THE BIG TRACY.
Figure 87 does not represent the Big Tracy in its best condition, but we think it about the mean, or an average illustration of its character. We have attempted to project all the sections of coal-strata on a scale of one-tenth of an inch to a foot; and this scale will approximate the actual thickness. But we have given the figures in all cases; though our artist, who is usually very correct, has not always put the distinguishing marks to denote feet from inches. A dot after the figure should denote feet, and befbre it inches. With this explanation the reader will be able to detect the error.
The Big Tracy, or K, ranges from 8 to 12 feet in thickness, and lies from 200 to 250 feet above the Diamond, or J; but between them there are two or three veins approaching the workable sizes, and among these is the "Clinton," which ranges from 2 to 3 feet in thickness.
We have represented this bed as divided or streaked with several benches of bone, and accompanied with a soft stratum or mining as a base; but this condition is changeable, and the vein is frequently found almost pure, or with but small strings of bone; when the bone is wanting, some of the upper benches are generally soft and Shelly, and productive of much waste in mining and the preparation for market.
This bed is also liable to " faults ;" and perhaps one-fourth of the entire area occupied by its strata will be found unproductive. The general form of fault or imperfection developed by the workings on this bed is a tend ency to crumble or waste. A considerable portion of the seam, under such circumstances, cannot be made available. "Dirt-faults," as repre sented in figures 111 and 112, are frequent in all the red-ash coal-beds; while rock-faults, as illustrated in figures 108 and 110, are more frequent in the white-ash beds.
It may be noticed, by an inspection of the transverse section across the field at Pottsville, and the several vertical sections taken in various parts of the anthracite regions, that K occupies but a small portion of this ter ritory. (See figure 74.) It is only found in the first four basins in the Schuylkill district, and does not extend to Tamaqua or Tremont; while it is not found in any of the other fields, except, it may be, in some portions of the Shamokin region.