Appalachian Geosyncline. 231 



rocks are thin, even -bedded shales and sandstones traceable for 

 long distances along their outcrops. They suggest for the 

 greater part of the deposits the smooth floor of an open shallow 

 sea. The dark mud-cracked shales of the Portage in central 

 New York appear to be delta deposits developed because the 

 subsidence was slower than the infilling, but the fine even grain 

 of the sediments shows that even these are far from the sources 

 of erosion and suggest that they once extended farther to the 

 north. 



Hate of Thinning at the Present Margins. 



A method of attacking the problem of the original limits is 

 found in the rate of thinning at the present margins. The 

 data for determining this is, however, somewhat meager and 

 not as accurate as is desirable for reliable results. It further 

 rests upon the assumption that the thinning shown near the 

 present margin continued uniformly to the original limits, an 

 assumption which certainly is not exactly true, but which on 

 the other hand serves as an approximation. The determina- 

 tions check fairly well with the other indications and thus the 

 arguments gain accumulative strength. The data of this 

 nature are as follows : 



The upper Devonian rocks in Pennsylvania and Maryland 

 thicken in general from northwest to southeast. In Maryland 

 the extreme easternmost outcrop is the thickest. Here there- 

 fore the axis of subsidence was to the east of the present eastern 

 outcrop and later complete erosion has advanced westward be- 

 yond the line of maximum thickness. This may possibly be true 

 in northeastern Pennsylvania also, but in Perry County on the 

 Susquehanna, in central eastern Pennsylvania, a thinning exists 

 toward the margin. Through Pennsylvania it seems prob- 

 able therefore that the axis passes under the anthracite coal 

 basins. This strikes east of the limiting outcrops in Maryland 

 and agrees with the indications from that region as to the 

 location of the axis. The eastern side of the original basin of 

 sediments seems consequently in eastern Pennsylvania not to 

 have been wholly destroyed. More carefully measured sec- 

 tions on the two sides of the coal basins are needed however 

 to establish this view. The data supporting it rest at present, 

 so far as known to the writer, upon the sections measured by 

 Claypole in Perry County. He notes the difficulty of obtain- 

 ing reliable measurements in this intensely folded and some- 

 what faulted district. Allowing for these difficulties as far as 

 possible, he states that above Newport on the Juniata river the 

 thickness of the Portage-Chemung rocks is 3300 feet. In the 

 southernmost outcrop, some sixteen miles distant toward the 



