CONCENTRATION OF IRON ORES. 1197 



result largely of the transportation of iron carbonate to the pitching troughs, 

 the iron being there precipitated, and partly of oxidation of ferrous carbonate 

 or silicate in place. Simultaneously with these processes the silica is dis- 

 solved and abstracted. 



It is to be noted that the precipitation of iron ores by .the above process 

 is due essentially to the mingling of solutions from two different sources. 

 The solutions from one source have taken a longer journey, or else have 

 passed through rocks more capable of abstracting oxygen, so that they 

 early acquired the properties of deep solutions — an excess of carbonic acid 

 and an absence of oxygen. Under such circumstances they carry iron car- 

 bonate precisely as do the deep solutions of the first concentration in ores 

 produced by ascending waters. The solutions from the other source pass 

 through material in which the iron has already been oxidized to the ferric 

 form, and therefore do not have the oxygen extracted. The course of this 

 water is not necessarily shorter than that of the other waters, but upon the 

 average is likely to be so. When the two solutions come together the iron 

 oxide is precipitated, in consequence of the mingling of solutions. While 

 for the Lake Superior iron ores apparently the solutions from both sources 

 were descending to the point where they became mingled, in iron-ore 

 deposits in other regions it may be found that the precipitation of the iron 

 oxide was due to the mingling of ascending solutions bearing iron carbonate 

 with descending solutions bearing oxygen. 



For instance, at certain localities the iron ores are magnetite instead of 

 hydrated hematite. In Chapter IX the various ways in which magnetite 

 mnj be produced are discussed, but it may be suggested that magnetites 

 may be due to the direct precipitation of ascending solutions bearing iron 

 carbonate and descending solutions bearing- an insufficient amount of 

 oxygen for transformation to the ferric oxide, the reaction being as follows: 



3FeC0 3 +0=Fe 3 1 +3C0 2 . 



Perhaps the most notable magnetite deposit in the United States is that 

 of the Cornwall mine in Pennsylvania. Here the ore is a replacement of 

 Silurian limestone, and rests upon a trap dike as an impervious basement. 

 The Mesozoic rocks are faulted against the limestone and ore. A jjlausible 

 suggestion as to the origin of this deposit is that ascending waters bearing 

 iron carbonate, moving up along the fault and through the limestone, met 



