THE IRON ORE RANGES OF MINNESOTA, AND THEIR 

 DIFFERENCES. 



X. Ei. VVinchell, late State Geologist of Minnesota.* 



1. There are two great iron-bearing formations. 



When in the forties of the last century, iron ore was <l 

 at Marquette, in ^he state of Michigan, nothing was known of the 

 relations it might have with the rocks in which it occurred, nor of 

 the rocks with each other. Neither the state geologist, Dr. Douglass 

 Houghton, nor the United States geologists, Foster and Whitney, paid 

 much attention to the structural geology of the region. Indeed, it 

 was one of the tenets of the famous report of Foster and Whitney 

 that the "Azoic*' was not susceptible of classification, nor of sub- 

 division, so far as it appeared on the south side of lake Superior, 

 and this idea was not dispelled until the region was examined by 

 the later state surveys of Michigan and Wisconsin, and by the geol- 

 ogists of the United States Geological Survey. Several of the field 

 geologists passed over the critical stratigraphic exposures without 

 comprehending their significance. Dr. Rominger, of the Michigan 

 survey, called attention to some non-conformities at Marquette which 

 would have led him to discover the duplicate nature of the iron- 

 bearing rocks if he had sufficiently appreciated their significance. 

 Foster and Whitney in making a survey of the iron district of Mich- 

 igan, also Irving for the later United States Geological Survey, and 

 Brooks for the Michigan State Survey, believed that not only the 

 rocks were in one great "azoic" series, but that the iron ore WHS 

 confined to one horizon. If they saw conglomerates, great conglom- 

 erates such as are now universally recognized as basal beds which 

 indicate non-conformities, (and some of them did see them) they 

 either believed them to be local breccias caused by igneous out- 

 breaks of granite or dioryte, or put them along with the ore into 

 the same formation. 



It was only after some examination of the geology of north- 

 eastern Minnesota by the members of the Minnesota Geological Sur- 

 vey that it was learned that, at least so far as concerns the state 

 of Minnesota, the iron ores are not all in the same formation. We 

 found that the rocks that contain the ore at Soudan are much older 

 than those that carry the Mesabi ore. We found that the M.sahi 

 rocks, which are the younger, run in a diverging course, from the 

 line of strike of the rocks that carry the Vermilion ore. Spurred by 

 this discovery, we organized a small party and visited Marquette, 

 where the greatest development had been made. We also examined 

 the Penokee-Gogebic rocks, in Wisconsin, and without going now in- 

 to the details so far as those iron regions are concerned, we con 



*The paper was given February 22, 1911, as a public addrei 

 Aitkin, and subsequently at the May meeting of the Academy Oi 

 ence. The figures were shown on a screen by Btereopticon. 



