simple denudation, without any faults in the neighbourhood, we can follow the beds of coal under the other rock without much regarding it; it is simply an additional matter of so much thickness interposed between the surface and the bed, and if the dip of of the two rocks be known, and the surface be levelled, the depth of the coal can be calculated with the greatest facility. When, however, the boundary of a coal-field is caused by a fault, the mere dip of the rocks is no longer a trustworthy guide to us. The "throw" of the fault, as also the angle of its inclination, or "hade," is à priori unknown to us; we have therefore commonly no means of judging how far the coal-bed is depressed on the other side of the fault below its level on this, and if we knew that, we should perhaps have no means of ascertaining how far the fault "haded," "overhung," or "inclined;" or, in other words, how far we ought to go beyond the broken end of coal on this side of the fault, before we felt sure that we were standing over the other end of the bed on the other side of the fault.
The determination of the nature and character of the boundary faults of the coal-fields, therefore, is one of the most important practical points which it is the province and the duty of a Geological survey to solve, since it is one of those least within the power of on individual observer, examining only small and isolated localities, to understand and explain.
I was at one time strongly inclined to entertain the supposition that the boundary faults of the Midland coal-fields were of the nature of cliffs rather than of fractures. It appeared from certain circumstances rather probable that the previously existing Coal-measures had been almost entirely destroyed and washed away, except in the parts where they are now seen at the surface; that in those parts they had been left as islands, round which the beds of the Permian and New red sandstone had been deposited, abutting against the old Coal-measure cliffs, their beds taking the place of those that had been swept away. Facts such as those we have seen in the Brereton district, where the coal cropped up into the New red sandstone, without the intervention of any fault, lent strength to this hypothesis. Soon after the commencement of the survey of the South Staffordshire Coal-field, however, my belief in this hypothesis was greatly shaken, and it was finally abandoned, so far as the supposition of any long line of lofty cliff was concerned, having a more or less nearly perpendicular face, with several hundred feet of New red sandstone deposited against it. The very form of the east and west boundary faults of this coal-field was against this supposition. Direct evidence against it, as to the West Bromwich district at all events, was soon obtained, and all doubt was finally set at rest when the boundary between the New red sandstone and Permian formations was surveyed. For it then appeared that sometimes a broad tract of Permian rock lay next outside the boundary fault, sometimes none at all. If, therefore, the gap caused by the supposed denudation of the Coal-measures had been first of all filled up by Permian, it would follow that that formation itself
