Page:The Journal of geology (IA journalofgeology21894univers).pdf/199

shales or other strata, probably are rarely exactly parallel to one another, or if so, the parallelism is purely coincidental. There are many causes which in places lead to the non-parallelism of the coal horizons. The original bottom of the sea may have been very uneven, as is well shown in the very irregular surface of the Saint Louis limestone in which the Coal Measures were laid down. Or, in two different seams the inequalities may be great, the extremes often occurring in the same locality, and thus presenting a much greater apparent unevenness than really exists. Erosion or currents may have altered the position of the seams or parts of them. The top of the seams which were originally level became subsequently depressed in the center more than at the margins. There are also other causes tending to widen the seeming discrepancies. (Figure 3).

Fig. 3.Coal Horizon as it now exists; parallel to shore line.

In another direction, at right angles to the old shore, the minor basins along the different horizons may appear to show no tendency to parallelism at all. The approach to the parallel condition is inversely proportional to the amount of deformation occurring in the region at the time of the formation of the coal beds. Instead also of the seam being continuous for a considerable distance across the coal basins, as may be inferred from Winslow's graphic representation, the productive coal strata should be confined to a limited marginal area and the coal horizon would only extend into the interior as a great stratigraphical plane, not easily recognizable perhaps, nor with any of the mineral itself to mark it. (Figure 4).

The conditions described apply particularly to the coal fields of Iowa and Missouri, where comparatively few disturbances of the strata have taken place. The relations are relatively simple. But