to the Ohio Shale Problem. 183 



limestone (chiefly) beneath the Onondaga limestone ; at the 

 former there are, according to Orton's interpretation, all of 

 1,600 feet of limestone and salt beds belonging in the same 

 interval. Moreover, analysis of the records indicates that beds 

 of great thickness in either well are either thin or entirely 

 absent in the other. In the Silurian and early Devonian, as in 

 the late Devonian and Waverlyan ages with which this dis- 

 cussion is chiefly concerned, the waters invaded alternately 

 from the east and south, or possibly north, and the deposits of 

 each overlap and finally pinch out in the direction of the 

 invasion. 



(5) This new interpretation of the " Devono-Carbonifer- 

 ous shale problem" seems to me to have great advantages over 

 those hitherto suggested. In the first place, it offers a much 

 more simple and, may I say it, rational correlation and classifi- 

 cation of the stratigraphic units involved. Next it does away 

 with that uncertainty (connected with the identification of the 

 assumed Chagrin element in the middle part of the series) 

 which has been so prominent a feature of the problem hereto- 

 fore. It substitutes sharp diastrophic boundaries between the 

 upper Devonian Chagrin and the overlapping base of the 

 Chattanoogan series for assumed and difficultly placeable tran- 

 sitions, and it does away with the necessity of explaining how 

 a sharply defined series of alternating gray and black Devonian 

 shales, that maintains its lithologic characteristics from central 

 Pennsylvania to Cleveland, could yet change its character mate- 

 rially in the next 40 to 50 miles to the west from Cleveland. 

 Third, it makes an essential unit of two faunas whose likeness 

 has been generally recognized — and is in truth undeniable — 

 but which have hitherto been conceived as widely separated 

 by a thick series of upper Devonian beds holding a very dif- 

 ferent fauna. Finally, it ascribes the local variation in strati- 

 graphic sequence chiefly to differential oscillation and invasion 

 of distinct seas and to consequent overlap in different direc- 

 tions of deposits representing the several invasions, and only to 

 a very limited extent to variations in kind and rate of deposi- 

 tion. 



