DISCUSSION OF CORRELATION 727 



Falls region. They are areally contiguous, contain the same 

 lithologic types, and throughout contain iron formation of remark- 

 ably uniform type and occurrence. To assume that they are not 

 of the same age requires not only the introduction of an entirely 

 hypothetical unconformity, but also the assumption that the 

 Negaunee formation has entirely changed its character to the exact 

 extent necessary to make it similar to the slate-iron formation of the 

 Crystal Falls-Iron River district. Such a thing is of course possible, 

 but extremely improbable. 



Nowhere in the Crystal Falls-Iron River district has the close 

 folding and erosion disclosed the distinctive Siamo slates of the 

 underlying Ajibik quartzite characteristic of the horizons below the 

 Negaunee. Neither has it disclosed any formations underlying 

 the Ajibik close at hand, with the exception of certain volcanics. 

 Somewhere they should appear on the erosion surface if the series 

 were really Middle Huronian. 



That there are two slates in the Iron River-Crystal Falls district, 

 as argued by Allen, is possible, but no trace whatever of an uncon- 

 formity has been found, and the differences in lithology which he 

 cites as evidence are completely duplicated by differences in 

 lithology in the Upper Huronian slates of the Marquette district. 



The principal argument for a change in correlation is based on a 

 supposed direct areal connection of the Negaunee iron formation of 

 the Marquette district with the iron formation of the Iron River- 

 Crystal Falls district. With some interruption, due to folding, the 

 Negaunee iron formation has been traced southwestward from the 

 Marquette district to a point in the northeast portion of the Crystal 

 Falls district near Amasa, on the west side of an area of underlying 

 Hemlock volcanics. Somewhere south of this area the character of 

 the iron formation changes, for before reaching the iron formation 

 belts in the slates (Upper Huronian) in the vicinity of Crystal Falls, 

 the formation is of different character, the underlying fragmental 

 phases are absent, and both walls are conformable slate. The diffi- 

 culty of drawing a line here, in the absence of exposures and com- 

 plete exploration, has long been obvious, and yet the contrast in 

 types and occurrence of iron formation seem to require that some 

 line be drawn. In successive maps the line has shifted more or less. 



