Page:The Journal of geology (IA journalofgeology21894univers).pdf/194

derstood in the same way, a "Coal Horizon" represents an even more limited expansion, where coal forming materials have accumulated. Practically it is one of the greater planes of sedimentation, marking a distinct episode in the deposition of a series of strata. Theoretically it represents not a phenomenon, but rather a set of conditions, a period during which the physical circumstances were similar over a considerable marginal portion of a geological province. From an economic standpoint it stands not for a continuous bed of mineral fuel, but a stratigraphical level where workable beds are more likely to occur than elsewhere, and where the coal is to be especially sought for in a wide belt fringing a great coal basin. It is not to be inferred, then, that the mineral is equally developed on a given

Fig. 1.Coal Horizon at time of formation; parallel to shore line.

horizon in all portions of this marginal border. In some places the accumulations of plant remains are much greater than in others; limited basins and troughs of unusual thicknesses are there found. Elsewhere the old vegetable materials are meagerly represented; only thin seams of coaly matter are there preserved. Wide intervals of sandstone and shale often separate adjoining basins, or ancient land elevations may cut off one area from another. (Figure 1). Yet, through all of the many irregularities of deposition and subsequent deformation there are nevertheless discernible, certain levels quite well defined at which coal beds are very much better developed than at others; clearly marked coal horizons they are, broad in extent and capable, in the case of the greater ones under favorable circumstances, of being traced over a large part of a given Coal Measure province. The coal may not be present in a continuous seam over the whole border district and probably never is; but along much of the