I9I3-] STEVENSON— FORMATION OF COAL BEDS. 37 



thick, is at the bottom and on it rests bituminous coal, 32 feet. The 

 coal in all the pockets is rather impure. Meek thought that the coal 

 beds had been let down by solution of the underlying limestone, but 

 studies by later observers make evident that the accumulations were 

 deposited in preexisting hollows. 



Ashley^- described a small area occupying a basin of different 

 type, eroded in the Merom sandstone of Sullivan county, Indiana. 

 This is in the upper part of the Coal Measures and is regarded by 

 him as evidence of a land surface. The coal is thickest in the 

 middle of this basin and thins away in all directions toward the 

 border. The lower coal beds in Indiana exhibit a tendency to this 

 basin shape, the thinning of coal toward borders of the " swamps " 

 being a common feature. But higher in the column, the areas in- 

 crease and at length the coal beds are practically continuous for long 

 distances. 



The condition, noted by Ashley in Indiana, prevails in the north- 

 ern part of the Appalachian basin, where extreme irregularity de- 

 creases after the close of the Pottsville, and the coal becomes reason- 

 ably continuous in greater areas, so that mining enterprises are 

 attended by less risk. But the irregularity was very great in the Potts- 

 ville. Reference has been made in another connection to Roy's 

 description of the mode in which the Sharon coal bed occurs, which 

 confirmed the statements made by Newberry, Read and others in 

 the Ohio reports. The same features characterize the Beaver beds 

 in Pennsylvania, of which Ashburner^^ says that in the northern 

 counties of the state they occur in " swamps," " swallows " or 

 " sumps," and that they are saucer-shaped ; the coal thins to a knife- 

 edge on the hillocks of sand but is reached again when those have 

 been pierced. I. C. White^* was able to study the vagaries of the 

 Sharon coal bed in a mine with 10 miles of workings. The coal 

 rests on I to 2 feet of fireclay, overlying the Sharon sandstone. 



'" G. H. Ashley, " The Coal Deposits of Indiana," 23d Ann. Rep. Geo!. 

 Surv. Ind., 1899, PP- 22-24, 532, 633, 666, 909. 



" C. A. Ashburner, Sec. Geol. Surv. Penn., Rep. R, p. 53 ; Rep. RR, 



pp. 95. 97- 



"I. C. White, Sec. Geol. Surv. Penn., Rep. Q, pp. 194, 202; Rep. QQ, p. 

 170; Rep. QQQ, p. 123. 



