94 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



Gresley^^ called attention to the persistence of slate partings in 

 the Pittsburgh coal bed as having an important bearing on the 

 origin of coal beds. Two of them, one fourth to one half inch thick 

 and separated by 3 to 4 inches of coal, are present in an area of 

 15,000 square miles. Under the lower one is a coal bench somewhat 

 more than 2 feet thick, while above the upper one is a bench varying 

 from 3 to 5 feet. The clay of the thin binders or slate partings is 

 extremely fine grained, mottled, non-plastic, contains macrospores 

 and indefinite plant remains, but no Stigmaria. 



Accepting in full the doctrine of transport, he assumes that, at 

 the close of deposition of the lowest bench, that mass of vegetable 

 matter lay practically level on the bottom of a vast lake or inland 

 sea. Such being the condition he finds difficulty in explaining the 

 overlying shale as due to fine material brought in by currents ; the 

 shale is uniform in thickness and composition over a great area, so 

 that the supply of material must have been uniform throughout; 

 there could have been no changes in currents or offshore conditions 

 during the period of deposition. The quantity is not less than 100 

 tons per acre. He finds equal dit^culty in the suggestions that the 

 shale consists of wind-blown dust, that it is a precipitate from solu- 

 tion, that it is concretionary. The supposition that these shales are 

 substitution or replacement formations or that there was a segrega- 

 tion of inorganic substances during solidification or the process of 

 coal- forming involves serious difficulties. " To suppose that such 

 shale bands were originally thin films of chalky mud, since chem- 

 ically converted into silica, alumina, iron, etc., would, I think, be 

 exceedingly unsafe." At the same time, he suggests that the globi- 

 gerina ooze, widespread " over the bottom of the Atlantic, where 

 deepest and farthest from land would seem to furnish us with about 

 the only way (as to physical conditions) in which our shale binders 

 in the 'Pittsburg' coal bed can be imagined to have accumulated." 

 If the lower slate binder was really deposited as silt by aqueous 

 transportation, the interesting query presents itself, How could the 

 succeeding 4 inches of coal be formed in situ? 



"^W. S. Greslcy, "The Slate Binders of the Pittsburg Coal Bed," Amcr. 

 Geologist, XIV., 1894, pp. 356-395- 



94 



