364 



MINERALOGY 



The waters of swamps and some springs carry ferrous salts in 

 solution, either connected with organic acids or as carbonate or 

 sulphate. When exposed to the air these salts oxidize and the iron 



FIG. 431. 



Limonite Pseudomorph after Marcasite from Richland County, 

 Wisconsin. 



is separated as ferric hydrate, which may be noted many times 

 as an iridescent oil-like film on the surface of stagnant water of 

 swamps. When the surface is agitated, the film of heavy oxides 

 sinks, and constantly accumulating'on the bottom they form the lake 

 and swamp deposits of " bog iron ore." It is not necessary to add that 

 such ore is usually impure, from the nature of its formation; it 

 is mixed with sand and organic matter and often contains consid- 

 erable phosphorus and sulphur. Numerous iron springs are so 

 charged with iron in solution that their banks and beds become 

 coated with a bright yellow gelatinous deposit of ferric hydrate. 

 Limonite is found in clays and as a residual deposit of ferruginous 

 limestones, the calcium carbonate having been removed in solu- 

 tion, while the ferrous carbonate was oxidized and remained in 

 place and concentrated as the chemical reactions continued. Such 

 limonites associated with limestones are found along the Blue 

 Ridge in western Virginia and in Tennessee, where they are mined 



