
We have faithfully tried to elucidate the propositions at large in another part of the work, and only briefly allude to them here in connection with the coal-beds, to bring the subject in bolder relief before the mind. But those who would understand us clearly, and who wish to gain a comprehensive knowledge of this part of our subject, should read Chapter IV.
The Skidmore at New Boston, in the Broad Mountain basin, is not in as perfect condition as the Mammoth. It is, however, as good as this seam is generally found, and better than its usual condition in the Southern coal field. The amount of good coal is over 6 feet, while that represented by white lines in this case is bone, which is not always a serious objection, except when the bone is not streaked in the benches of coal. A mass of hard bone or slate between the benches can always be removed and prevented from mingling with the prepared coal. But when thin streaks of bone or slate are intercalated in and through the solid benches, it becomes impossible to separate them from the prepared or marketable coal.
The thickness of D is 9 feet in this basin, which is over its average thickness, but less than its maximum thickness in the Mahanoy and Lehigh basins.
C is not often in a workable condition, and seldom as pure as we find it here, with nearly 7 feet of merchantable coal, divided by two small bone partings. In the Black Creek basins of the Lehigh group it is rather larger, but in the Beaver Meadow basin it has not been developed. The Buck Mountain bed, or B, of our nomenclature, though not in its maximum dimensions, is here in its best condition. It is almost a solid bed of coal, 18 feet thick, with a few unimportant streaks of bone and slate. In this respect it does not much resemble its celebrated counterpart of the Buck Mountain, which is divided by a massive slate, and which is a distinguishing feature of this bed wherever found, within our experience, except in this single locality.
There cannot, however, be any mistake in regard to the identity of the bed, since in every other respect it is a perfect counterpart. The absence of slate in this coal is not more remarkable than its absence in the Mammoth above it, and can only be attributed to the general and extra ordinary purity of the coals of this basin.
The lower seam A we have not made the subject of special inquiry, but we understand it is here in workable condition, and from 6 to 8 feet in thickness,—which is its maximum size.
The Broad Mountain coal formation extends westward of the New Boston basin ; but hitherto no coal has been developed in a workable condition, though we have reason to believe it exists.