106 THE MAKQT'ETTE IKON-BEAKING DISTRICT. 



two di.stiiict series, and tor tin- first time we find tlie g-reen schists separated 

 from the iron-bearing rocks and jdaced nncont'ormably beneath them in the 

 same series witli the granite. 



18SG. 



iBViNci, K. 1>. Origin of the feirugitioiis schists and iron ores of tiie Lake 

 Superior region. Am. Jour. Sci. (3), Vol. XXXII, 18SG, pages 255-272. 



Irving continued the discussion in the folh.)wing year, when he pubHshed 

 an article devoted exclusively to the origin of the iron ores of the Lake Supe- 

 rior region, but mainly of those of the Penokee district. The conclusions 

 reached in this paper had already been foreshadowed in the bulletin on the 

 Enlargement of Quartz Grains in Quartzite, etc. In its introduction the 

 author gives the status of the problem at the time of the publication of his 

 paper. Two theories had been proposed to exj)lain the origin of the ores 

 and jaspers of the district in question — an eruj)tive and a sedimentary 

 theory. Of the former, advocated by Wadsworth among later geologists, the 

 author states that the phenomena cited in its favor are with one exception — 



mainly trivial matters occurring within tlie spa(;e of a few inclies, or feet, at most, 

 and * * * all are more easily explicable as irregularities in original deposition, 

 as irregularities due to the crumpled condition of the strata, or, and this chiefly, as 

 due to infiltrations of iron oxide and silica into cracks in the rocks, and the replace- 

 ment of rock material by such substances — ou theories of original sedimentation of 

 the iron beds than on those of an eruptive origin. * * * The occurrence of frag- 

 ments of the banded jasper in the immediately overlying (juartzite deserves more con- 

 sideration, siuce it certainly indicates that, to some e.xtent at least, these substances 

 bad reached their ])reseiit condition at an early day. But cooling from a state of 

 fusion is not the oidy way of reaching rapidly the indurated condition, and a former 

 fused conditiou seems to be 7iegatived at once by tlie nature of the material, — quartz 

 and iron oxide. (P. 250.) 



Ho then dismisses the eruptive tlieorA- as improl)al)le, and proceeds to 

 argue in favor of a sedimentar}- origin for the ores in question, first stating 

 briefly the nature of the sedimentary theories already proposed. 



Those who have maintained the theories of a sedimentary origin have relied 

 chiefly upon the common intimate interlamination of siliceous and ferruginous mate- 

 rials; upon the manife.st restriction of the ores and jaspery schists to definite strati- 

 graphical horizons; upon their interfolding with other members of the same series. 



