466 J. BARRELL ORIGIN OF THE MATJCH CHUNK SHALE 



without other considerations, they might sometimes lead into consider- 

 able errors of interpretation. 



INFERENCES AS TO GEOGRAPHIC CONDITIONS OF ORIGIN 



Floodplain origin of the Mauch Chunk. — Either mud-cracks, rain- 

 prints, footprints, or impressions of roots in situ were found from the 

 bottom to the top of the formation in the eastern outcrops contiguous to 

 the southern and middle anthracite coal fields, the region of its maximum 

 thickness. . They wei'e also found to occur from the northeasternmost 

 limit, on the Lehigh river, to the southwesternmost limit, at Dauphin, on 

 the Susquehanna. 



The significance of such a widespread distribution of the marks of 

 subaerial exposure has been fully discussed by the writer elsewhere.* 

 It seems unnecessary to repeat the argument, and it need only be said 

 that reasons were shown for considering that such evidences pointed to 

 a continental and more particularly to a fluviatile origin. It is to be 

 concluded, therefore, that the Mauch Chunk sediments were brought by 

 the flood waters of one or more rivers and spread over the river plains 

 in times of flood. In this region of greatest subsidence and accumu- 

 lation and far from the margins of the basin the sea seems to have at 

 no time extended, but to have been kept out by the abundance of the 

 river sediment and the consequent rapidity of the upbuilding, keeping 

 pace with subsidence. 



Relations of land and sea. — The evidence has been given in the first 

 part showing the marked thinning out of the Mauch Chunk around the 

 northern half of the northern anthracite field. It has also been seen 

 that but an insignificant thickness of strata represents the formation 

 through the northern tier of counties in Pennsylvania, and that it is 

 entirely probable, as held by Eogers, that it was never deposited in 

 greater thickness in that region. The lower portion, however, is repre- 

 sented in the southwestern quarter of the state by the northern edge of 

 the Greenbrier limestone, which thickens greatly on passing west and 

 south. Under the Greenbrier limestone in Pennsjdvania lies a thin 

 horizon of red shales, 140 feet thick under the Broad Top coal field, 50 

 feet thick at the most southwestern outcrop in the state. It is not 

 known, however, that these shales are continental and they may well be 

 marine, since a fairly wide zone of marine terrigenous deposits may 

 skirt the land. 



From these relations it is seen that during earlier Mauch Chunk time 



* Mud-cracks as a criterion of continental sedimentation. Journal of Geology, vol. 14, 

 1906, pp. 524-568. 



