SILURIAN. 249 



to this series, that State being apparently north of the embayment. The embayment covered 

 Ohio, Michigan, and probably a part of Indiana and extended into Wisconsin. 



Following the deposition of the Lower Monroe came a retreat of the sea and eohan deposits 

 of quartz sands accumulated upon the limestone foundation. These are now seen in the 

 Sylvania sandstone, the source of the material of which probably was the St. Peter sandstone. 

 The Upper Monroe invasion was from the northwest, and it brought with it a wholly new fauna, 

 in which the prevailing element was of Devonic aspect. A large proportion of the species of 

 the Anderdon and Amherstburg beds is most nearly related to the Schoharie fauna, the similarity 

 being often so great that species have been described as Schoharie or Onondaga forms. 



For a list of species more nearly related to Middle Devonian forms than to 

 known Silurian see the publication cited. On these species Sherzer and Grabau ''^^^ 

 comment as follows : 



This list of species shows that the Anderdon-Amherstburg fauna is most nearly related to 

 the Schoharie fauna of eastern New York, and that it probably represented the stock from 

 which that fauna was derived. Coral-reef conditions existed in Michigan and Ontario at that 

 period, the eastern extension of these conditions being first manifested in the water-lime deposits, 

 and later in the Akron, and finally ^in the Cobleskill. This latter marks the period of reestablish- 

 ment of connection with the Atlantic, and we find that this formation is especially characterized 

 by an Atlantic fauna in its more eastern development (Halysites, etc.). The faunas mingled 

 in the neighborhood of the Schoharie region." 



With the opening of the Atlantic connections the late Siluric gastropod and cephalopod 

 fauna entered this region and became characteristic of the succeeding Manhus-Lucas deposits, 

 while the typical Anderdon fauna soon disappeared. A comparison of the Anderdon and the 

 Lucas fauna shows scarcely a common species. In the Amherstburg, however, there is more 

 or less of the commingling of the two faunas. That the junction with the Atlantic was effected 

 while the Amherstburg beds were forming is shown by Siluric gastropod and cephalopod elements 

 in its fauna, and their absence from the Anderdon fauna. The correlation of the Amherstburg 

 and Cobleskill thus seems evidenced. 



Considering the Anderdon fauna as a whole, we see a blending of types of the Siluric with 

 those of Devonic affinities. Recognizing that this fauna is interpolated between two Siluric 

 faunae, we are forced to admit that here is an example coming perilously near satisfying the 

 demands of Barrande's theory of colonies. Somewhere the Siluric fauna must have developed 

 into the Devonic, while in other regions the Siluric fauna still lingered. That this Devonic 

 aspect is that of the mid-Devonic fauna of America rather than the lower shows that this 

 evolution was progressing along different lines from that of the Helderbergian fauna. This 

 latter fauna is alien to North America, as is weU known, having come to us from Europe. 

 Somewhere in northwestern America an indigenous Lower Devonic fauna existed, which in 

 turn gave rise to the Middle Devonic faunas of America. This indigenous American Lower 

 Devonic must have been much like the Middle Devonic fauna, seeing that the indigenous Upper 

 Siluric is already so far advanced as to have a decided mid-Devonic aspect. 



It might of course be argued that the Upper Monroe is the indigenous Lower Devonic of 

 America and that it existed contemporaneously with the Helderbergian fauna. On such an 

 interpretation the Sylvania marks the Siluro-Devonic hiatus, and the upper hiatus representing 

 the folding and erosion of the entire of Monroe and earher rocks falls into the Oriskany. That 

 period, as we know it, was scarcely long enough for the accomplishment of such extensive 

 erosion as is implied in the pre-Onondaga hiatus, though it is known that a considerable amount 

 was accomplished during that time. The strongest argument against such an interpretation is 

 however, the Siluric character of the fauna of the Lucas dolomite and the evident correspondence 

 of the Amherstburg and Cobleskill horizons. 



a Grabau, A. W., Bull. New York State Mus. [No. 92, 1906], p. 131. 



