116 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



toward the west in its lower portion, so that if the direction were 

 retained, it would be continuous with a tributary of the main stream 

 within less than 900 feet. Several branches were followed to their 

 heads ; in each case the channel became shallower and at length dis- 

 appeared in the roof. At the sides of all the channels, one finds inter- 

 locking coal and clay and the adjacent coal is always tender, finely 

 fractured but pure. The indurated clay, filling the channels, passes 

 upward into shale. 



Platt^-* has described a complicated channel of this type, seen by 

 him in Westmoreland county of Pennsylvania. A " rock fault " in 

 the Millwood Colliery on the Pittsburgh coal bed was traced for 

 more than 1,200 feet. The roof is a grayish clay shale, which, at the 

 edge of the " fault," descends suddenly through the bed and spreads 

 out on the underclay. The sides slope at 20 degrees and upward. 

 The width of the clay deposit averages 100 feet, but in one cross- 

 heading the maximum, 120 feet, was found. Along the median line, 

 wedge-shaped masses of the Pittsburgh sandstone replace part of the 

 clay. Coal is found in the clay at the sides but not elsewhere. Close 

 to the fault, the coal is twisted, hard, lusterless, and has so much 

 slate as to be worthless for fuel. This condition changes gradually 

 away from the clay and at 400 feet the coal equals that from other 

 mines in the region. This description by Piatt is that of a filled 

 channel, originally occupied by a stream during the whole period of 

 the Pittsburgh coal, a stream subject to floods and carrying muddy 

 water which left its silt on the vegetation during overflow. The 

 stream became insignificant during deposition of the overlying shale 

 and its narrowed channel was obliterated during the early stages of 

 the Pittsburgh sandstone. Similar " washouts " occur in other 

 mining properties within the district, but their relations have not 

 been worked out. 



Descriptions of such channels abound in the reports in several 

 states. Ashley and Udden have recorded instances like those of 

 southwestern Pennsylvania, where the old channel way was filled 

 with a conglomerate mass of pebbles, lumps of clay and coal, with 



''' W. G. Piatt, Sec. Geol. Surv. Penn., Rep. H4, 1878, pp. xxv, xxvi. 



