GEOLOGICAL EXPLOIJATIONS AND LITEKATl'lJE-lSli.!. 141 



BiRKiNBiNE, John. Iron ores. Mineral I{e.sources of the United States, cal- 

 endar year 1891, Washington, 18'.l.!, pages l(>-4«. 



We find in this paper- an abstract of the reports of (A D. Lawton, 

 conimi.ssioner of mineral statistics for ^Michigan, explaining tlie occurrence 

 of the ores in the Maniuettc district. IMie rocks associated witli the ores 

 are stated to be built up of the detritus of the Laurentian rocks. The ore 

 formation is folded, the ore bodies being- found in the troughs of the folds, 

 with a hanging wall of quartzite, which is often sei)arated fr. .111 'the ore by 

 "soap rock" (}>. 17). 



Van Hise, G. E. The snccessiou in tiie Marqnette iron .listrict of Micliigan. 

 Bull. Geol. Soc. Am., Vol. V, 1893, ])age.s 5-0. 



The first announcement of results reached In' the Lake 8ui)erior 

 division of the United States Geological Sitrvey hi the detailed examina- 

 tion of the Marquette area was made in 1893 by Prof. C. R. Van Hise, in 

 a short paper read at the Madison meeting of the Geological Society of 



America. 



The oldest group of the district is called the Basement Complex, follow- 

 ing Irving's suggestion. It consists of granite, gneisses, hornblende-schists 

 and mica-schists, green schists, and greenstone-conglomerates, which appear 

 to be surface volcanics, in part lavas and in part tuffs. The schists are 

 intruded by granite and gneissoid granite. No sedimentary rocks are known 



in the group. 



Unconformably on this gr(.up is the Lower JMarciuette, consisting, in 

 ascending order, of conglomerates and (luartzites, and the iron formation, 

 including actinolite-magnetite-schists, ferruginous cherts, jaspers, etc. 



The Upper Marquette rests upon any one of the lower formations. 

 Broadl}- speaking, it is a shale, mica-slate, and mica-schist formation, 

 although at its base are often found quartzites and conglomerates. Where 

 the underlying rock is the iron-bearing formation of the Lower ]\Iarquette 

 the basal member of the upper series consists of a recomposed iron-bearing 

 formation, which may, in consequence of secondary concentration, afford 

 workable ore bodies. Another iron-ore horizon occurs at from several hun- 

 dred to 1,000 feet above the base of the series. This formation is not much 

 unlike the corresponding formation in the lower series. 



