THE PROTEROZOIC ERA. 179 



change, brought about by dissolving, recombining, and precipitating, 

 was the production of beds of iron ore, chiefly hematite. 1 



The sections which follow give some idea of the present conditions 

 of the formations. 



Sections of the Huronian. — The following Huronian sections, in selected locali- 

 ties where the rocks have been well studied, give some idea of the range of the 

 system. 



The Huronian series of the Marquette region of northern Michigan 2 . rests un- 

 conformably on the Archean, and includes, in ascending order, (1) the Mesnard 

 quartzite, with basal conglomerate, and some slate, 110 to 620 feet thick; (2) 

 the Kona dolomite, with some clastic layers, 425 to 1375 feet thick; (3) the We we 

 slate, including, besides slate, some conglomerate, quartzite, and schist, 550 to 

 1050 feet thick; (4) the Ajibik quartzite, which is sometimes schistose, 700 to 

 900 feet thick; (5) the Siamo slate, with some graywacke and quartzite, 600 to 

 1200 feet thick; and (6) the highly ferruginous Negaunee formation, which includes 

 sideritic slate, grunerite-magnetite schist, ferruginous chert, jaspilite, and iron 

 ore, and has a thickness of 1000 to 1500 feet. 3 The sideritic slate appears to have 

 been the dominant original type of rock in the Negaunee formation and most of 

 the other phases of this formation have been developed from it. The series 

 also includes some volcanic rocks. The beds are now closely folded and generally 



Fig. G4. — Section of Huronian in the Marquette (N. Mich.) region. Aim, Mesnard 

 quartzite; Alk, Kona dolomite; Alio, Wewe slate; Ala, Ajibik quartzite; Als, 

 Siamo slate. Length of section, 3 miles. (Van Hise, Bavley, and Smyth, U. S. 

 Geol. Surv.) 



much metamorphosed. (Figs. 64 and 65 illustrate the positions of the formations 

 in this region.) 



In the Menominee (Michigan and Wisconsin) 4 region, the Huronian system 

 (known also as the Lower Menominee) is unconformable on the Archean, and is 

 divided into three formations: (1) the Sturgeon quartzite, with basal conglom- 

 erate, 1000 to 1250 feet thick; (2) the Randville dolomite, including some 

 silicious dolomite, calcareous quartzite, cherty and argillaceous rocks, 1000 to 



1 For discussion of the iron ores of the Huronian of the Lake Superior region, see 

 Van Hise, 21st Ann. Rept. TJ. S. Geol. Surv., Pt. Ill; Irving, Am. Jour. Sci., Vol. 

 XXXII, 1886, p. 255; Van Hise, Vol. XXXVII, 1889, p. 32, and Vol. XLIII, p. 116; 

 and Monographs XIX, XXVIII, XXXVI, and XL, U. S. Geol. Surv. 



2 Van Hise, 15th Ann. Rept. U. S. Geol. Surv., and Van Hise, Bayley, and Smyth, 

 Mono. XXVIII, U. S. Geol. Surv. 



3 According to the classification last suggested (p. 161) the division between the 

 Middle and Lower Huronian is between (3) and (4). 



4 Van Hise and Bayley, Menominee (Mich.) folio, U. S. Geol. Surv. 



