268 THE PENOKEE IKON BEARING SERIES. 



tlie Penokee series. A clierty ferriferous carbonate was the original rock. 

 From this the iron oxides have been in the main the first to form. Before 

 their formation was complete, when actinolite is present, it has begun to- 

 develop, and is included in the oxides, and particularly in the magnetite. 

 Its growth is most pronounced at the beginning of the rearrangement 

 and introduction of silica, as would be expected from its requiring this 

 material to produce it. The average amount of silica is much greater than 

 in the original clierty carbonates, and it is also more largely in the 

 f()]-jn of quartz. The final stage in the development of these rocks has 

 been, then, a rearrangement of silica already present and an addition of 

 silica to a greater or less degree at the time when most of the quartz 

 formed. Cherty carbonate located elsewhere may have been the source 

 whence the most of this silica was derived, as will be more apparent after 

 the orie-in of the ores in the Penokee series is considered. Interbedded 

 with the Animiki(^ rocks are also heavy beds of diabase and gabbro, altera- 

 tion of which may also have furnished silica for the silicification. The like- 

 ness of the Animikie and Penokee iron formations is further exhibited in a 

 striking manner by the fact that, in the most typical exposures, the 

 Animikie series upon Gunflint lake, in upper horizons, are almost pure 

 chert}' carbonate; in intermediate horizons, ferruginous cherts and carbon- 

 ates ; and in the lower horizons, actinolite slates, jaspers, and ferruginous 

 and concretionary cherts, which, however, often contain siderite. 



SECTION IV.— THE IRON ORES.' 



Position of the orc.^ in the Iron-hearing member. — The iron ores are all 

 located, as far as known at present, in that part of the iron-bearing forma- 

 ti«m between Sec. 33, T. 45 N., R. 1 W., Wisconsin, and the east line of T. 

 47 N., R.. 45 W., Michigan, a distance of about 30 miles. The greater 



'A brief acroiint of the coutfiits of section IV was published in advance in tlie Am. .lour. Set , 

 3d scries, vol. xxxvii, i)p. a2-18, by the junior anthor. Dr. Irving had not oonsidercd this p.art of 

 the subject. 



This section indicates the iliveloiimeut of the mines at the time it was written. A recent visit 

 to the l>( iiokce range cinibles nie to say that later developments accoril with tlic conclusions herein 

 coutivined, l)ut the mining of the yl^ars since this section was written has wholly clianged tlie maps of 

 the mines. 



