I9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 489 



quired low gradients and they carried little coarse material, which 

 was dropped on the border of the low area, while the streams, 

 flowing sluggishly across the flats distributed the abundant fine 

 materials and, as they shifted their courses, sorted the stuff, giving 

 the lenses of clay and sand. The source of the red material is 

 to be sought at the west, for the reds are present mostly on the 

 west side of the bituminous region — even the great Pittsburgh reds 

 extend only a little way east from the middle line of that region. 

 During contraction of the area of deposition, the Mississippian 

 beds became exposed to erosion while the calcareous deposits of 

 the low-lying Cincinnatia had been converted by solution in their 

 exposed portions into red clays such as one sees in so much of the 

 Great Valley within Virginia and Pennsylvania. During deposition 

 of the Pittsburgh reds, residual soils of the northern land must 

 have been an additional source of supply. It seems wholly pref- 

 erable to regard the Pennsylvania reds as derived from reworking 

 of deposits already red. At the same time, one cannot suppose 

 that the local and widely separated patches in the Monongahela, 

 Washington and Greene were derived from a distant source. Prob- 

 ably they mark the sites of ponds into which the sluggish streams 

 carried muds due to decay of limestones and mingled with those 

 from the fine shale of the red region. 



Surface Markings on Shales. — There are no recorded observa- 

 tions of surface markings on Pennsylvanian red shales within the 

 Appalachian basin, though such markings are sufficiently abundant 

 in the Acadian region. But sun cracks and ripple marks are 

 common enough on shale beds of other types. Footprints were 

 found by Mason*'" on slabs of slate from the roof of the Alammoth 

 coal bed. The surfaces show occasional ripple marks along with 

 the tracks of a four-footed animal arranged in regular sequence. 

 This appears to be the only case recorded within the Appalachian. 

 There is little reason to expect such discovery in the bituminous 



•* W. D. H. Mason, " On the Batrachian Foot-tracks from the Ellan- 

 gowan Shaft in Schuylkill Co., Penn.." Proc. Amer. Phil. Soc, Vol. 17, 

 1878, pp. 716-719. 



