ANNUAL REPORT FOR 1875. 83 



at the junction of Lake Huron and Lake Michigan, occupying the 

 entire island of Bois Blanc, occupying most of the small peninsula 

 west of St. Martin's Bay, and skirting the Straits of Mackinac along 

 the northern shore of the southern peninsula of Michigan. It over- 

 lies the Salina formation, and, consequently, is in the exact strati- 

 graphical place of the Lower Helderberg. Speaking of these same 

 rocks, Dr. Rominger says (Geological Survey of Michigan, vol. I., p. 

 28, of his division of the subject), " Taking into consideration the 

 stratigraphical sequence, surmounted above by well characterized 

 Upper Helderberg strata, underlaid below by beds of perfect litholog- 

 ical resemblance to the Onondaga salt group, we may safely take th e 

 intermediate beds as contemporaneous with the Lower Helderberg 

 gro x up." 



Now the formation, under discussion in the state of Wisconsin, has, 

 so far as the underlying strata are concerned, the same stratigraph- 

 ical relations as the Water-lime subdivision of the Lower Helderberg in 

 New York, Canada, the Appalachian region (and westward to the 

 Mississippi river), and on the islands and adjacent shores of Mackinac 

 straits. The Salina or Onondaga salt group underlies it here as else- 

 where. 1 



As an exception to the general statement, if Mr. Worthen is cor- 

 rect, not only the Salina, but other formations are wanting below the 

 Water-lime in southern Illinois, down to the Cincinnati limestone. 

 But in this limited region of Wisconsin, the underlying Salina is 

 present, and crops out all around the Water-lime in a narrow circular 

 rim. 2 The ancient Silurian sea slowly receded from the Archaean 

 peninsula in northern central Wisconsin, with its outlying islands 

 and reefs, depositing in turn the Potsdam sandstone, the -Calciferous 

 or Lower Magnesian limestone, the St. Peters sandstone, the Trenton 

 limestone, the Galena limestone, the Clinton and Niagara groups, the 

 Salina, and, finally, at the last point of emergence, the Water-lime base 

 of the Lower Helderberg. 



There is a stronger argument from stratigraphy than that of the 

 normal relations of the Salina and the Water-lime. 



If the formation in question is Hamilton, then the absence of the 

 intermediate strata must be accounted for, and reconciled to the facts 

 of local, as well as North American geology. Between the Salina 



1 Recent investigation has shown that the rocks here called Salina are themselves 

 probably the Lower Helderberg. T. C. C. 



2 On the maps published previous to the more careful investigations o.f the present 

 survey, the formation was represented as here stated, but upon confessedly imperfect 

 evidence. It is now shown that this mapping was unwarranted. T. C. C. 



