280 THE PENOKEE IRON BEARING SERIES. 



remember tliat tliey are composed of eliert and iron peroxide; that they 

 are as regiihxrly bedded as the unahered carbonates, and tliat they readily 

 cleave alony tlie bedding. Above these slates tor a lono- distance, and 

 particularlv well exposed in T. 47 N., R. 4() W., Michigan, constituting 

 the upper horizonof the ore t'ormation, are partly or wholly imaltered 

 chert^• iron carljonates. While the above section is known to occur at 

 several of the moi-e important mines, it can not certainly be said to l)e 

 common to all of them. Also the I'espective tliicknesses of the ill-delined 

 belts are very different at different mines. A general statement may be 

 made that at most of the mines a cross section of the iron formation shows 

 the proportion of unaltered iron carbonates to increase in passing from 

 lower to higher liorizons and appears to be greatest in quantity at the 

 uppermost horizon, it is true, liowever, that almost solid carbonate occurs 

 in three places at a relatively low horizon, aUhough none of them are 

 known to be at tlie l)as(' of tlie member, whih' one is certainly underlain 

 by a ferruginous chert. Also at several localities a completely altered and 

 brecciated chert is found at very high horizons. 



TJip character of flic arc. — The iron ore of the Penokee-Cloffebic rang-e 

 is a soft red somewhat hydrated hematite. By chemical analyses it is 

 shown to be more or less manga niferous. Mu-ch of it is so friable that it 

 can l)e liroken down witli a common jiick, although as taken from the 

 mines a large jiortion of it is compact enough to hold together in tolerably 

 large lumps. These lumps are porous, often more or less nodular, and 

 often also rougldy stratifoi-m. The strata conform in a general way to the 

 strike and diji of the formation. Mingled with this 'soft hematite in a few 

 mines is a small (piantity of ajihanitic hard steel-blue hematite, which 

 breaks with coiichoidal fi-acture and is of nmiarkable purity. In general 

 this exce]itionally hard material is found in contact with or clo.se to the 

 diorite dikes of the mines. The following analyses and facts as to com- 

 position are mainly taken from a report by Ih: John Birkinbine:' 



'John Birkinbine : The iron ores eaat of the Mississippi river ; Mineral Eesources of the United States, 1886, pp. 67-73. 



