BITUMINOUS AND ANTHRACITIC. 457 



ceous grit, or even, occasionally, a siliceous conglomerate. In 

 these instances, the inclosed vegetable remains are for the most 

 part fragments of the larger stems or branches of gigantic arbo- 

 rescent plants, their fronds and leaves being less abundant. 

 These fragments occur in all postures, as respects the plane of the 

 bedding, horizontally, obliquely, or perpendicularly ; and betray, 

 in their broken condition and irregular mode of dispersion, 

 the sudden and tempestuous character of the currents which 

 drifted and entombed them. Though the arenaceous rocks, hav- 

 ing these features, sometimes rest in immediate contact with the 

 upper surface of the beds of coal, they more frequently lie at a 

 moderate distance over them, an argillaceous, laminated slate in- 

 terposing to form the actual roof. A further indication of the 

 violence of the currents, which strewed these coarse materials over 

 the coal, is sometimes to be detected in the composition of the 

 lowest portion of the overlying bed of giit or sandstone, in which 

 a large amount of coal, in the state of pow^der or sand, is dissem- 

 inated in the rock, giving it a dark, speckled appearance. This 

 is of very common occurrence in the anthracite coal strata of 

 Pennsylvania, where the coarse grit not unfrequently rests imme- 

 diately on the coal. It implies, I conceive, the erosion of a certain 

 portion of the upper surface of the soft, carbonaceous mass by the 

 friction of the sandy current. The coaly matter, thus disturbed, 

 would subside with the first layers of the sand, with which it was 

 mingled. Mr. Logan has mentioned a still more striking proof 

 of the energy of the movements which occasionally occurred, 

 during the formation of the coal-measures. He gives an account 

 of actual boulders, or rounded pebbles of coal, in the Pennant 

 grit, and other coarse strata of the coal-field of South Wales. 



Of the direct contact of Coal-Beds and Marine 

 Limestones. 



In the preceding account of the sti-ata immediately below^ and 



above the seams of coal, I intentionally omitted to introduce the 



limestones, which occasionally compose the floor or the roof, 



toinetirue-j in direct contact with the coal. The portion of the 



30 



