Grout and Broderich — The Huronian. 205 



advanced too far to leave any such structures. The thin 

 sections show crystalline iron oxides scattered in rough 

 lines across the fine-grained chert. (Fig. 4.) 



Conditions of Deposition. 



Van Hise and Leith have argued that the Biwabik for- 

 mation was deposited in shallow water. 7 This is clearly 

 indicated by the flat forms of the pebbles in the intraf or- 

 mational conglomerate. They find further evidence in 

 the dominance of quartz over carbonates, the structural 

 differences between the Mesabi and Cuyuna ranges, and 

 the lack of outcrops farther north. These shallow-water 

 conditions are those that at the present day favor algal 

 growth and deposition. 



Nature of the Organisms. 



The classification of the organisms producing the 

 structures is a matter of less certainty. Specimens were 

 submitted to Dr. Walcott, who wrote us as follows : 



1 ' The structure shown in the specimens is very much like that 

 which occurs in the siliceous limestones of the pre-Cambrian in 

 the Grand Canyon, Arizona, and somewhat like those found in 

 the pre-Cambrian Belt series of Montana. The Grand Canyon 

 forms are referred to Collenia. I am at a loss to explain the 

 origin of such structure without the influence of some organic 

 agency acting in conjunction with diffusion and concretionary 

 phenomena. ' ' 



Samples were also sent to Dr. Charles Schuchert, who 

 wrote in description: 



"If I had gotten such structures in any Ordovician limestone 

 I would have called them algae related to Cryptozoon, In New- 

 foundland I got not only regulation Cryptozoon, but, as well, thin 

 finger-like or pipe-like bushy forms. Evidently the iron was 

 segregated out of the marine waters through the metabolism of 

 the algal growths, and it is surprising to see how it is laid down 

 in thin concentric bands." 



If the forms are referred to Collenia(l), the lower, 

 more columnar ones may be given the specific name biwa- 

 bikensis, from their occurrence near the base of the 

 Biwabik formation, while the specific name f errata for 

 the smaller, upper beds may be an appropriate reference 

 to the iron content of the chert. 



University of Minnesota. 



7 Van Hise, C. E., and Leith, C. K.: The Geology of the Lake Superior 

 region. U. S. Geol. Survey, Mon. 52, 214, 604, and 613, 1911. 



