302 E. 0. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



limestone belonging to the Tribune formation of the Chester group. 

 This is the third formation of the Chester group, the Saint Genevieve 

 being the first and the intervening Cypress sandstone, 100 feet or more 

 in thickness, being the second. In something like 20 pounds of Tribune 

 oolite, representatives of no less than 40 of the 70 species originally de- 

 scribed by Hall from the Spergen limestone of Indiana were found. 

 Many of these Tribune examples perhaps might be distinguished from 

 their Spergen progenitors, but I doubt if any paleontologist would think 

 of applying even subordinate designations to them if he did not know 

 that they occur in a much younger formation than the Spergen limestone.* 

 Since this fauna in the Tribune is always associated with other fossils 

 whose age is in no case open to misinterpretation, I question again if it 

 is worth while, from the stratigrapher's standpoint, to undertake the 

 task of discriminating between the Tribune and Spergen representatives 

 of the fauna. 



Finally, I have observed in southwestern Missouri a fifth, and possibly 

 a sixth, occurrence of the Spergen fauna in limestone lenses of a shale 

 thought to be late lower Pottsville in age. One of these may correspond 

 in age to the occurrence in Montana of a mixed Spergen and Pennsyl- 

 vanian fauna reported many years ago by Meek. 



Value of fossil evidence in correlation not seriously impaired by recur- 

 rence of faunas. — It may appear to some that these cases of recurring 

 faunas — or, as Williams probably would call them, shifting faunas — 

 must greatly impair the generally accepted view as to the time value of 

 fossils. As I understand the cases, they do not seriously affect the value 

 of fossil evidence as a whole. Neither does it seem to me that the corre- 

 lation of geographically separated deposits, by which I mean the deter- 

 mination of their contemporaneity, is hopelessly complicated even in the 

 case of faunas known to have two or more distinct time values. Intelli- 

 gently administered, the evidence of the fossils of a formation containing 

 a fauna characterizing two or more formations that are separated by in- 

 tervals in which it does not occur is scarcely less diagnostic than in the 

 case of faunas appearing but a single time in the visible geologic scale. 

 In my opinion a fauna reappearing after a long absence must indubitably 

 have undergone some probably recognizable mutation in at least the com- 

 bination of forms, if not also in the specific characters of the original 

 elements. 



The circumstances that have permitted invasions of the interior seas 

 by extracontinental faunas, and also those that brought about intercom- 

 munication between the more or less distinct inner basins, can in most 

 cases, I believe, be determined with a reasonable degree of probability 



