466 INDEX TO THE STEATIGEAPHY OF NORTH AMEEICA. 



division, while sandstones are also quite important. Coal beds and fire clays play a minor 

 part, although economically the former are exceedingly important. Ah these beds are won- 

 derfully irregular in distribution. They thicken and thin with amazing rapidity and grade 

 from one to another horizontally as well as vertically. * * * The beds of coal vary in thick- 

 ness up to 7 feet, with an average of about 4 feet. Their areal extent is usually not above 

 a few hundred acres and many of them are much below this maximum. Many of them lie 

 in depressions in the St. Louis limestone, and their limits are determined by this formation. 

 The Cherokee beds form the eastern line of outcrop of the Lower Coal Measures from Van 

 Buren to Webster counties. Their maximum thickness wUl probably average 500 feet, although 

 over most of their area they are considerably thinner. * * * 



The Appanoose formation is typically developed in Appanoose and neighboring counties, 

 as well as across the Slate line in Missouri. The beds composing this division are much more 

 regular in structure than those underlying them, and the coals, while thinner, are more contin- 

 uous and dependable. The most important of these coals is the Mystic seam, which underlies 

 an area of about 1,500 square miles in the two States. * * * 



Accompanying this coal seam are several relatively thin limestone beds marked, like the 

 coal, by great continuity and uniformity of character. Some shales and a conglomerate are 

 also present. * * * 



The upper division of the Des Moines, the Pleasanton, is not of great importance in Iowa. 

 WhUe in Kansas these beds attain a thickness of about 200 feet and carry some important 

 coal seams, they thin to the north and in Iowa are characteristically barren of coal and scarcely 

 distinguishable from the next lower division. They thin rapidly in Guthrie County and 

 probably do not extend beyond the northern limit of this county. 



It is not to be understood that these three divisions are sharply set off one from the other, 

 for they are for the most part conformable each to each, and hence there are no clear dividing 

 lines. The divisions are made because each phase exhibits certain well-marked features which' 

 set it off in a general way from the others. 



J-K 15-16. EASTEBN INTERIOR COAL REGION. 



The following statement on the eastern interior coal region was prepared for 

 this volume by G. H. Ashley : 



The Pennsylvanian of the eastern interior coal region consists of about 2,000 feet of rocks, 

 occupying a simple structural basin that includes most of lUiuois, southwestern Indiana, and 

 part of western Kentucky, an area of about 50,000 square mUes. The rocks consist of shales, 

 sandstones, coals, clays, and limestones, usually in members of small thickness, many times 

 repeated. Soft shales and sandstones predominate. Coals up to 13 feet thick, usually under- 

 lain by stigmarial clays, occur at about forty horizons, and limestones up to 20 feet thick are 

 fuUy as numerous. 



The basin, while mainly structural, was enlarged and extended by pre-Peimsylvanian erosion 

 in the underlying Carboniferous rocks. To the north especially erosion removed successively 

 the several Mississippian formations, and in part of northern Illinois the Pennsylvanian rests 

 on Ordovician rocks .^*' 



The oldest Pennsylvanian rocks of the region correspond in age with the Pottsville of the 

 Appalachian region and are confined to the central and southeastern parts of the basin in 

 southern Illinois °"° and probably southern Indiana and the eastern part of this region in Ken- 

 tucky, this basin probably having been for a time connected with the northern Appalachian 

 field. These rocks are predominatingly sandy, with shale, clay, thin coals, and in many places 

 conglomerate. The sandstone is massive and resistant and now forms a belt of rugged hiUs 

 along the whole eastern and southern margin. The oldest of these rocks are probably of lower 

 Pottsville age, and the top not older than Sharon, the basal member of the upper Pottsville. 

 Unconformably above these in southern Indiana are 100 feet or less of rocks of later Pottsville 

 age, more shaly in character and containing workable coals in small basins, as well as valuable 



