COMPARISON WITH POTTSVILLE UNCONFORMITY 215 



Cleveland shale as contributing elements to the Chattanooga shale. This 

 does not seem to us indicative of any particular time gap between the 

 Berea and Bedford. 



At the base of the Pottsville in northern Ohio precisely the same sort 

 of channeling of the underlying surface is observable as at the Berea base. 

 This seems to us a feature produced entirely subsequent to the general 

 long period of erosion which truncated the gently warped surface of the 

 Mississippian rocks before Pottsville deposition began. After the base- 

 leveling was complete the surface was channeled by the currents, which 

 transported the gravel and sand which constitute the basal Pottsville. 

 The channeling does not of itself suggest a time gap; it is the previous 

 warping and the truncation of the warped beds by long erosion, so that 

 the rock just under the Pottsville varies much in horizon across tlie 

 State, which demonstrates the unconformity and proclaims its impor- 

 tance. It is just this sort of evidence which absolutely fails in the case 

 of the Berea base. 



Conclusion 



The matter is here discussed and this view emphasized because of the 

 present tendency on the part of several geologists to draw the line be- 

 tween tlie Devonian and Mississippian at this Bedford-Berea horizon and 

 to quote the imconformity as evidence justifying the procedure. If this 

 were a m.ere local question affecting only Ohio and Michigan, I should 

 not have discussed it. I readily admit that, in so far as considerable 

 parts of tliose two States are concerned, this is perhaps the most con- 

 venient and easily recognizable horizon at which to draw this line. I 

 also willingly admit that it is a better place to draw it than at the base 

 (){' the Beilford shale, where it was placed by Orton and where there is no 

 break at all, so far as I can discover. But it would seem that this con- 

 venient line in Ohio is located at the expense of the geologists of Ken- 

 tucky, New York, and Pennsylvania, in which States it is far less easy 

 of recognition or else not recognizable at all. But this is a long ami in- 

 volved question, with many phases, and my purpose is simply to empha- 

 size one of these. 



In ni;iny districts the line between the Devonian and Mississippian is 

 a confessedly difficult one to draw. Nowhere is this more true than in 

 Ohio, where difference of opinion cimcerning its proper location has long 

 prevailed and still prevails. Unless we are totally at fault in our at- 

 tempted locations of this boundary, we must conclude that eitli'T in many 

 localities the Devonian passed into the Mississippian without any con- 



