ARTHUR WINSLOW — THE MISSOURI COAL MEASURES. 



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underlying strata which were accumulated in slightly inclined positions. Figure 30 

 represents the resulting conditions, provided deposition is continued and subsidence 

 is arrested. The number of deposits cannot be taken to represent, strictly speaking, 

 so many individual and separated strata, as each one may be made up of a varying 

 number of layers of different materials; they simply indicate the limits reached 

 by the deposits in successive intervals of time. The apparent dip and the conse- 

 quent unconformity of the coal layer C C upon these underlying strata is also much 



Figure 30 — Ideal Representation of a complete Cycle of Deposition of Coal Measures, and of their 



Mode of Accumulation. 



exaggerated by the excessive vertical scale. If reduced to the natural scale, neither 

 the dip nor the unconformity at any one point would be perceptible. 



The natural results of such a growth of sediment is that a coal bed should he 

 thicker near the margin, where its accumulation began, than in the interior, and the 

 thickness of the bed at any one point will depend upon the length of time during 

 which subsidence was arrested and the accumulation was allowed to go on. The 

 coal bed may expand over the whole area, as is represented in figure 30, and may 

 there accumulate through a thickness of several feet, and then be cut short by a 

 suhmergence to the point C, when another cycle of deposition will begin similar to 

 the first. 



Changes in the amount and character of the sediment supplied at any time dur- 

 ing such a cycle would cause corresponding changes in the thickness and character 

 of the strata. A rapid, continuous, or frequently recurring subsidence would pre- 

 vent the accumulation of coal, or would allow of its formation only over narrow 

 marginal areas. A subsidence after the coal bed had expanded overa half or other 

 fraction of the submerged area would fix a limit to that individual bed at such point, 

 and it would be buried beneath the strata of the succeeding cycle of deposition. A 

 varying rate of subsidence over different areas would also affect the character of the 

 deposits. Where the rate was greatest, deep-water or marine conditions would he 

 more prevalent, and where the rate was slow shallow-water conditions would pre- 

 vail generally and coal beds would he more frequent. If the rate of subsidence over 

 the interior were constantly greater than that over the marginal area the firsl formed 

 ami lowesl beds would gradually acquire a westerly dip, while the upper beds were 

 horizontal, and the aggregate thickness of the deposits would be increased toward 

 the interior, although the thickness of an individual stratum, or of a heterogeneous 

 deposit formed during any interval of time given, would he thinner, proceeding from 

 margin to interior, [f subsidence were arrested along the margin and continued in 

 t he interior, t he deposits would thin to a feather edge along this margin. On the 

 other hand, if t here were elevation along t he margin and subsidence in t he interior, 

 the succeeding deposits would thin out within what were previously the marginal 



