CHEMICAL RELATION OF IRON AND MANGANESE. 369 



Manganese, on the other hand, is rarely found as sulphide, and 

 there is reason to think that the sulphide never represented the 

 original form of any large sedimentary deposits of manganese ore 

 (seepages 364 to 365). It seems probable, therefore, that from 

 a solution of iron and manganese in surface waters the iron might, 

 where the conditions are favorable, be precipitated as sulphide 

 (FeSg) and the manganese might be carried on in solution to be 

 deposited somewhere else as oxide or carbonate. Subsequently 

 the oxidation of the ores would give rise to oxide of iron from 

 the sulphide and oxide of manganese from the carbonate; and 

 the two ores, though occurring at the same horizon, would be 

 separated by a greater or less distance. 



After the deposition of the sulphide of iron, the conditions 

 might change and permit the deposition, in the same place, of 

 the carbonates of iron and manganese together. This is an easy 

 case to imagine, and where such a deposit was exposed to sur- 

 face influences, the resulting product would be oxide of iron from 

 the underlying sulphide and a manganiferous iron oxide from the 

 overlying isomorphous carbonates. Hence another possible cause 

 of the frequent association of pure iron ores and manganiferous 

 iron ores. It is possible also that after the solution of iron and 

 manganese had been freed from the former by precipitation as 

 sulphide, the manganese might be carried on and laid down as 

 carbonate on a previous deposit of iron sulphide, and when such 

 a combination was oxidized, the result would be oxide of iron 

 and oxide of manganese in beds closely associated but yet dis- 

 tinct. 



By supposing the iron sometimes to be deposited in sea water 

 as glauconite, a manner in which manganese is not laid down (see 

 page 365), a further means of separation of the two metals would 

 result. 



Thus by alternating the conditions of the deposition of iron 

 and manganese in different forms, a great variety of methods of 

 association and separation of the two metals can be produced. 



The above discussion refers not only to the deposits of iron 

 and manganese ores of notable size, but also to the iron and man- 



