PENNSYLVANIA AND DEPOSITS THEREIN. 71 



though it increases to nearly a mile near the mouth of the stream. At the 

 level of the gradation plan there is a general width of about i mile. This 

 gradation plain is capped by a deposit of sand and gravel, with an average 

 thickness of perhaps 40 feet, that serves to accentuate the terrace-like appear- 

 ance, for it fills up small trenches that have been cut in the gradation plain 

 prior to the gravel filling. 



This statement, and the description of the preglacial Clarion 

 above, prove that the Clarion was the preglacial dominant stream in 

 Western Pennsylvania. The above trenching cut its rock floor so 

 far below those of its affluents that they are strongly refreshed for 

 a few miles from their mouths, and seem to leap into it. This ap- 

 plies also to the Allegheny. The recency of the Clarion trenching 

 is indicated both by the freshness and steepness of its rock walls, 

 Leverett concludes his description of this trench with the words : 



It is hardly necessary to state that just above the level of this gradation 

 plain the bluffs are far more worn and receding than in the inner or canyon 

 valley lying below it. 



This freshness of the Mississippian-Pennsylvanian measures 

 along the Allegheny is paralleled in the denser outcrops of the 

 same in the Anthracite basins — notably at Morea. Fig. 10 shows 

 that the resistance of the anthracite bed is greater than that of the 

 top rock, and Dr. Kiefer's analyses^ tell that the carbon ratio (38.37) 

 of the beautifully polished surface was the highest of all the sam- 

 ples : that samples taken 60 feet below the surface came next with 

 38.20, and the mealed anthracite, ground up by the ice and found 

 directly against the polished surface and below 8 feet of gravelly 

 drift, showed 11.05 ^^''^ ii.i8. This is vastly different from the 

 "black dirt" of an imglaciated outcrop with its low ratio of 1.23. 

 Dr. BarrelP reported, in strength tests : 



Samples near the surface of North Crop are as strong, if not stronger, 

 than those at a depth of 55 feet. Sample No. 3 taken 250 feet below surface 

 was an especially hard, solid piece of coal, and gave fairly uniform results ; 

 but its average is not different from that of the more fissile samples taken at 

 the North Crop. 



This bears upon the finding of fresh pieces in the Kansan 

 gravels, and especially of crystallines from the preglacial surface 

 which were given the usual concentric shells of weathering before 

 incorporation in the glacier ; but which have been irregularly gla- 



