282 THE PKNOKEE IRON BEARmO SERIES. 



The sliape of the deposits and tlieir relations to tlie 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 the oxidation of iron 

 carbonate in place alone. All of the unaltered siderite now found contains 

 a much larger quantity of silica than the ores, so much so as to make these 

 carbonates themselves entirely valueless. Also the red banded slates rather 

 than the ores described (pp. 202-205) give every evidence of being a 

 material which has resulted from the oxidation of such carbonate in place. 

 Further, the large amount of manganese which the ores (especially the 

 south deposits) occasionally contain is much greater than the amount con- 

 tained in an}' carbonate fr(jm 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 certain tliat the ores are not carbonates of iron which 

 have altered in place alone, it is almost as certainly true that the siderite 

 ot the belt has been the source whence the iron oxides for these ores have 

 been derived. This statement is based upon the facts furnished by the 

 detailed description of the rocks of the iron formation as a whole and those 

 de^'eloped by the discussion which has preceded. That manganese is 

 always present iii the iron carbonates, frequently in some quantity, and is 

 yet more abundantly found in the ores, is an additional strong- indication 

 that the iron carbonate has been the source whence the ore deposits derived 

 their ii-on oxide. 



Since, then, the iron ores can not be explained by oxidation of carbon- 

 ates alone in place, and, since the carbonate was the source whence they 

 were derived, they are necessarily concentrations of iron oxide, combined 

 perhaps with iron oxide furnished by oxidation of carbonate in place. If 

 this explanatit>n is adopted, however, it is necessary to explain not only the 

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

 whole lower j)ai-t of tlu^ iron-bearing formation. The explanation must 

 account f<(r the great increas<' in the amoimt of silica in the lower horizon 

 of the ore formation as compared with the original chertv carljonate ; for 

 its almost total absence in the ore; for the concentration of the iron oxide; 

 for the almost complete absence of carbonate at the lower horizons ; for the 



