38 Van Hise — Iron Ores of the Penokee- Gogebic Series. 



the more important mines, it cannot certainly be said to be- 

 common to all of them. Also the respective thicknesses of the 

 ill-defined belts are very different at different mines. A gen- 

 eral statement may be made, that at most of the mines a 

 cross section of the iron formation shows the proportion of un- 

 altered iron carbonate to increase in passing from lower to 

 higher horizons. It is true that almost solid carbonate occurs 

 at three' places at relatively low horizons, although none of 

 them are known to be at the base of the member, while one is 

 certainly underlain by a ferruginous chert Also at several 

 localities a typical chert is found at very high horizons. Figs. 

 2 to 8 illustrate as wide variations of the relations of the ore- 

 bodies to the surrounding rocks as is anywhere found ; yet all 

 are alike in essential points. 



Character of the ore. — The iron ore is a soft, red, somewhat 

 hydrated hematite. By chemical analyses it is shown to be 

 more or less manganiferous, the manganese occasionally run- 

 ning to a high percentage. Much of it is so friable that it can 

 be broken down with a pick, although as taken from the mines 

 it is compact enough to hold together in tolerably large lumps. 

 These lumps are porous, often more or less nodular, and also 

 often roughly stratiform. The strata conform in a general 

 way to the strike and dip of the formation. Mingled with this 

 soft hematite, in a few mines, is a small quantity of aphanitic 

 steel-blue hematite. The south deposits carry upon an average 

 more manganese than the north deposits, the average in the 

 South Iron King being above 1 per cent, while from the South 

 Colby, ore has been taken which contained as much as 30 per 

 cent metallic manganese. 



The Origin of the Ores. 



We have before us the character of the iron ores, the shape 

 of the deposits, their relations to the rocks surrounding them, 

 the nature of the rocks of the iron formation above the ore 

 horizon, and the character of the formations above and below 

 that bearing iron. An attempt will now be made to suggest 

 an explanation of the character and location of the ore-bodies. 



The shape of the deposits and their relations to the strata 

 of the iron formation are such as to exclude the idea of original 

 sedimentation in place ; neither can they be considered as the 

 result of oxidation of iron carbonate in place alone. All of 

 the unaltered iron carbonate now found contains a much larger 

 quantity of silica than the ores, so much as to make them 

 entirely valueless. Also the red banded slates found at the 

 middle horizons give every evidence of being a material which 

 has resulted from the oxidation of the bedded carbonate in place.- 



