CHEMICAL RELATION OF IRON AND MANGANESE. 36 1 



along the same geologic horizon, and yet in close proximity with 

 each other, the explanation is entirely inadequate, for the deposits 

 are too close to each other to have been formed from different 

 supplies of surface waters. 



(2) It might be supposed that the iron or the manganese 

 had been leached out of a deposit of the mixed ores, leaving 

 one free from the other and depositing the dissolved ore some- 

 where else. This explanation, except in special cases, also 

 appears inadequate, because the reagents in surface waters, which 

 dissolve iron and manganese, seem to affect both about equally, 

 so that if one were dissolved, the other should be taken up in the 

 same way. Doubtless small differences could be found in the 

 behavior of the organic and inorganic compounds in surface 

 waters towards iron and manganese minerals, but they would be 

 small as compared with the more active reactions which go on. 



(3) It might be supposed that a separation could be produced 

 by secondary concentration such as segregation, replacement, etc. 

 This has doubtless sometimes been the case, but where the con- 

 centrating action is not assisted by a difference in the chemical 

 behavior of the two substances, the separation would only be 

 on a small scale. Even in the case of concentration by replace- 

 ment of limestone, if iron and manganese both acted in the same 

 way during the replacement, it would be expected to find them 

 deposited in an intimate mixture. Though this secondary con- 

 centration, therefore, unassisted by other agencies, would not 

 produce all the results found in nature, yet, when it is thus 

 assisted, it often plays an important part. 



(4) The fourth, and what seems the most important, factor 

 in the separation of iron and manganese, is that, though very 

 often they are precipitated in the same form from the same 

 solution, yet sometimes they are precipitated in different forms ; 

 and even when precipitated in the same form, the precipitation 

 of one sometimes requires different conditions from the precipi- 

 tation of the other. This fact will explain the alternate associa- 

 tion and separation of iron and manganese, not only when no 

 secondary concentration has gone on, but also in cases where 



