I9I3-] STEVENSON— FORMATION OF COAL BEDS. 57 



Virginia and indicate a different history for the bed in the two 

 regions, showing that coal accumulation persisted for a much longer 

 period at the north than at the south. The conditions afford no little 

 justification for the recognition of each bench as an independent bed. 

 The irregularities of surface indicated by variations in the lower 

 benches at the north as contrasted with the general regularity of the 

 breast or upper portion show that in all probability the area of 

 accumulation increased landward toward the north by advance of 

 the marsh area. But increasing slate partings of extreme irregu- 

 larity indicate sufficiently that small streams often flooded the area 

 with muddy water. 



The continuous area of the Pittsburgh coal bed was estimated by 

 H. D. Rogers*^ at 14,000 square miles, the space embraced within 

 the outcrop. I. C. White,** however, after study of oil-well records 

 of West Virginia and Ohio discovered that the bed is wanting in a 

 rudely triangular space within those states and that the available area 

 is not more than 8,000 square miles. As the coal approaches the 

 central area of fine sandstones and red muds, the structure becomes 

 unrecognizable and the bed thins to disappearance. The constancy 

 of the Pittsburgh coal bed is apparent rather than real. 



Abrupt changes in thickness and structure are the rule in all coal 

 beds. They are not startling in the bituminous region, except to 

 those who have invested in mines, since the beds rarely exceed 10 

 feet ; but they are very notable in the southern and middle anthracite 

 fields. At one locality in the former, the Mammoth coal bed has 105 

 feet of coal in 114 feet of measures; at 8,246 feet toward the east it 

 has only 42 feet in 49 feet; in both the coal is concentrated, there 

 being but ten members in each section ; but, within a short distance, 

 one finds 40 feet of coal in 53 feet of measures and the section con- 

 sists of 43 members.*^ Variations of this type are reported from all 

 coal areas in the United States and they are commonplace in Europe. 



** H. D. Rogers, " An Inquiry into the Origin of the Appalachian Coal 

 Strata," Reps. Amer. Assoc. Geol. and Nat., Boston, 1843, p. 446. 



** I. C. White, " Stratigraphy of the Bituminous Coal Field in Pennsyl- 

 vania, Ohio and West Virginia," U. S. GeoL Surv. Bull 65, 1891, p. 64. 



** C. A. Ashburner, " The Geology of the Panther Creek Basin," Sec. 

 Geol. Surv. Penn., 1883, pp. 96, 98. 



