I9II.] STEVENSON— FORMATION OF COAL BEDS. 83 



clav is not essential for vegetable growth, but more than three 

 fourths of the coal beds have it. The " bullions," composed of fossil 

 wood, occasionally show rootlets working their way through the 

 decaying wood, separating the fibers which now surround them. 

 The fossil wood is often parallel to the bedding of the coal, a condi- 

 tion familiar in prostrate forests and in peat accumulations. Erect 

 trunks and stems are unusual both in coal and peat. The underclay 

 was the land surface which supported vegetation like the forests 

 of swamps where warmth and moisture prevail. 



If coal is to be considered as derived from drifted material, 

 he is puzzled to discover what has become of the shells and fishes, 

 which must have abounded in the tracts of water in wiiich the 

 deposits were laid down. To float some of the large trees either 

 vertically or horizontally, with their outspread roots having a radius 

 of 15 to 20 feet, would certainly require enough water to accommo- 

 date fishes and mollusks. Remains of fishes are not necessarily de- 

 stroyed by embedding them in coal-forming material, and shells are 

 as capable of resisting destruction as fish spines are. Shells and 

 fish remains occur often in impure cannel. The " bullions " have 

 yielded no shells, and fish remains are very rare in pure coal. That 

 the trees were forest growth is proved by the splendid specimens in 

 the ]^Ianchester and other museums. 



Estuarial swamps with intermittent subsidence, permitting de- 

 position of sand and mud, would explain alternations of coal and 

 other strata, whichever theory of coal accumulation be accepted. 

 Marine conditions frequently followed directly upon formation of 

 a coal bed ; fishes of shark-like types are in shales directly overlying 

 coal at many horizons. But shells and fish are unknown in the 

 underclay. 



Orton,^- in his description of the coal-fields of Ohio, considers 

 the various theories of formation ; some of them appear to be based 

 on merely local conditions, others are extravagant and only a very 

 small proportion of the explanations seems to have been the result 

 of careful observation in extensive areas. 



In a coal-field, one finds a system which can be explained onh' 

 by subsidence. Limestone is found above and below coal beds and 



*-E. Orton. Geol. Survey of Ohio, Vol. VII,, Antioch, 1893, pp. 256-262. 



83 



