428 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



EXPANSE OF COAL HORIZONS 



Very unexpectedly lowan coal measures in yielding up their 

 secrets lately brought into perfect harmony the two diametrically 

 opposed hypotheses concerning the disposition of coal beds, a 

 problem which was the subject of bitter controversy for many years 

 the world over. On the one hand it was contended that there 

 existed a strict parallelism in the stratigraphic relations of coal 

 seems. On the other hand it was held that coal beds were always 

 set at an angle to one another ; they split, came together, diverged. 



Stratigraphic analysis of a circumscribed Iowa coal basin dis- 

 closed the fact that the two opposed views were not really con- 

 tradictory. Paradoxically as it seemed both were true. The main 

 difficulty was that the two conclusions had only resulted from an 

 approach of the theme from distinct angles. The one idea proved 

 to represent a cross-section of the coal-bearing strata taken parallel 

 to the general course of the shore-line ; the other conception repre- 

 sented the structure at right angles to the strand-line.-'^ 



In other words approach to parallelism, or divergence of coal 

 seams is a direct function of local diastrophism during Carbonic 

 times. Instructive and interesting as this generalization is its 

 practical aid to prospecting and development is of infinitely greater 

 significance. Its importance in mining economy can hardly be 

 over estimated. 



Thus practically visualized coal horizons come to have an in- 

 dustrial meaning not usually ascribed to them. In stratigraphy, a 

 geological horizon is a level recognizable over a considerable area, 

 having a more or less well-defined stratigraphical position, dis- 

 tinctive as to lithologic features, and characterized by a particular 

 set of fossils. In a broad sense the term is almost equivalent to 

 formation, and is used about as indefinitely. In its more limited 

 meaning it is applied properly to a minor part, or zone, of the 

 smallest stratigraphic unit having a commonly accepted specific 

 name. Understood in the same way, a "Coal Horizon" represents 

 an even more limited expansion, where coal-forming materials 

 have accumulated. In reality it is one of the greater planes of 

 sedimentation, marking an episode in the deposition of a series of 

 strata. Theoretically it represents not a phenomenon but rather a 

 set of conditions, or a period during which the physical circum- 

 stances were similar over a considerable marginal portion of a 

 geologic province. From an economic angle it stands not for a 



"Journal of Geology, Vol. II, pp. 178-186, 1894. 



