Grout and Broderick — The Huronian. 



201 



cores show a minute banding in red and white bands. 

 The outcrops at the east end of the range show some beds 

 of the same red color, but over large areas the color has 

 been changed to gray, apparently by the alteration of 

 hematite to magnetite. The iron oxide in the chert con- 

 sists of scattered grains, arranged so as to produce the 

 banded appearance. 



Associated beds carry quartz, amphibole, and magne- 

 tite as predominant constituents, with many other min- 

 erals in small amounts. The occurrence of carbonates 



Fig. 2. 



Fig. 2. Sketch of smooth vertical cross section through the upper algal 

 beds (natural size), Biwabik formation. 



and graphite may be in some way related to the organ- 

 isms giving structure to the chert, but the detail of that 

 relation is not clear, for neither mineral has been 

 detected in the cherts that show the best structures. The 

 graphite, however, probably indicates that the conditions 

 of accumulation of most of the formation were not unfa- 

 vorable to organic growth. 



