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



the Pittsburgh coal bed and are illustrative of those shown by nearly 

 all beds. They are associated with equally marked chemical differ- 

 ences, which will be considered on a later page. 



The coal in all benches has a laminated structure, due perhaps 

 in some cases to pressure but in others to some other cause. The 

 writer has traced laminae, which tapered to nothing in each direction 

 along an entry ; whether or not this is characteristic, he cannot say. 

 Any one who has attempted to determine this matter in a coal mine 

 must have recognized that the intense application required should be 

 devoted to something more important. H. D. Rogers concluded that 

 in pursuing any brilliant layer, not more than one fourth of an inch 

 thick, one may observe that its superficial extent is too great to per- 

 mit the supposition that it had been derived from the flattened trunk 

 or limb of any arborescent plant. It is certain, however, that pres- 

 sure cannot account for the alternation of brilliant or glance laminae 

 with those of dull or matt coal, which one finds almost invariably. 

 Usually these layers are very thin, but in many instances they are 

 several inches thick. Sometimes this lamination seems to be due to 

 the presence of mineral charcoal, which covers every surface ob- 

 tained by splitting, but at others the charcoal is clearly without 

 influence, for it lies in all directions. This mineral charcoal is a 

 common constituent of all the fuels from anthracite to peat, but it is 

 not an essential constituent, for layers of glance several inches thick 

 have been found without it and Orton''* has described a coal bed of 

 workable thickness which shows no trace of it. 



Fragments of plants, sometimes large, occur in coal. Occasion- 

 ally they have been converted into fusain but more frequently they 

 appear as glance coal, — though even these occasionally enclose more 

 or less of the charcoal. Ordinarily they are flattened, the interior 

 having disappeared while the cortex remained to be converted into 

 glance. At times, they are merely impressions on the apparently 

 structureless mass of coal, recalling the conditions observed in many 

 peat deposits, where the great bulk of vegetable material has been 

 changed into the flocky ulmic mass, while enclosed stems of trees, 



** E. Orton, " Mines of Muskingum and Licking Counties," Geo!. Surv. 

 Ohio, Vol. v., 1884, p. 881. 



