NATURE OF COAL HORIZONS. 



185 



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 on 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 con- 

 dition 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 dis- 

 tance across the coal basins, as may be inferred from Winslow's 

 graphic representation, the productive coal strata should be con- 

 fined 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 particularlv to the coal fields of 

 Iowa and Missouri, where comparatively few disturbances of the 

 strata have taken place. The relations are relatively simple. But 



