PRE-C AM BRIAN FORMATIONS OF GWINN IRON DISTRICT 561 



Van Hise and C. K. Leith/ These authors had made no detailed 

 survey of this district and attempted merely a summary of the 

 information from other sources available to them at the time their 

 monograph was written. They describe the Gwinn district as a 

 southeastern-pitching synclinorium about two miles long and from 

 one-half to two miles wide, the structure being unknown toward 

 the southeast because of the deep overburden. They correlate 

 the pre-Cambrian sedimentary rocks with the Upper Huronian 

 (Animikee) series and describe them as (i) a basal "quartz slate 

 and quartzite grading down into arkose or decomposed granite" 

 which is overlain by (2) the Michigamme slate carrying the Bijiki 

 iron-bearing formation in "lenses and layers" near its base. 



Recent studies by the writer for the Michigan Geological Survey 

 based on field mapping and an examination of the records of several 

 hundred diamond drill holes show clearly that the Gwinn district 

 contains at least two unconformable series of sedimentary rocks. It 

 seems probable that the upper series, which will be described as the 

 Princeton series, is equivalent to the Upper Huronian of the 

 Marquette district, that the lower series, which will be described 

 as the Gwinn series, is equivalent to the Middle Huronian of the 

 Marquette district, and that the Lower Huronian series, while not 

 present in the Gwinn synclinorium, is represented by certain frag- 

 ments of quartzite and cherty slate in the conglomerate at the base 

 of the lower or Gwinn series. 



Without the information afforded by records of drill holes and 

 other exploratory operations, any statement of the geology of the 

 Gwinn district would probably be misleading and in any event 

 necessarily fragmentary and incomplete. Outcrops are not plenti- 

 ful except in certain restricted localities and are limited to the north 

 two-fifths of the district. The records of drill holes, carefully com- 

 piled by geologists of the Cleveland Cliffs Iron Co. and the Oliver 

 Iron Mining Co., are the main reliance for mapping the formations. 

 Only a few of the drill samples were seen by the writer, but each 

 of the formations is somewhere exposed either in outcrops or in 

 excavations and was studied on the ground. It will be seen on the 



' C. R. Van Hise and C. K. Leith, Monograph 52, U.S. Geological Survey, 

 pp. 283-86. 



