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



Seaweeds have cellular structure alone. They decompose cjuickly 

 whether exposed to atmospheric oxygen or protected from it. They 

 are soon transformed into a fluid, black material which penetrates 

 the sands along the seashore. He thinks it possible that remains of 

 marine algas may have been thrown casually on swamps and that 

 their decomposition products, added to those of the decomposing 

 materials, may have enriched them and may have given cannel. 



J. Geikie** sees in the alternation of coal and limestone, evidence 

 of prevailing subsidence, while the coal seams indicate frequent re- 

 currence of land surfaces. The cannel's and iron-stones show that 

 many wide lakes and lagoons existed. He finds lines and ribs of 

 cannel associated with splint and even ordinary coals, while the can- 

 nel itself passes into common coal or black shale or even into black- 

 band ironstone. The varying conditions are due to the mode of accu- 

 mulation. Cannel was formed under water, for it contains fresh or 

 brackish water fossils. The expanse of fresh water was surrounded 

 by wooded flats; slimy vegetable mud, with, in places, ferruginous 

 matter, was deposited where the streams entered. Along the shores 

 were marsh plants, while farther back were trees and fern under- 

 growth. The last gave ordinary coal, the marshy plants were con- 

 verted into splint, while the slime became cannel, oil-shale or even 

 iron-stone. 



Stevenson,^^ as the result of studies in the Upper Coal Measures 

 (Monongahela) of Ohio and West Virginia, came to the conclusion 

 that at the close of the Lower Barren Measures (Conemaugh) the 

 northern part of the Appalachian area basin was a half-filled trough 

 separated from the western coal areas by the Cincinnati fold. He 

 accepted the in situ doctrine without reserve. The conditions ob- 

 served in the Upper Coal Measures prove a succession of gradual 

 subsidences interrupted by intervals of repose, during each of which 

 a lid of coal was formed over all or part of the basin. The sub- 

 sidence could not have been paroxysmal, for, as the shore line sank, 



^''J. Gcikie, "On the Geological Features of the Coal and Iron-stone- 

 bearing Strata of the West of Scotland," Jonrn. Iron and Steel Inst.. Vol. 

 III., 1872, pp. 13, 14. 



^■■'J. J. Stevenson, " Tlie Upper Coal Measures West of the Alleghany 

 Mountains," Ann. Lye. Nat. Hist., N. Y., Vol. X., 1873, pp. 226-252. 



42 



