464 J. BARRELL — ORIGIN OF THE MAUCH CHUNK SHALE 



subaerial delta formation. His quandary in this respect is clearly due 

 to his inability to reconcile his logical conclusion from the facts with the 

 still prevailing dictum that sedimentation implied the presence of per- 

 manent bodies of water, the only exceptions of consequence believed in 

 being the exposure through tidal ebb of accumulating mud-flats marginal 

 to the seas. 



Other geologists, not having had occasion to study the formation with 

 the same detail as Eogers and Lesley, have apparently been less impressed 

 with the evidences of subaerial exposure, so that the description of 

 Eogers, now half a century old, is still the most detailed, and probably 

 but few geologists are aware of the extent of these evidences. 



The belief that all the Mauch Chunk sediments were of marine or 

 littoral origin has been the only interpretation definitely expressed in 

 the literature up to 1906. To mention the opinion of the two living 

 geologists most widely acquainted with the formation: Willis, in his 

 paper on the physical interpretation of all the Paleozoic sediments of the 

 Appalachian basin, states that the Mauch Chunk "represents a height 

 of land which was elevated, eroded, and distributed in the Carboniferous 

 sea," and that in the following Pottsville times "the marked contrast in 

 the sediments is significant of a change in depth and slope of the sea 

 bottom. Tide flats of the Mauch Chunk epoch were submerged, and 

 their practically level surface was replaced by one having a decided 

 seaward inclination."* 



J. J. Stevenson, in his summary discussion of the Lower Carbonifer- 

 ous, also speaks of the limits of the sediments as indicating the limits of 

 the water body, and states that "at the east one finds evidence of con- 

 tinued lowering of the mainland and of continued advance of the sea 

 upon a low shoreline."! 



In 1906 Grabau published in a valuable paper a broad discussion, 

 which contains, so far as the writer is aware, the first definite statement 

 as to the fluviatile and non-marine origin of the Mauch Chunk shales. J 

 This conclusion in regard to the Mauch Chunk is drawn, however, with- 

 out detailed argument, but two pages being given to this formation, thus 

 leaving room for the present paper. His opinion appears to be ulti- 

 mately founded upon the growing appreciation by geologists of the im- 

 portance of fluviatile formations in the stratigraphic column, and from 

 the absence throughout the greater portions of the Pocono and Mauch 

 Chunk formations of direct evidence of marine origin. 



• Paleozoic Appalachia. Maryland Geological Survey, vol. iv, 1902, pp. 66, 69. 

 t Lower Carboniferous of the Appalacbian basin. Bull. Geol. Soc. Am., vol. 14, 1903, 

 pp. 94, 95. 



t Types of sedimentary overlap. Bull. Geol. Soc. Am., vol. 17, 1906, pp. 632-634. 



