To account for this increase of the seams, we can only give the theory of division, or a splitting of the beds, as in the case of the Twins. The lower coal-beds are invariably divided by heavy slates, as in the case of the Buck Mountain; and the probability is that the slates have increased in thickness and widely separated the seams which are counted as a unit in other fields or regions.
Figure 62 is a vertical section obtained from develop ments made at the Burnside colliery, two miles south of Shamokin, and from data furnished by William H.
Marshall, of Shamokin. It appears to be a general type of the central basins, though not a fair representation of the southern basins, or that portion of the field in the vicinity of Locust Moun tain, which partakes of the character of the Mahanoy region; nor does it present a correct representation of the western end of the field, in the vicinity of Trevorton, where the veins are rather thicker than those shown in this section.
We think, however, it is about as fair a type of the central portions of the Shamokin region as can be offered; but the beds are better and more productive in the Mount Carmel basins, including the Locust Ridge anticlinal, than those portrayed in figure 62. The Mammoth, westwardly, maintains its twin form, but in the eastern and southern portions it is consistent with its general size and dimensions, or divisions, and is very productive in coal, and excellent in the quality of the same. At or near Tre
vorton the Twins consist of two large veins, Nos. VIII. and IX. of Rogers, each from 16 to 20 feet in thickness, and divided by a considerable body of slate and rock, of indefinite thickness, but enough to entitle the Twins here to be ranked as separate and distinct beds.
Farther east they depreciate in size, but come closer together. The Twins in the vicinity of Shamokin range from 7 to 9 feet thick each, and are divided by rock and slate which varies from 10 to 30 feet in thickness. The accompanying illustration, figure 63, will convey a general impression of the character and form of the Twins, or Mammoth, in this locality. It is not singular to this region, as we find the same splitting process in effect at Mount Laffee, in the Schuylkill region, and at several other points. But it is not the normal condition of the Mammoth, and may be considered a serious imperfection wherever found.
There is considerable variation in the relative sizes of the Twins; but generally the lower portion of the vein is the largest, and varies from 8 to 20 feet in thickness. The upper section is also variable, and ranges from 6 to 16 feet in thickness.
We give a concise description of the order and size of the veins as developed in the Trevorton district,—adapting the nomenclature of that region with that used in figure 62, with which it may be compared for a proper conception.