DISCUSSION OF CORRELATION 



705 



beKeves that the evidence thus far available, while suggestive, does 

 not constitute a sufficient basis for throwing over the present corre- 

 lation, which brings out the main features of succession in common 

 in the different districts, and that a revision introduces difficulties 

 in local correlation quite as great as any in the old correlation. 

 In order to bring out clearly the possibiHties in the situation, the 

 two views are presented below respectively by R. C. Allen and 

 C. K. Leith. 



A REVISION OF THE CORRELATION OF THE HURONIAN GROUP 

 OF MICHIGAN AND THE LAKE SUPERIOR REGION 



R. C. ALLEN 



The Huronian group comprises at least three unconformable 

 series of pre-Cambrian sedimentary and associated igneous rocks 

 separated from the younger Keweenawan series by a great uncon- 

 formity and from the older Archean system by another more pro- 

 found unconformity. The position of the group has been defined 

 by the United States Geological Survey and by a committee of 

 Canadian and United States geologists^ (1904) as follows: 



Paleozoic 



Unconformity 



Keweenawan 



Algonkian Huronian 



Archean 



Keewatin 

 Laurentian 



Unconformity 



Upper 



Unconformity 

 Middle 



Unconformity 

 Lower 



Unconformity 



Eruptive contact 



Former correlations of the Huronian group as well as the 

 entire pre-Cambrian of the various districts of the Lake Superior 

 region in the United States are mainly the results of detailed field 

 study and mapping by the United States Geological Survey. These 



^ Jour. GeoL, XIII (1905), 89-104. 



