296 ANNALS NEW YORK ACADEMY OF SCIENCES 



quently in regions far removed, show a basal sandstone or conglomerate 

 with more or less pronounced evidence of preceding erosion or uplift. 

 Such events though often far reaching must in the broadest sense have 

 been only local, but we may fairly expect the more subtile changes of 

 environment, such as depth, temperature, currents, which govern the 

 character and distribution of animal life, to have extended far beyond the 

 area of great disturbance, and we may look for evidence of such disturb- 

 ance in the fossil floras and faunas, when none is to be found in the rocks 

 themselves or their relation to one another. In fact, such paleontologic 

 evidence may have been all but universal. Conglomerates and uncon- 

 formities and faunal changes occur at other horizons than the division 

 lines between systems, but such evidence will be of great importance in 

 deciding the point in question. 



In addition to the kinds of evidence already mentioned as often accom- 

 panying the transition from one geologic system to another, that is, an 

 interval of erosion, a basal sandstone or conglomerate and a well-marked 

 faunal change, there are also certain other considerations of a more ad- 

 ventitious or incidental nature. For practical purposes, it would be 

 unfortunate if this line (that between two systems), which of all lines it 

 is desirable to represent on a geologic map, were taken where it could 

 with difficulty be recognized in the field, as in the middle of a uniform 

 lithologic interval, or where the evidence would often be concealed, as 

 would be the case in some regions if it were assumed to lie between two 

 formations of soft and easily disintegrated material. Furthermore, the 

 importance of convention also enters the consideration. Other things 

 being equal, it is clearly preferable to take for the boundary which is 

 sought the same horizon at which it has been drawn elsewhere, if that 

 horizon can be determined. 



The boundary between the Berea sandstone and the Bedford shale in 

 some degree satisfies all these requirements. It is easily recognized and 

 easily traced; it also appears to be the locus of an unconformity. 2 The 

 Berea answers to the basal conglomerate of theory. While the Bedford 

 seems properly to form part of the great shale series which preceded it, 

 the Berea marks the change to another and different type of sedimenta- 

 tion. The passage from Bedford to Berea is also marked by an abrupt 

 and strong faunal change. Not only are the two faunas widely different, 

 but the Bedford has a preponderant Devonian facies and the Berea a pre- 

 ponderant Carboniferous facies. To some extent, the faunal change at 



2 This erosional unconformity was clearly stated by Newberry as early as 1874 (Ohio 

 Geol. Survey, Geol., vol. 2, p. 91). More recently it has been mentioned by J. B. Hyde 

 (Jour. Geol., vol. 19, 1911, p. 257) and described by W. G. Burroughs (idem, p. 655). 



