THE CARBONIFEROUS SYSTEM. 163 



dimensions is very unreliable. Its characteristic features are well shown 

 in the cut west of Barnesville, where my attention was called to it by 

 Prof. Stevenson. At one end of the excavation it is barely six inches in 

 thickness, while at the other it shows a total of five feet four inches, con- 

 sisting of coal, one foot; shale, four inches; coal, four inches; shale, four 

 inches; coal, four inches; shale, two feet; coal, one foot. Seven miles 

 east from Barnesville, where it is 'exposed in a railroad cut, it is about 

 one foot thick, and parted in the middle by a thin layer of limestone. 

 Near St. Clairsville, in the same count}', it is rudely worked, and shows 

 three feet of very impure coal, resting almost immediately upon a foot 

 of limestone. Near Bridgeport, opposite Wheeling, it. is three feet six 

 inches thick, roofed by six inches of impure blackband, which is over- 

 lain by two feet of alternating bands of bituminous and ordinary shale. 

 On a run four miles west from Bellaire, and just south of the railroad, it 

 suddenly thickens out and becomes a confused mass of coal and shale, 

 not less than fifteen feet thick, and totally worthless. 



In Harrison and Jefferson counties it is never more than two feet 

 thick, and is seen only near the tops of the highest hills, and has no 

 economical value. (Stevenson.) 



Coal No. 12 is generally found in the highlands which border the Ohio 

 opposite Wheeling. It is usually from a few inches to two feet in thick- 

 ness, but is locally, near the Ohio, developed into a dry coal six feet in 

 thickness, but heavily charged with pyrites. 



Coal No. 13 lies some seventy feet above the last, is very thin, and has 

 no practical value. It is limited to the summits of the divide east .of 

 the Ohio, on and south of the Baltimore and Ohio Railroad. 



DISTEIBUTION OF THE LIMESTONES OF THE UPPEB COAL MEASURES. 



If it is true, as has been supposed, that Coal No. 10 of the Belmont 

 section is the Cumberland coal Cf the south-eastern counties of the State, 

 it will be seen to thicken toward the west, and its basin may be said to 

 lie west of the Ohio. This is also indicated by the limestones with 

 which this coal is associated, and which are thinner and less numerous 

 in the Upper Coal Measures of West Virginia and Pennsylvania, after 

 passing above the " Great limestone," which there lies over the Pittsburgh 

 seam. We are in these facts furnished with evidence of another change of 

 the center of gravity, if we may use the term, in the progressive sub- 

 sidence of the Alleghany coal field. Following back the history we have 

 traced, and measuring the spread of marine conditions by the reach, and 

 their continuance by the thickness of the limestones, and tracing the 

 local depressions of the different epochs by the thickening of the de- 



