boundary is formed by an unconformable overlap of the New red sandstone resting horizontally, or nearly so, on the slightly inclined edges of the Coal-measures. Near Littleworth there are some clay pits, in which blue bricks are made from some beds that appear to be the upper red clays of the Coal-measures, like those at Rumour Hill, at Essington Wood, and at Walsall Wood If this be so, and our identification of these measures be correct, it will follow that all the Essington and Wyrley Coal-measures will there be found below, and probably, therefore, all their coals and ironstones.
The reader will recollect that the southern margin of the coalfield was formed by the Permian rocks resting on the upper Coal-measures in apparent conformity, both being horizontal, or dipping at the same imperceptible angle to the south. Both the north al south boundaries of the coal-field, then, are formed by the simple denudation and removal of the superior rocks, and the consequent appearance of the lower ones at the surface of the ground; the only difference in the two cases being that in the one the superior foals were in apparent conformity to the inferior, while in the other they were distinctly and obviously unconformable.
CHAPTER XII.
The Boundary Faults and the Rocks surrounding the Coal-Field.
We will now examine the east and west boundaries of the coalfield. One difference between these and the north and south boundaries will strike us at the first glance of the map. Instead of being irregularly and deeply indented, and conforming to the natural surface of the ground, advancing or receding with its hills and valleys, the cast and west boundaries are regular and equable, preserving a certain mean course with great persistency, and when curving, doing so gradually, and with a wide and steady sweep, forming a curve of large radius. The north and south boundaries are like an indented coast line, the east and west are like artificial roads, going nearly straight across the country, with but little respect to the variations in its surface. These boundaries, indeed, are not the result of mere denudation alone (or of that action by which the present configuration of the surface has been produced), but of denudation combined with great longitudinal faults, which at the very period of their formation produced a sudden change in the ground along their line, so that no subsequent lowering of the level of the ground, though it may slightly shift the position, can much alter the character of that change. This difference in the nature of the two boundaries has a very important practical bearing. When the boundary of a coal field has been formed by
