40 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



a limestone. Many times an exposure was incomplete, some por- 

 tion of the little group was concealed but enough was seen to make 

 recognition definitive. The coal was observed so often that, when its 

 place was concealed, its presence was assumed. The bed was mined 

 at that time near Freeport in Pennsylvania and the deposit was 

 named Upper Freeport. Either coal or very black shale was ex- 

 posed so often in this position both in Pennsylvania and Ohio that 

 barren spaces were regarded as due merely to petty local conditions 

 and the supposedly continuous deposit was called the Upper Free- 

 port coal bed. In like manner, the other horizons became known as 

 coal beds and widespread accumulation of coal at each horizon an 

 accepted fact, without reference to either quantity or quality of the 

 material. 



But detailed study of individual coal beds proves that in all 

 there was great irregularity. The Pittsburgh, Waynesburg and 

 Washington, in the upper portion of the series, approach as nearly to 

 continuity as one may conceive, for they are always present in ex- 

 posures and records within an area of thousands of square miles ; 

 but the Pittsburgh shows remarkable variations in thickness ; it thins 

 away to nothing from all sides toward the central part of the area 

 while at times only its underclay remains to mark the horizon. The 

 Waynesburg and the Washington horizons are persistent, coal or 

 black shale being present, but there is often only a trace of coal, 

 while the variations in structure of the deposit are extreme. Some 

 Conemaugh coals are practically continuous, according to natural 

 exposures, in Ohio within an area of not far from i.ooo square 

 miles, but they are rarely seen in Pennsylvania ; others are present 

 on the east side of the region and rarely appear on the west side. 

 The Allegheny conditions are similar; one bed attains great com- 

 mercial importance within an area of perhaps a thousand square 

 miles in Ohio, but in Pennsylvania and West Virginia, it is only 

 occasionally important and it is practically wanting in considerable 

 areas. And the statement is true of other coal horizons. The evi- 

 dence goes to show that there were periods, longer or shorter, 

 during which proper conditions existed, so to say, contempora- 

 neously in many localities but did not exist in very many others. The 



