552 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i, 



Mississippi and the filled channels so often disclosed when a stream 

 in flood cuts across its " bottom." The distinct evidence of sorting 

 of materials in the red shales, where clay and sand are in dove- 

 tailing lenses, as well as in some conglomerates, where hardly 

 enough fine material remains to bind the pebbles, leaves little room 

 for doubt that the work was done by streams moving rapidly in 

 some cases, slowly in others. The pebbles are not flat, such as one 

 may find on a shore, but oval or sub-spherical, river pebbles, and 

 their gradual decrease in size as well as number in certain direc- 

 tions shows that the materials were rehandled many times. The 

 rounded pebbles of coal and carbonaceous shale prove equally with 

 those of quartz and sandstone that the deposits, whence they came, 

 cropped out and were exposed to attack by streams of water. The 

 marine limestones, with one exception, are in definite, long, narrow 

 and com.paratively insignificant areas, and pass, at the borders, where 

 those remain for observation, into sandstone, chert or shale, the 

 condition being that of an estuary surrounded by lowland, whose 

 rivers bring a minimum of sediment. The shallowness of the water 

 by which sediment was distributed and the short duration of the 

 flooding are disclosed by wave marks, sun cracks and footprints of 

 animals, occurring at so many horizons, while the moderate depth 

 of the estuaries, in which limestone was formed, is apparent from 

 the shore conditions of the limestone. The testimony of the fauna 

 is confirmatory; that life needed not deep water, for it persisted to 

 the very shore line in Ohio. Unconformability by erosion or by 

 overlap marks the contact of Pennsylvanian with the underlying 

 Missis'sippian in almost the whole basin, showing that the great part 

 was dry land. 



The record appears to show that the Appalachian basin, between 

 the Alps-like Appalachia at the east and the low-lying Cincinnatia 

 at the west, was divided longitudinally by the flat-topped and only 

 moderately high Alleghania. The deepest portion of the eastern 

 valley lay close to the foot of Appalachia, whence the surface rose 

 westward almost inperceptibly to the crest of Alleghania. The 

 western valley extended as a plateau with its low line crossing eastern 

 Ohio in a south-southwest direction and deepening southwardly. The 

 thickness of deposits in the two valleys is no index to the difiference 

 in altitude of the surface ; the eastern valley is coincident with the 



