Devonian Shales of Ohio and Pennsylvania. 39 



ter and is called Olentangj shale. It is of course a well-known 

 fact that the whole body of Devonian shales thins toward the 

 west, but that does not alter the conditions stated above. The 

 source of clastic material was to the east, probably Appalachia, 

 and it stands to reason that the amount deposited should de- 

 crease in proportion to distance from its source. 



The cause of the black color of the shales is admittedly due 

 to a high percentage of carbonaceous matter. Various theories 

 have been presented to account for its presence. Twenhofel,* 

 in a paper recently published, throws a good deal of light on 

 the probable conditions existing at times of black shale forma- 

 tion. His article also includes an excellent summary of other 

 theories. J. M. Clarke infers deep water conditions of a more 

 or less isolated basin and refers to the report of Andrussow on 

 the bionomic conditions obtaining at present in the Black Sea. 

 Professor Schuchert thinks that conditions favorable to the 

 formation of black shale imply the existence of ' cul de sacs ' 

 where incidentally the presence of many sulphur bacteria ex- 

 plains the abundance of pyrite and marcasite usually found in 

 such shales. Ruedemann, on the other hand, does not think 

 enclosed basins are necessarily demanded to explain the pres- 

 ence of carbonaceous matter but merely quiet conditions of 

 deposition and hence distance from the shore line. 



We can safely assume then that during upper Devonian 

 times the island of Cincinnatia presented low-lying shore lines. 

 Furthermore the erosion of the Ordovician and Silurian lime- 

 stones from it would not be likely to furnish much detrital 

 matter. The extensive swamps reaching far inland and over- 

 grown with pteridophyte vegetation supplied an immense 

 amount of slowly decaying plant material to the streams slug- 

 gishly entering the sea. For a long period this was not carried 

 any great distance out, except at intervals, but transferred and 

 distributed mainly along the shore by littoral currents. Dur- 

 ing Chemung times, however, strong currents tending toward 

 the east became more pronounced and more consistent. The 

 material was swept further away from the land and finally 

 toward the close of the Chemung even reached the eastern 

 border of the state. 



Devonian Shales of Pennsylvania. 



Venango : In his exhaustive study of the oil districts of 

 western Pennsylvania, J. F. Carll delimited a series of rocks 

 consisting of shales and three prominent oil-bearing sandstones 

 under the term Yenango oil sand group. f These rocks out- 



* Twenhof el, W. H. , Note on Black Shale in Making, this Journal, xl, 

 p. 272, 1915. 



t See Eeports of Progress of 2d Pa. Geol. Survey, I, 1875 and I^, 1880. 



