USEFUL METALS 207 



pitches which shows that the transverse folding was intense. 

 The structure of the district is further complicated by intrusivea 

 of various ages. Unlike the other districts the iron ores occur in 

 the Achean, Lower Huronian and Upper Huronian formations. 

 It differs also in that the chief producer of iron ores is the Archean 

 formation. The ores occur in the upper portion of the Archean 

 terranes known as the Soudan iron formation. It consists of 

 cherty iron carbonates, ferruginous cherts, pyritic quartz rocks, 

 jaspilites and ore bodies. The ore bodies are usually near the 

 base of the formation and occur in synclinal troughs in the green- 

 stones. At Soudan the ore bodies are underlain in part by the 

 greenstones and in part by the associated porphyries (Fig. 110). 



The Lower Huronian iron-bearing terrane is known as the 

 Agawa formation. The ores occur chiefly in the eastern part of 

 the district on Hunter's Island, Canada. The formation con- 

 sists of ferruginous cherts, ferruginous slates, jaspilites and iron- 

 bearing carbonates. 



The Upper Huronian iron-bearing terrane is known as the Gun- 

 flint formation. It is confined to the northeast part of the district 

 west of Gunflint Lake. It consists pf carbonaceous slates, fer- 

 ruginous slates, and jaspilites, in the vicinity of Gunflint Lake, 

 but to the southwest it passes into amphibolitic and magnetic 

 quartz rock due to the metamorphism of the associated gabbros. 



3. The Cuyuna District: This range is southwest of the 

 Mesabi range in Minnesota. The formations include quart zites 

 and their altered equivalents, iron formations, slates, together with 

 intrusive granites and diorites. The ore occurs in lenses from 

 100 to 250 ft. in thickness. 



4. The Marquette District: This district, according to C. R. 

 Van Hise, W. S. Bayley and H. L. Smyth, comprises a compara- 

 tively small east and west belt situated wholly within the state 

 of Michigan. The geological section comprises formations 

 ranging from the Archean to the Cambrian. The iron-bearing 

 formations occur in all four of the horizons at which iron ores 

 are found in the Lake Superior region; viz., The Archean, the 

 Negaunee formation of the Middle Huronian, and in two horizons 

 in the Upper Huronian, the one at the base of the Goodrich 

 quartzite and the other in the Bijiki-formation. Negaunee is the 

 chief iron-producing district. It consists of cherty iron carbonates, 

 ferruginous slates, ferruginous cherts, jaspilites, griinerite magne- 

 tite schists, detrital fragmental rocks and iron-ore bodies (Fig. 111). 



