IMPORTANCE OF THE UNCONFORMITY 213 



the lake and then eastward to the Pennsylvania line, the Berea rests on 

 the Bedford shale. The Bedford is both a thin and a weak formation, 

 susceptible of easy and rapid erosion. It seldom exceeds 100 feet in 

 thickness and seldom falls below 50 feet, except along the channel bot- 

 toms in the northwest corner, in the deeper of which it is entirely cut 

 out. To believe in a considerable time gap between the two formations, 

 one is forced to believe that along this entire long line oscillation took 

 place, with no warping whatever. The land was either so low that no 

 erosion took place, and the channels seem to negative that, or else it was 

 so evenly uplifted that the same amount of erosion took place everywhere. 

 There is nowhere, so far as known to us, an oscillation involving so long 

 a line without some local warping being involved. We have had occa- 

 sion in the past ten years to study many of the unconformities within 

 the Paleozoic series in New York. Without exception, they are charac- 

 terized by the fact that the formation which follows the break rests on 

 underlying beds which vary in horizon from place to place. The oscilla- 

 tion has been always accompanied by gentle, very gentle, warping, and 

 the upwarped beds have been truncated by erosion before renewed deposit 

 took place. This is the one feature which they all have in common, and 

 the one feature unexplainable on any other theory than that of a pro- 

 tracted erosion interval between the two sets of deposits. It is wholly 

 lacking at the Berea base in Ohio. We are personally unable to conceive 

 of the possibility of an oscillation of the type, involving such a long line, 

 without the appearance of some warping, with ensuing truncation should 

 an erosion interval follow. 



Comparison with the Pottsville TJnconfoumity 



The break at the base of the Pottsville in Ohio furnishes an excellent 

 ilhistration of this ordinary type. It is a real break. It rests on the 

 gently truncated edges of several of the Waverly formations, as it is fol- 

 lowed across Ohio. In the Cliagrin A'alley, east of Cleveland, the Potts- 

 ville base is only 100 feet above the Berea summit and rests on the 

 ()niiigc\ ille. Eastward other bed.s conie in. and when the Pennsylvania 

 line is reacheti the I'ottsville is some 3o0 feet above the Berea, the thick- 

 ness being a<l(!e(l, heij l)y l»e(|. in passing east. West I'loni the Chagrin 

 \'iilliy tlu! same thing takes place, and somewhat more rapidly than in 

 ihe otlier direction. Over most of northern Ohio the Pottsville rests on 

 various beds of the lloyalton i'orniation. West of the Cnyahoga tiie Potts- 

 ville-Berea interval is from .")<»() to I0() feet. \\\ the time lentral Ohio is 

 readied the entire Blaek Hand and Logan formations have come in and 

 the interval expanded to .something like 1,000 feel. 



