19"-] STEVENSON— FORMATIOX OF COAL BEDS. 21 



were many alternate periods of movement and of total or compara- 

 tive rest. Limestones indicate periods of comparative tranquillity. 

 Some of the coal beds are of great extent. The Pittsburgh bed had 

 been traced around an area of 14,000 square miles and there are 

 isolated basins holding that bed far southeast from the main area, 

 so that the Pittsburgh coal must have covered a surface of not less 

 30,000 square miles. The uniformity in thickness and the absence 

 of abrupt variations are as remarkable as the area. These features 

 " seem strongly adverse to the theory which ascribes the formation 

 of such deposits to any species of drifting action." 



The alternation of laminae of bright and dull coal ; the lenticular 

 form of the bright layers ; the predominance of mineral charcoal in 

 the dull laminae seem to be almost conclusive arguments in favor of 

 belief that the vegetable matter grew where it was deposited. He 

 finds it difficult to understand why the coal does not consist principal- 

 ly of the larger parts of trees if any drifting agency brought the 

 materials together. The leaves and smaller parts would be detached 

 before the trunks could become waterlogged. 



But the beds have subordinate divisions, coal, clay, impure coal, 

 so persistent in great areas that miners can recognize their bed at 

 great distance from their own locality ; only one method of accumu- 

 lation can explain this. " I cannot conceive any state of the surface, 

 but that in which the margin of the sea was occupied by vast marine 

 savannahs of some peat-creating plant, growing half immersed on a 

 perfectly horizontal plain, and this fringed and interspersed with 

 forests of trees, shedding their offal of leaves upon the marsh. Such 

 are the only circumstances, under which I can imagine that these 

 regularly parallel, thin and widely extended sheets of carbonaceous 

 matter could have been accumulated." The purity of the coal is 

 inconsistent with any notion of drifting of the vegetable matter, 

 "which according to any conceivable mode of transportation, would 

 be accompanied by a large amount of earthy matter, such as abounds 

 in all delta deposits and even mingles with the wood as in the raft 

 of the Atchafalaya." 



The underclay, irregular in structure, accompanies nearly every 

 coal bed in the Appalachian basin and usually contains Stiguiaria 



21 



