SlLURlC FAUNA FROM SOUTHERN MICHIGAN 553 



of colonies. Somewhere the Siluric fauna must have developed into the De- 

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

 aspect is that of the niid-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 

 well known, having come to us from Europe. Somewhere in northwestern 

 America an indigenous Lower Devonic fauna existed, which in tv;rn 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 Helder- 

 bergian 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 earlier rocks falls into the Oriskany. That period, 

 as we know it, was scarcely long enough for the accomplishment of such ex- 

 tensive 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 Am- 

 herstburg and Cobleskill horizons. 



NOMENCLATURE AND SUBDIVISION OF THE UPPER SILURIC STRATA OF 

 MICHIGAN, OHIO, AND WESTERN NEW TORE 



BY A. C. LANE, CHARLES S. PROSSER, W. H. SHERZER, AND A. W. GRABAU 



[Ahstracf] 



Contents 



Page 



The Monroe formation 55;^ 



Location and general characteristics 553 



I. The Lower Monroe 554 



Proposed name — Bass Islands series ._ 554 



a. The Greenfield dolomite 554 



6. The Tymochtee shales and limestones 554 



f. The Put-in-bay dolomites 554 



(/. The Raisin Elver dolomites 554 



II. The Middle Monroe 555 



III. The Upper Monroe 555 



Proposed name — Detroit River series 555 



d. The Flat Rock dolomite 555 



6. The Anderdon limestone 555 



c. The Amherstburg bed 555 



(7. The Lucas dolomite 555 



Proposed classification 550 



The Monroe Formation 



LOCATION .WD GENERAL CHARACTERS 



The highest Siluric strata of America are represented by the Monroe forma- 

 tion of Michigan, using this term in its limited sense for the post-Salina Siluric. 



