322 N. H. WINCHELL ORIGIN OF LAKE SUPERIOR IRON ORES 



idea of metamorphism as now understood, it contains the germinal 

 thought of Whittlesey^s hypothesis, namely, segregation from ferruginous 

 rocks that preexisted. 



The Work of E. D. Irving 



Irving was the first who made careful microscopical and chemical dis- 

 criminations among the ores themselves and the associated quartz and 

 other minerals. He was more exact than some of his followers, and more 

 correct than all of his predecessors, at least along the lines which he 

 investigated.^^ He did not treat of the titanic magnetites, and he did 

 not distinguish structurally between the formations that carry ores of 

 different ages. He hence reduced all the then known Lake Superior ores 

 not only to one geological epoch, but to one chemico-sedimentary origin. 

 Hence, as has been remarked already, he did not reach the origin of the 

 iron, although he elucidated its present mineral condition and its struc- 

 ture. Irving^s studies were based essentially on the jaspery hematites 

 of the Archean at Marquette, the Vermilion ores not then having been 

 discovered, and he believed the results of his worJv were equally applicable 

 to the ores of the Penokee-Gogebic region. Irving^s work proved to be 

 epoch-making, as it furnished the basis of an important series of publica- 

 tions by the geologists of the United States Geological Survey, all of 

 whom follow his main ideas, with the sole exception that latterly it was 

 established to their satisfaction that the ores are of at least two different 

 ages. 



Irving specifically rejected eruption and metamorphism, and did not 

 mention segregation; and, as stated by him, "it followed that we were 

 restricted to some theory which should account for the precipitation of 

 most of them essentially in their present conditions, with perhaps some 

 slight internal rearrangement, or to one in which the production, from 

 some form of sedimentary deposit, of the conditions now obtaining, 

 should be assigned to metasomatic processes, carried out, in part at least, 

 at a very remote period." Further examination and study led him to 

 conclude that "these ferruginous rocks were once carbonates, analogous 

 to those of the Coal Measures." This conclusion was hailed at once as 

 tending to establish the Azoic as a great zoic age, in which flourished an 

 abundance of life of different kinds, a prototype of the Carboniferous. 



This conclusion was based on the actual existence of considerable 

 quantities of carbonate of iron, associated with chemically deposited 

 secondary silica in the terranes in which some of the ores are found. 

 This he styled cherty carbonate, and this term figures largely in the 



" American Journal of Science, vol. xxxii, October, 1886. 



