These minor rolls or basins which we allude to generally occur on the long, gentle planes of the south-dipping strata, in which they are small undulations. They occur more frequently towards the outcrops of the coal-seams than towards the centres of their main basins; but they are occasionally found in the centre of deep basins and the steep sides of com paratively high angles of dip; but in all such cases the peculiarity which we propose to make a specialty almost entirely disappears : we allude to the enlargement of the coal-seams in all these minor rolls when they occur in gently dipping strata,—say at 35° and below, but more particularly about 15°. Figure 95 will illustrate more clearly the form and character of these minor rolls.
In figure 95 we have given six illustrations of the same seam—for instance, the Primrose—on dips ranging from 5° to 50°, which represent generally the form and character of those small interior or subordinate basins or undulations of the strata on the south dips, and their consequent effects on the coal-seams. We have chosen the Primrose because it is a single and regular seam, and not subject to the frequent and extraordinary enlargements of the Mammoth, yet subject, as we represent, to frequent changes and enlargements, such as affect, more or less, all the seams in the anthracite measures.
Those undulations frequently have an influence on the overlying and underlying seams, where the waves or rolls are of sufficiently great extent; but small depressions do not so much affect the overlying strata. Sedi mentary deposits naturally fill the depressions faster than the elevations, and the normal condition of all sedimentary strata is an increased thickness in the synclinal and a decreased thickness in the anticlinal axes when subject to like influences: consequently, the small depressions become filled to the general horizon of the surrounding deposits.
The upper section at 5° dip in figure 95 represents some of the undu lations of the Primrose above water-level in Brown's old Oak Hill colliery, where this seam makes several extensive and wave-like rolls, in creasing and decreasing the dimensions of the coal from 8 to 20 feet. These extensive waves affect the Orchard, which lies about one hundred feet above, but to a less degree.
The second section illustrates nearly the undulations or rolls in the Mammoth and Primrose, on the Mammoth Vein Consolidated Coal Company's lands, between St. Clair and Wadesville, as found in the Hickory colliery, formerly operated by the Messrs. Milnes.
The third section represents a rare form of undulation, and one that happens more frequently near the outcrops of coal-seams than on their regular dips. It is found on the outcrops of the Orchard, on the Oak Hill
tract, and at several other 'points on different seams, and is illustrated on a large scale by the undulation of the Mammoth at Wolf Creek, or the eastern point of the Jugular basin.
The fourth section, on 30° of dip, is similar to a roll or enlargement of the Primrose at the Warrenton colliery of Messrs. Miller & Maize at Silver Creek, where this seam appears to obtain its maximum size in 40 feet of thickness.
The fifth and sixth forms of undulation are sometimes met with in all the anthracite seams, and offer serious obstacles in the way of sinking " slopes." They are of very frequent occurrence in all the eastern Virginia coal-fields, as we shall notice in our description of those formations.
We find in all the foregoing minor basins an enlargement of the coal seams ; but in all deep basins rather a decrease than an increase of the coal. Yet the enlargement is not confined to the temporary basins on the angle of dip, or towards the outcrop of gently undulating seams : all our moderately deep or medium basins present the same feature, as in the New Boston or Broad Mountain basin, and in all the medium Lehigh basins. We therefore accept it as a fact that the coal-deposits are more uniform, and are in their maximum dimensions, when formed in basins of medium depth. We take the Broad Mountain basin as the medium, and will illustrate its form, dimensions, and character, before offering it further in evidence of a theory advanced in the early chapters of this work on the formation of coal.
This is a small but uniform basin, containing the white-ash beds in their maximum dimensions and in their most perfect condition. Its extreme length is about six miles, and its maximum breadth 2000 feet. It is 3G() feet deep to the base of the Mammoth, 730 to the Buck Mountain, or B, and 860 to the lowest seam, and contains six workable coal-beds, with an aggregate thickness of 107 feet of coal.
The "Seven-feet" seam is the upper bed in this basin, and lies about 20 feet above the Mammoth, which ranges from 60 to 80 feet in thickness. Seventy-five feet below E we find D, or the Skidmore, which is 9 feet thick ; and 62 feet below D we find C, which is 6 feet thick. Below these 200 feet is the Buck Mountain, or B, 18 feet thick, and in fine condition. The lower workable seam is A, 100 feet below B. A is here from 6 to 8 feet thick : a small seam exists below it, in the conglomerate, but is not considered a regular or permanent bed.