Van Hise — Iron Ores of the Penokee-Gogebic Series. 39 



Further, the large amount of manganese which the ores, espe- 

 cially the south deposits, contain is greater than that found in 

 any carbonate from which analyses have been made ; and the 

 average content of manganese in the ore is much greater than 

 the average of the carbonates. While it is thus true that the 

 ores are not carbonates of iron which have altered in place, it 

 is almost as certainly true that the iron carbonates of the belt 

 have been the source whence the iron oxides for these ores 

 have been derived. The nature of the evidence upon which 

 this statement is based has been suggested; but the conclusion 

 would be much clearer if there were place to give a detailed 

 description of the rocks of the iron formation as a whole. 



Since, then, the iron ores cannot be explained by oxidation 

 of carbonate alone in place, and since the carbonate was the 

 source whence they were derived, they are necessarily concen- 

 trations of iron oxide, combined perhaps with iron oxide fur- 

 nished by oxidation of carbonate in place. If this explanation 

 is adopted, however, it is not only necessary to explain the 

 presence of the iron oxide in its peculiar position, but the nature 

 of the whole lower part of the formation. The explanation 

 must account for the great increase in the amount of silica in 

 the lower horizon of the ore formation as compared with the 

 original cherty carbonate ; for its almost total absence in the 

 ore; for the concentration of the iron oxide; for the almost 

 complete absence of carbonate of iron at the lower horizons ; 

 for the red banded slates and carbonates in the middle horizons ; 

 and for the relatively much more abundant unaltered carbonate 

 in the upper horizons. 



A particular occurrence of iron ore. — Before attempting to 

 give a general explanation of these facts, it will first be well to 

 refer to one of several occurrences of narrow belts of iron ore 

 in natural exposure. At Sunday Lake outlet, in Sec. 13, T. 

 47 ~N., R. 46 W., Mich., the actual transformation from cherty 

 iron carbonate to iron ore is seen in all its phases. In clefts, 

 joints and partings along the bedding of the exposure are nar- 

 row seams of hematite. In passing from the seams, the hema- 

 tite becomes mingled with some chert ; going still farther, the 

 chert increases in quantity until a ground mass of silica contains 

 many rhombohedra of iron oxide. The iron oxide then grad- 

 ually passes into siderite. This siderite is in perfect rhombo- 

 hedra, and it is evident, in thin section, that the iron oxide 

 adjacent is pseudomorphous after it. The rock has now passed 

 from an ore into a sideritic chert, which is of a light-gray color, 

 apbanitic texture, and breaks with conchoidal fracture. The 

 latter rock is manifestly in its original condition. The pro- 

 cesses by which the seams of iron oxide occupied the space once 

 taken by sideritic chert are plain. The iron carbonate has de- 



