Grout and Broderick — The Huronian. 199. 



Art. XIV. — Organic Structures in the Biivabik Iron-bear- 

 ing Formation of the Ruronian in Minnesota; 1 by 

 Feank F. Geout and T. M. Beodeeick. 



The emphasis recently given 2 to structures attributed 

 to algae and other organisms in pre-Cambrian rocks may 

 make it desirable to record, in more detail than has been 

 done, the structures that occur in the Upper Huronian 

 (Animikie) rocks of the Mesabi range in Minnesota. 

 The Biwabik iron-bearing formation extends along the 

 range for nearly 100 miles, and throughout the east half, 

 recently studied by the Geological and Natural History 

 Survey of Minnesota, the outcrops and drill cores show 

 structures closely resembling those now attributed to 

 algae. They will probably be found to extend over the 

 whole range. The engineers of the Oliver Mining Com- 

 pany mention them as "contorted beds" in reporting 

 the character of drill cores. Leith has shown 3 in a 

 colored plate in the monograph on the Mesabi district, a 

 fair sample of the red banded cherts, but suggests that 

 their contorted nature may be due to movement. 



The Biwabik Formation. 



The formation as a whole is from 400 to 800 feet thick, 

 and though all parts of it are called taconite, a number 

 of divisions are recognizable. In the eastern end of the 

 range the lowest member is about 20 feet thick, and con- 

 sists largely of a chert with coarse wavy bands (fig. 1). 

 Above these basal members of the formation is a black 

 slate grading upward into a slaty chert. Above this is 

 taconite with the texture of an intraformational con- 

 glomerate. Throughout the eastern area where outcrops 

 are numerous, the conglomerate makes a bed about 100 

 feet thick and is noteworthy for the abundance of mag- 

 netite it contains. About 10 feet below the top of this 



1 Published by permission of the Director of the Minnesota Geological 

 Survey. 



2 Walcott, Charles D. : Pre-Cambrian Algonkian algal flora, Smiths. Misc. 

 Coll., 64, No. 2, 111, 1914. 



Idem : Notes on fossils from limestone of Steeprock series, Ontario, Geol. 

 Survey Canada, Mem. 28, 16, 1912. 



Moore, E. S.: The iron-formation on Belcher Islands, Hudson Bay, etc. 

 Jour. Geology, 26, 412, 1918. 



Schuchert, Charles: A Text-book of Geology, Pt. II, Historical geology, 

 570-575, 1915. 



3 Leith, C. K.: The Mesabi iron-bearing district of Minnesota, U. S. 

 Geol. Survey, Mon. 43, PI. XII A. 



