262 EDWARD STEIDTMANN 



Allen^ reports that the stratigraphic succession in the Iron River 

 district of Michigan from the Brule river northward consists of fine- 

 grained, ellipsoidal basalts, and green schists probably Keewatin; 

 Lower Huronian cherty and slaty dolomite, quartzite, and slates; 

 Upper Huronian slates and graywackes, containing iron formation 

 lenses in at least four horizons, and associated with basic igneous 

 rocks, mainly extrusive. Unconformities between the major units 

 are inferred but not known, owing to the drift-covered condition of 

 the contacts. 



The rocks are complexly folded, but the details of the structure 

 are obscure. The major axis of folding is northwest-southeast in 

 the eastern part of the district, east and west in the central portion, 

 and southwest to northeast, farther west. 



The Upper Huronian is divisible into three main belts or 

 stratigraphic horizons running parallel to the major structure, that 

 near the Brule River on the south being the lowest. The southern 

 belt consists dominantly of slates and iron formation lenses, the 

 productive portion of the district. The next overlying belt is ill 

 defined, consisting dominantly of extrusive greenstones. The 

 uppermost belt, lying farthest north, consists of slates, graywackes, 

 basic extrusives, and iron formations. 



The iron formations comprise slaty and cherty iron carbonates, 

 ferruginous cherts and slates, magnetic, chloritic, sideritic slates, 

 altered greenalite, and secondary soft, hydrated hematite ores. 

 The ore bodies occur mainly as irregular sheetlike masses parallel to 

 the bedding or as pockets related to structural basins. They are 

 bounded by phases of lean iron formation, extrusive basic rocks, or 

 carbonaceous shale, or any combination of these. 



Buckley^ confirms Haworth's conclusions that the pre-Cambrian 

 rocks of Missouri consist of contemporaneous granites and rhyolites 

 intruded by diabase dikes. About 200 feet of slate and con- 

 glomerate are exposed on Pilot Knob. The igneous rocks are 

 correlated by Buckley with the Laurentian and the sediments with 

 the Huronian, but the reasons for this correlation are not evident. 



' R. C. Allen, "The Iron River Iron-bearing District of Michigan," Mich. Geol. 

 and Biol. Surv., 1910, 144 pp., 17 pis., 18 figs. 



* E. R. Buckley, "Geology of the Disseminated Lead Deposits of St. Frangois and 

 Washington Counties, Missouri," Missouri Bur .Geol. and Mines,IX (igoS) , Ft. i, 259 

 pp., 39 pis., 10 figs. 



