EXTENT or PALEOZOIC CONTINENTAL SEAS 311 



continued through the Utica and later stages of the Eden, the elevation 

 must have been very broad and without marked local flexing. Erosion of 

 the Catheys prior to the submergence of the west and north flanks of the 

 dome by the late Cincinnatian or Leipers sea is indicated chiefly by the 

 local absence of some of the upper layers. There are no clastic deposits 

 worth mentioning at the contact of the Leipers and Catheys, The plain- 

 est feature of the physical evidence of the hiatus between the two is the 

 unquestionable landward overlap of the Leipers. The most western ex- 

 posures of the contact contain a number of clearly distinguishable beds 

 in the lower part of the Leipers that are entirely absent in the Nashville 

 hills; and on the south flank, as near Fayetteville, the Leipers is repre- 

 sented by only the uppermost member, the lower Platystrophia bed."^ 



LATE CINCINNATIAN EROSION IN NORTHERN APPALACHIAN AREAS 



While the erosion of the interior areas during the closing Cincinnatian 

 stage of the Ordovician was so gentle that the process is not obviously 

 indicated by resulting clastic deposits, erosion of a more vigorous type 

 prevailed in northern Appalachian regions. This is indicated by land or 

 delta deposits of sandstone (Oswego, "red Medina," and Juniata sand- 

 stones) in New York and central Pennsylvania, in which no trace of 

 marine fossils has yet been found. The fineness of the sand indicates a 

 low gradient for the streams that carried it, but the thickness of the de- 

 posits, which in Pennsylvania locally aggregate over 2,000 feet, shows 

 that the contributing area was large and its elevation considerably greater 

 than the average for the continent at that time. 



EROSIVE AGENCIES RELATIVELY INEFFECTIVE IN NEGATIVE AREAS 



Eeasoning from observed thicknesses and reputed original extent of 

 clastic deposits, it is supposed rather generally that at various times of 

 emergence great sheets of sediments were removed by erosive processes. 

 Geologists holding this view naturally assume also that the continental 

 seas were of great extent and often of very considerable depth ; that they 

 endured with merely local interruption, from one period to another — as, 

 for instance, from the beginning of the Cambrian to the close of the 

 Ordovician. Further consequent assumptions (1) that the local absence 

 or thinning of sediments of certain ages signifies their complete or partial 

 removal in the process of peneplanation, and (2) that the geographic 

 variations observed in the composition of fossil faunas and in the char- 

 acter of marine deposits are due to local changes in contemporaneous 



' The upper Platystrophia bed, indeed none of the members of the McMillan formation 

 forming the top of the Cincinnati section, has been recognized in middle Tennessee. 



