1913-] STEVENSON— FORMATION OF COAL BEDS. 41 



greatest unbroken area, after the close of the Pottsville, in which 

 coal accumulated, was that at the Pittsburgh horizon, the coal having 

 been proved up in an area of approximately 8,000 square miles. 

 Originally it was greater, for erosion has removed much. The 

 Sewanee coal bed of the New River seems to have a great con- 

 tinuous area, but the measured sections are somewhat widely sepa- 

 rated; they suffice to prove identity of horizon, but they do not 

 justify either assertion or denial of continuity. 



Accepting, however, the extreme conceived area for original 

 extent of the Sewanee or the Pittsburgh, one is compelled to recog- 

 nize that accumulation of coal was not in process at any time in an 

 area of more than 30,000 square miles and that it never was in 

 process simultaneously in all parts of that area ; that at most horizons, 

 conditions were favorable to accumulation in areas of a few square 

 miles to some hundreds of square miles while in perhaps the greater 

 part of the regions the conditions were unfavorable. In fine, that 

 the conditions were very much like those existing to-day. And this 

 has always been the case. The Triassic coals were formed in 

 narrow areas ; the inconstancy of Upper Cretaceous coals in New 

 Mexico, Colorado and Utah is proverbial — they are spoken of as 

 lenticular ; Tertiary brown coals exhibit the same features, which 

 are equally characteristic of Quaternary deposits as well as of peat 

 accumulations of this period. At all periods, conditions favorable 

 to accumulation of coal have existed in comparatively small areas, 

 more or less widely separated. This will be considered in another 

 connection. 



The relation of coal to the immediately adjacent rocks is so inti- 

 mate that they must be regarded as one: a coal bed consists of the 

 floor, mur, Liegende ; the coal, houille, Kohle; the roof, toit, Han- 

 gende, each of which must be examined in detail. 



The Floor of the Coal Bed. 



Miners, long ago, recognized that coal beds ordinarily have a 

 clay floor or seat, but the fact was announced as generalization first 



