204 ECONOMIC GEOLOGY 



and bogs. The residual limonite ore bodies occur in many 

 scattered localities both in the Blue Ridge and the Appalachian 

 mountains either in the Cambrian quartzites at or near their 

 contact with the overlying limestones or in the limestones them- 

 selves. The former are called mountain ores and the latter the 

 valley ores (Fig. 107). 



Gossan limonites. These are derived from the disintegration 

 and oxidation of the sulphides of iron and copper where the more 

 soluble constituents have been transferred downward for the 

 enrichment of the underlying ore bodies or carried elsewhere in 

 solution. The ore in such deposits sometimes contains 41 

 per cent. iron. Corinth and Vershire, Vermont; Cooper, Vir- 

 ginia; and Ducktown, Tennessee, represent such deposits (Fig. 

 108). 



2. Lake Superior District. This field is by far the most im- 

 portant iron-ore district of the world. It furnishes more than 

 four-fifths of the iron ores of the United States and therefore 

 more than four-fifths of the pig iron. The ores are mostly 

 hematite, although there is some magnetite in the Marquette 

 field in Michigan (Fig. 109). 



The Lake Superior region may be further subdivided into 

 seven districts each worthy of a detailed description: (1) 

 Mesabi district in Minnesota; (2) Vermillion district in Minne- 

 sota; (3) Cuyuna district in Minnesota; (4) Marquette district in 

 Michigan; (5) Crystal Falls district in Michigan; (6) Menominee 

 district in Wisconsin; and (7) Penokee-Gogebic district on the 

 Michigan and Wisconsin boundary. 



1. Mesabi Range. The Mesabi iron range comprises by far 

 the most important district in the Lake Superior region, and it 

 produces more iron ore than all of the others combined. 



According to C. K. Leith, the geological section comprises at 

 its base Archean greenstones, hornblende schists and porphyries 

 which are separated from the Lower Huronian terranes by an 

 unconformity. The Huronian terranes comprise the equiva- 

 lents of the Ogishke and Knife formations of the Vermillion dis- 

 trict, slate-graywacke formations and conglomerates in nearly 

 vertical position; and granite intrusives in the lower formations. 

 These are separated from the Upper Huronian by another uncon- 

 formity. The Upper Huronian terranes comprise the Biwabik 

 iron formation, the Pokegama quartzite and quartz slate; and 

 the Virginia slate. This slate is separated in turn from the 



