I9I3-] STEVENSON— FORMATION OF COAL BEDS. 55 



Variations in Structure. 



A coal bed is apt to vary rather abruptly in structure, local condi- 

 tions having been as efficient during accumulation of coal as they are 

 now during accumulation of peat. A coal bed may consist of two 

 or more divisions, the benches or bancs, separated by partings, which 

 are often more variable in thickness and composition than the coal 

 itself. In some treatises, these benches are referred to as separate 

 beds — and with good reason, as will appear after consideration of 

 the varying character of the partings and the often contrasting com- 

 position of the coal in successive benches. Occasionally, however, 

 definite structure persists throughout a considerable area. Thus 

 the Pittsburgh bed, at the bottom of the Monongahela formation, 

 shows roof division, overclay, breast-coal, parting, bearing-in-coal, 

 parting, brick-coal, parting, bottom-coal. 



This structure can be recognized in the northern part of the area 

 along a west-northwest line of not less than 170 miles from the 

 eastern to the western outcrop in Maryland, Pennsylvania and Ohio, 

 exposures being practically continuous for 120 miles. It is distinct 

 in an area on each side of the line not less than 40 miles wide for 

 much of the distance and much wider on the eastern side. Yet even 

 this remarkable bed, when traced beyond the limits given, shows that 

 it too is variable. Bownocker^^ has made clear that on the western 

 side, in Ohio, the structure changes abruptly at a little way south 

 from the long west-northwest line. The change first appears in 

 southern Belmont county, where the roof division disappears and the 

 breast-coal becomes irregular. Within a few miles, the bed consists 

 of coal, clay, coal, there being no recognizable trace of the upper 6 

 parts and the clay parting is often a foot thick, whereas in the 

 typical section the partings are all thin, seldom more than half an 

 inch. The condition, first observed in southern Belmont county, 

 prevails southward on the western side for 90 miles. At some 

 localities, the section resembles that seen farther north but analysis 

 of the parts shows that they are not the same. 



'" J. A. Bownocker, Geol. Surv. Ohio, 4th Series, Bull. 9, 1908, pp. 10-12. 



