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



in considering the problem of coal accumulation, one has not to deal 

 with vast areas, since coal never was accumulating at any one time 

 throughout a great basin. 



Relation of Coal Beds to Black Shale. 



Coal beds vary in character; frequently coal passes gradually 

 into black shale containing laminae of bright or dull coal; occa- 

 sionally, the passage is almost imperceptible to the eye, the increase 

 in ash causing no marked change in appearance. It is a common 

 observation that, in the Coal Measures, black shale is almost certain 

 to be replaced with coal somewhere. At the Uniontown horizon, in 

 the Monongahela, one finds usually a thinly laminated black shale 

 containing scales and teeth of small fishes and some laminae of 

 coal ; but at many localities within its area of several thousands of 

 square miles, this becomes a coal bed which though impure is of 

 local importance. Any coal bed is liable to show this change. The 

 Buck Mountain bed, near the bottom of the Allegheny in the anthra- 

 cite area, is worthless within a space of many square miles; the 

 Mammoth bed degenerates westwardly and at times is little better 

 than carbonaceous shale. Coal beds as they approach the border of 

 their area are apt to show a greatly increased number of thin part- 

 ings, usually mud but sometimes sand. Not rarely lenses of sand 

 are intercalated, which may be of considerable extent. Such 

 changes seem to indicate proximity to upland, whence streams came 

 loaded with sediments. They suggest conditions like those which 

 are seen within five or six miles west from New York, where one 

 finds many times a small area of clean peat surrounded by impure 

 material containing layers of mud. 



The origin of the black shale is not always clear, but it is a sedi- 

 ment. The carbonaceous matter, in some cases, came in with the 

 sediments as plant fragments, but in others it came rather from 

 animal matter. An illustration of the former condition is found in 

 the work by Scott,^'' who made dredgings in Lakes Ness, Oich and 



" T. Scott, " The Lochs of the Caledonian Valley," Scot. Geogr. Mag., 

 Vol. VIIL, 1892, pp. 94. 95- 



