REVISION OK STKATIGRAPHIC CLASSIFICATION 301 



SO closely allied to well known Utica shale fossils that unless they are 

 very thoroughly compared the distinctions may easily be overlooked. 

 Since the lithologic character of the three formations likewise is prac- 

 tically identical, it is not surprising that they have been regarded as con- 

 temporaneous. However, the facts in the case are that the Sylvan and 

 Maquoketa shales overlie unquestionable Eichmond faunas, while the 

 Utica lies just above the Trenton at the base of the Cincinnati series. 



Recurrences of the 8 per gen fauna. — We will now briefly consider the 

 most remarkable case of recurrent faunas so far discovered in America. 

 This fauna was first described by Hall in 1858. Since that date it has 

 been known as the Spergen, the name being taken from Spergen Hill, 

 Indiana, where Hall's original material was collected. Essentially it is 

 an association of diminutive molluscs — ^mainly gastropods and pelecy- 

 pods of many species — found almost invariably in oolitic limestone. 



The first appearance of this diminutive fauna that we can definitely 

 locate in the geological column is in the Warsaw formation. Here, how- 

 ever, the fauna is represented by comparatively rare individuals of only 

 a few of the characteristic species. This may be due to the fact that 

 conditions favoring the deposition of oolitic limestone did not then pre- 

 vail in the areas in which the Warsaw is found. 



The second appearance in the Mississippi Valley is in the Spergen 

 limestone, in the Indiana outcrops of which the fauna was first discov- 

 ered. In the oolitic limestones of this formation it occurs locally in 

 extraordinary development, not only in the way of individuals, but also 

 in species. 



Following the Spergen oolites, the section passes through 300 feet or 

 more of Saint Louis limestone, distinguished lithologically from the beds 

 next beneath and above it by its more compact texture, darker color, and 

 more cherty character. Though the Saint Louis limestone is rather gen- 

 erally fossiliferous, the distinctive Spergen fauna is wholly unknown in it. 



The formation next above the Saint Louis is the Saint Genevieve lime- 

 stone, the beds making up the lower and middle parts of which are 

 prevailingly oolitic. N'ow, many of these Saint Genevieve oolites are 

 simply crowded with typical Spergen shells. Hand specimens might be 

 selected that could not be distinguished from samples of typical Spergen 

 rock. Out of 43 species found in the oolitic beds of the Saint Gene- 

 vieve, no less than 35 have been recognized as belonging to the Spergen 

 fauna. Fortunately one or more of a half dozen corals and crinoids that 

 experience has proved to be strictly characteristic of the Saint Genevieve 

 have never failed to reward a few minutes of search. 



The fourth appearance of the Spergen fauna occurs again in oolitic 



