THE FIRST, OR POTTSVILLE, BASIN.
The first and most extensive basin lies between the Sharp Mountain, 1, and the Gate Ridge anticlinal, h, and underlies Pottsville and Port Car bon in this vicinity. It is nearly a mile wide from outcrop to outcrop, and is the largest basin in the anthracite fields, extending from a point east of Middleport to the end of the Dauphin Fork, a distance of not less than 50 miles. Along the entire distance it preserves its peculiar character. The north-dipping strata are always perpendicular or inverted at their outcrops, and descend, in all probability, nearly 3000 feet before a change from the perpendicular is made. This feature of the South, or Sharp Mountain is manifest in every water-gap along its line, but more pecu liarly so west of Middleport. At this point the field is suddenly increased in breadth to double its dimensions farther east, by an abrupt shifting of the Sharp Mountain range to the south. In the oblique corner formed by this offset of the conglomerate, several axes originate; and in the vicinity all the anticlinals of the western portion of the field start out, but none of them are so persistent as the Gate Ridge anticlinal and the Southern basin, whose extensive range we have just mentioned.
Figure 74 correctly illustrates the dip of the strata near Pottsville; while figure 94 represents the basin at Black Spring Gap in the Dauphin Fork. Between these points the strata change but little from the vertical, and the veins generally are so crushed and distorted that little workable coal exists in these north dips of the South basin. It is plainly evident that the contracting forces were mainly exerted on the deeper basins,— originally deeper, no doubt, but particularly so on the southern deeply depressed strata; that is, the contracting forces were exerted on the deep and, consequently, weaker axes, in favor of the higher and less corrugated strata, as we explained in the early pages of this work. The effects of
this contracting force not only tend to depress further the already-depressed basins, but also to elevate the anticlinals or ridges.
If we take a book and lay it open about the middle before us, it will very nearly represent the strata of a gently inclining basin, the leaves being the strata. If we depress the middle, we have a representation of the action of the crust-contractions : the leaves are elegy ated and come together as the middle is depressed. And if we apply force to the cover, for the purpose of closing the book, we see the effects of the forces which have contracted not only our• coal-basins, but a wide extent of the strata east of the Alleghanies. The axes are the weakest points, and the strata naturally fold from these points, whether anticlinal or synclinal, as a book folds or hinges on its back.
In the section presented, we have drawn a line from ni to n, represent ing the proper or real thickness of the coal measures, or their depth as strati fied in its original position. It is plain that the uptilting of the strata or depression of the synclinals naturally increases the depth of the basins, since the strata are either brought together like the leaves of a book when closed, and thus presenting the breadth of the book, instead of half its thickness, as the depth of the axis, or the strata are less acutely folded, and the depression filled with subsequent sediment to the water-level.
According to our measurement, the actual thickness of the anthracite coal measures is between 2000 and 2500 feet, while the actual depth of the Southern basin is over 3000 feet. This will be manifest by the sections presented. Vertical section figure 75 gives the minimum thickness of the measures at right angles : in some localities they are perhaps one-fourth greater in thickness.