PREVIOUS OPINIONS 465 



The particular reasons advanced for the conclusions are based on: 



First. The general absence of marine fossils, especially in the eastern 

 outcrops (page 630), the presence of a 1-foot seam of coal 80 feet 

 above the base of the Pocono in Lycoming county (page 631), and, in 

 the Mauch Chunk, "the ripple-marks, sun-cracks, rain-drop impressions, 

 and footprints of vertebrates — all signs of floodplain deposits" (page 

 633). 



Second. The progressive overlapping westward of the Pocono and 

 upper Mauch Chunk. 



'V^Tiile these facts are undoubtedly of great significance, they are sug- 

 gestive rather than conclusive and need support by detailed observations 

 and discussions, as may be shown: 



First. Sandstones and conglomerates are frequently barren of fossils, 

 whether they be continental or marine. Marine fossils- are found in cer- 

 tain shaly layers of the Pocono in Garrett county, Maryland, and in 

 Huntington and Bedford counties, Pennsylvania, as noted by Grabau. 

 No characteristic distinction is stated between the sandstones which are 

 surely marine and those which are surely continental, the increased 

 coarseness on the eastern side being noted, but greater coarseness toward 

 the source of supply is true of both marine and fluviatile detritus. The 

 presence of marine fossils in one place or of a coal bed in another is not 

 evidence that any great part of the contiguous strata are either marine or 

 continental, since in land-delta and shallow-sea deposits a widely fluc- 

 tuating strand line is to be expected on account of the very flat profile 

 of both the land surface and sea bottom. Finally, so far as geological 

 literature is concerned, ripple-marks, sun-cracks, rain-drop impressions, 

 and footprints of vertebrates, spoken of by Grabau as "all signs of flood- 

 plain deposits," are rated as more characteristic of tidal flats than of 

 floodplains. 



Second. Transgressive overlap toward the source of the sedimentary 

 material is taken by Grabau as positive evidence of the presence of an 

 invading sea (page 570), and progressive overlap of beds of shale or 

 sandstone barren of marine fossils away from the source of supply is 

 considered as still more positive evidence of fluviatile and terrestrial 

 deposition of the sedimentary beds in question (pages 635, 636). 



While the present writer is in general agreement with the conclusions 

 in regard to the particular formations cited, yet he does not believe in 

 the universality of these principles without qualification, or that they 

 alone will suffice to prove the marine or non-marine deposition of shales 

 and sandstones in general. On the contrary, it would seem that, applied 



XL — Bull. Gbol. Soc. Am., Vol. 18, 1906 



