THE LOWER HURONIAN. 327 



over, a duplicate succession of the iron formation and other rocks could be 

 determined, showing the structure to be synclinal. 



The iron-bearing' formation on the Canadian lakes is locally consid- 

 erably folded, but in no cases does this plication reach the extreme that it 

 does in the Soudan formation. In a way we may consider the plication 

 of the rocks as a measure of the frequency and intensity of the folding. 

 Therefore this very noticeable difference in the folding of the two iron- 

 bearing formations, which are essentially of the same petrographic character, 

 is indicative of the lesser age of the one under consideration, which has 

 been assigned to the Lower Huronian. 



The iron formation at the northeast end of Ogishke Muncie Lake is 

 also in a syncline, the formation appearing on both the southeast and north- 

 west shores of the lake The formation is here a carbonate-bearing slate 

 and is especially prominent on the southeast shore, where at several places it 

 occurs in high cliffs with a deep brown ocher-colored crust. When this is 

 removed it discloses a clean, white, almost pure carbonate rock forming the 

 main mass of the ledge. No crumpling could be detected in the formation 

 itself, although the folding of the formation in general is shown by the 

 synclinal structure of the lake basin. 



PETROGRAPHIC CHARACTERS. 



The iron-bearing Agawa formation consists of two petrographic facies, 

 a carbonate-bearing slaty facies, and a chert, jasper, iron-oxide, and slate 

 facies. These do not occur together, but as they occupy the same relative 

 position, at the base of the Knife Lake slates, they are supposed to belong 

 to the same horizon. The presumption is that the carbonate-bearing facies 

 is of essentially the same kind of material as that from which the chert, 

 jasper, and iron-oxide facies has been derived as the result of processes 

 of metamorphism similar to those which have taken place in the production 

 of the normal jaspers and ii'on oxides from the ferruginous cherts in the 

 other iron formations of the Lake Superior region (p. 192). 



The first phase of the iron-bearing formation, the carbonate-bearing 

 slates, are best developed on the southeastern shore of Ogishke Muncie 

 Lake. There they lie at the base of the Knife Lake slates, resting imme- 

 diately upon the Ogishke conglomerate, which has been derived from tlie 

 subjacent Ely greenstone. These carbonate-bearing slates have been 

 traced along the southeast shore of the northeast arm of this lake by means 



