GEOLOGICAL EXPLORATIONS AND LITEIIATURE— 1S8C. 107 



and upon their apparent gradation in places into plainly fragmental deposits. These 

 conditions being talien to indicate original sedimentation, different authors have 

 imagined the unaltered deposits to have been argillaceous carbonates like those 

 of the coal measures; to have been brown ores, like those found under bogs, or 

 accumulating in shallow lakes, at the present day; or to have been magnetic sands 

 like those of modern sea shores. All of these theories appear to regard the silica of 

 thejaspery schists' and ores as having been sand; its present nonarenaceous, nonfrag- 

 mental condition being taken to be the result of metamorpliism. (P. 2")."'..) 



Tlie studies of tlie author were coniined largely to the Penokee- 

 Gogebic district, where the rocks are less disturbed than they are in the 

 Marquette district, but their conclusions are made to cover also the ores of 

 the latter area. 



While holding the sedimentary origin of the ores and jaspers, it 

 is shown that the close association of these rocks with nonraetamor- 

 l)hosed quartzites, graywackes, etc., precludes the notion of a metamorphic 

 origin for the former rocks. Moreover, "all theories of a formation of 

 these ferruginous rocks by metamorpliism or recrystallization in situ from 

 some sort of sedimentary deposit seem to regard the jaspery or clierty 

 material as representative of a fragmental siliceous ingredient in the 

 original deposit. On these theories this substance has been recrystallized 

 from a fragmental material" (p. 259). But the microscope shows that the 

 jaspers an<l cherts are composed largely of chalcedonic silica, like that 

 deposited from solution. It contains intermingled with it fragmental grains 

 of quartz that liave lost none of their original angularity, and which are 

 easily distinguished from the chemically precipitated chalcedony. Hence, 

 the metamorphic theory is abandoned and a chemical theory advocated in 

 its place. According to this theory the original sediments were ferruginous 

 carbonates. I'he least altered of the ferruginous schists still contain car- 

 bonaceous material, and often little rhombohedra of siderite, and the amount 

 of this carbonate present varies inversely with the amount of disturbance and 

 alteration the rocks have suffered. In the Penokee district a bed of hematite 

 was traced directlv into one of these carbonate-bearing schists. In this and 

 the other unfolded Huronian iron districts there is excellent proof that the 

 origin of the ores and jaspers was as indicated. In the Marquette district 

 the complication of the structure obscures the evidence to a considerable 



