150 GEOLOGY. 



Keewatin, greenstone schists and spherulitic greenstone, cut by dikes of gabbro, 

 diabase, and granite, and schistose quartz-porphyry (Quinnesec schist) below, 



Fig. 40. — Figure showing the structure of Archean and Huronian formations in one 

 locality in the Marquette region of northern Michigan. The Archean formations 

 are: sy, syenite, and ms, Mona schist. The Huronian formations are: Aim, Mes- 

 nard quartzite; Alk, Kona dolomite; Alw, Wewee slate; Ala, Ajibik quartzite; 

 Als, Siamo slate; and Aln, Negaunee formation (iron-bearing). Length of section, 

 3 miles. (Van Hise, U. S. Geol. Surv.) 



and (2) Laurentian, granites, gneisses, hornblende, greenstone, and mica schists 

 above. The lower division is similar to the schists of the Marquette region, and 

 represents metamorphosed igneous rocks of diverse origin (flows, tuffs, ashes, etc.). 1 



In the Mesabi district of northeastern Minnesota 2 the Archean consists prin- 

 cipally of various greenish rocks (Keewatin), including dolerites, basalts, diorites, 

 micaceous and chloritic schists, with spherulitic structures common in the more 

 massive portions, all of igneous or meta-igneous origin. With the greenstones 

 there are some granites and porphyritic rhyolites. 



In the Vermilion district , z the Archean consists of (1) Keewatin, the Ely green- 

 stone, and the Soudan formation, and (2) Laurentian granites. The first is 

 altered extrusive rock of andesitic and basaltic types. Locally they retain 

 spherulitic and amygdaloidal structures, but they are largely schistose. The 

 Soudan formation is meta-sedimentary, consisting of basal conglomerate derived 

 from the Ely greenstone below, and an iron-bearing portion above. The latter 

 consists of banded white chert, red jasper, Carbonate-bearing chert, griinerite- 

 magnetite schist, magnetite, pyrite, and hematite. Iron ore derived from the 

 Archean is extensively mined in this region. The original forms of the iron 

 were the carbonate and the silicate (sedimentary). Its change to iron oxide 

 (chiefly hematite), and its concentration, as now found, were effected subse- 



41. — Fisrure showing the relations of the Arc 





Fig. 41. — Figure showing the relations of the Archean and Huronian rocks at a locality 

 in the Vermilion district. Archean formations: JRe, Proterozoic formations, Pro, 

 Ogishkee conglomerate, and Prk, Knife Lake slates. Length of section, about 9 

 miles. (Clements, U. S. Geol. Surv.) 



quently 4 (see Iron Ores, Chapter IV). The granites are intrusive in the Ely 

 greenstone and the Soudan formation. 5 Figs. 41 and 42 show the relations 



1 Van Hise and Bayley, Menominee folio, U. S. Geol. Surv.; also Bayley, Mono. 

 XLVI, U. S. Geol. Surv. 



2 Leith, Mono. XLIII, U. S. Geol. Surv.; Wmchell, Geol. and Nat. Hist. Surv. 

 of Minn., Vol. IV. 



3 Clements, Mono. XLV, U. S. Geol. Surv. 



4 Van Hise, 21st Ann. Rept., U. S. Geol. Surv., Pt. III. 



5 Leith, Mono. XLIII, U. S. Geol. Surv.j Minn. Geol. Surv., Vol. IV, pp. 522-46. 



