206 



ECONOMIC GEOLOGY 



A lean ferruginous chert, the chert and iron occurring in alternate 

 bands, or irregularly mixed. (2) Iron ore bodies. (3) Ferrous 

 silicate and carbonate rocks. (4). Ferruginous slates. 



The iron was originally present in part as a carbonate but mostly 

 as the silicate, greenalite. Surface waters charged with carbon 

 dioxide attacked the greenalite forming the carbonate of iron and 

 orthosilicic acid. The iron carbonate was either taken into 

 solution and precipitated by contact with solutions bearing an 

 abundance of oxygen, or else it was oxidized and hydrated in 



FIG. 110. Iron mine, Soudan, Minnesota, shows old open pit with 

 jasper horse in middle. (By permission of the Macmillan Conpany, from 

 Ries' Economic Geology.) 



situ. The removal of the orthosilicic acid in solution is cited as 

 responsible for the slumping of the beds at the contact of the wall 

 rock and the ore bodies. 



2. The Vermillion District: According to J. M. Clements 

 this district comprises a narrow belt of Archean rocks in places 

 overlain by those of the Huronian series extending in a north- 

 easterly direction from Vermillion Lake, Minnesota, to Gunflint 

 Lake on the International Boundary, a distance of about 92 

 miles. The district is one of extremely complex folding. Super- 

 imposed upon the longitudinal folds are cross folds with steep 



