THE NATURE OF COAL HORIZONS. 



The Coal Measures of the Mississippi valley occupy an area 

 of more than 120,000 square miles. They extend in an almost 

 unbroken field from the western flank of the Appalachians on 

 the east to beyond the Missouri river on the west. Separating 

 the area into two somewhat unequal parts is the Mississippi 

 river along the course of which are exposed older rocks. The 

 Coal Measures may have been at one time continuous over the 

 central portion, but the outcrops of the more ancient rocks along 

 the borders of the Mississippi is probably not due entirely to 

 unaided erosion but in part to slight folding, an anticlinal axis 

 coinciding approximately with the line of the great river. 



The geological history of the eastern and western areas, 

 which are sometimes called respectively the Central and the 

 Western Interior coal fields, is probably similar, though in some 

 particular phases there is a divergence which dates back prior to 

 the close of the Lower Carboniferous. The northern portion of 

 the Coal Measures west of the Mississippi forms a broad bay-like 

 expansion opening to the westward. Beyond the Missouri river 

 the strata are hidden from view by newer sediments. The most 

 productive portion of the Western Interior coal field is a marg- 

 inal zone extending from northcentral Iowa southeastward to 

 northeastern Missouri, thence sweeping to the westward around 

 the Ozark uplift into Indian Territory and continuing on into 

 central Texas. The interior portion of the bay-like expansion 

 of Coal Measures is as a general thing unproductive, though a 

 few thin seams of coal do occur. 



Everywhere throughout the region the stratigraphical details 

 present great simplicity, being almost free from the effects oi 

 orographic movements. The lithological characters of any one 

 locality are repeated again and again in the same monotonous 



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