PRE-CAMBRIAN FORMATIONS OF GWINN IRON DISTRICT 565 



Cheshire) mine and during the time which has since elapsed has 

 extended its holdings by purchase and lease until it now controls 

 all of the known workable ore bodies with the exception of the 

 Stegmiller, which is mined by the American (Oliver) Mining Co. 

 Since the building of the beautiful and principal village of Gwinn 

 by the Cleveland Cliffs Iron Co. the name of the district has been 

 changed by common usage from Swanzy to Gwinn. There are 

 five producing mines in the district. This number will be six in 

 19 14, and probably eight in 191 5. Concrete shafts have been sunk 

 to two additional ore bodies but it is not known when these will 

 be equipped for mining operations. 



NOTES ON THE STRUCTURE OF THE GWINN SYNCLINORIUM 



The Gwinn synclinorium contains two unconformable series of 

 sedimentary rocks, having a combined thickness of from 800 to 

 1,000 ft. Outliers of flat-lying Paleozoic (Cambrian or Ordo- 

 vician) sandstone and limestone occur throughout this area. The 

 pre-Cambrian beds are remnants of formations, originally much 

 more extensive, which have escaped erosion by downfolding or 

 depression in the Archean basement. 



The synclinorium is constricted to not more than three-fourths 

 of a mile in width in the vicinity of the N.W. j of section 29, 

 T. 45, R. 25. North of the constricted portion, the rocks are 

 folded and faulted in a complex manner but south of it the struc- 

 ture is apparently somewhat less complicated. 



The southern three-fifths of the synclinorium is a spoon-shaped 

 basin four miles long with a maximum width of about two miles. 

 The deepest part of the fold is adjacent to the northeast limb 

 where the Archean granite is reached in many drill holes at depths 

 of from 1,000 to 1,200 ft. (see cross-section III-IV). Drilling along 

 the southwest limb indicates a number of sharp drag folds pitching 

 northwest. The folds on the opposite limb are not so sharp and 

 are apparently simple cross-folds. The most prominent one appears 

 in the S.E. J of section 35. The syncKnorium practically terminates 

 against a faulted zone on the southeast. It is not possible to deter- 

 mine from present information the full extent of this zone nor the 

 character of the faulting. The rocks in the faulted area are largely 



