418 Williams — Fossil Faunas and their use in 



established by thorough study of the successive formations in 

 which they occur, range through a third and often a half of 

 one of the standard geological systems. 



With such a long range for the life period, it has been in 

 some cases established that the variation between the earliest 

 and latest known representatives of the species is not greater 

 than the variation among representatives of the same species 

 found buried together in the same stratum of rock. Having 

 established such long range and such long persistence of spe- 

 cific characters, and associated with plasticity of the varietal 

 characters for a few well studied species, it seems reasonable 

 to infer that simple specific identity cannot be taken as proof 

 of contemporaneity within narrower bounds than the life 

 endurance of the species. 



A second reason for not resting implicit confidence on this 

 method of correlation is the frequently observed fact, that 

 parts of the geological column of different sections, which 

 upon satisfactory stratigraphic grounds are known to be strati- 

 graphically equivalent, contain different fossils. If two such 

 faunas were to be observed one above the other, the difference 

 in fossils would be naturally interpreted as difference in time, 

 while the fact of stratigraphic equivalency proves their con- 

 temporaneity. Since the non-identity of fossils is consistent 

 with contemporaneity, the reverse is to be inferred, viz : that 

 identity of fossils may be consistent with non-contempora- 

 neity. 



A third observation also may be cited against the principle : 

 In a continuous section it is often the case that the topmost 

 beds of one formation are followed abruptly, but without 

 apparent unconformity, by the next following formation con- 

 taining a majority of new species. 



The abrupt change in the fossils, in such a case, cannot be 

 taken as evidence of the total stopping of the old fauna or of 

 the beginning of the new, but rather as the result of a shift- 

 ing laterally of the conditions in which the faunas have lived, 

 across the locality in which the sedimentation was made. 



It is thus shown that a sufficient amount of doubt is thrown 

 upon the reliability of simple identity of single fossils in deter- 

 mining equivalency of stratigraphic horizon (except in a broad 

 sense) to call for a discussion of some more accurate method 

 of correlation. 



In the following pages is given a synopsis of the results of a 

 prolonged series of investigations directed toward the dis- 

 covery of the true time element in fossil faunas. It will be 

 assumed that the reader is familiar with the distinction between 

 a stratigraphic formation (which is a more or less uniform 

 lithologic unit, and constitutes the thing which is represented 



