1913-] STEVENSON— FORMATION OF COAL BEDS. 53 



The Coal. 



The passage from mur to coal is gradual at most localities ; but it 

 appears to be rather abrupt where the seat is a sandstone or con- 

 glomerate. This latter statement is made with reservation, as the 

 writer has had few opportunities to make determination, since coals 

 with sandstone floors are seldom of economic importance within the 

 areas which he has studied. No reference to the condition appears 

 in the literature to which he has had access ; but the records of cores 

 in the anthracite area lend countenance to the suggestion, for in 

 many cases, a mere film of clay separates the coal from sandstone or 

 conglomerate and the coal is good to the bottom. At some localities 

 in the bituminous region, a coal bed is clean apparently to the con- 

 tact with underclay, but in most cases the bottom coal is so impure 

 as to be unmarketable. For the most part, one finds a transition 

 layer, the faux-miir, between coal and clay; it may be very thin or it 

 may be several inches thick, and it may consist of inferior coal or of 

 coaly shale. 



In broad areas, where the f aux-mur is distinct, there is, neverthe- 

 less, an abrupt separation of the coal bed from the underlying clay ; 

 but this is not original, it is the result of disturbance. One finds 

 this condition even in the western part of Pennsylvania and eastern 

 Ohio, where the rocks vary so little from the original horizontality 

 that the dips on the sides of the low anticlinals rarely reach half a 

 degree and often for long distances are much less. Yet even there 

 one finds that the coal has slipped under the pressure and that the 

 contact between coal and clay is slickensided. This is the familiar 

 condition everywhere, so that one seldom is able to determine the 

 exact relation of coal to mur or the relation between plants of the 

 mur and those of the coal. But the opportunity fell to the lot of 

 Grand'Eury^^ during his study of the Loire basin. He says that in 

 coal beds, at their mur and in their more or less shaly partings there 

 are roots belonging to various species and that many a time he had 



^' C. Grand'Eury, " Du bassin de la Loire," C. R. Vllle Cong. Geol. Int., 

 1900, pp. 531, 532; " Sur les conditions gienerales et I'unite de formation des 

 combustibles mineraux de tout age et de toute espece," Couiptes Rendus, Vol. 

 138, 1904, pp. 740-744. 



