194 



ECONOMIC GEOLOGY 



carbon dioxide serve as a solvent for the iron in both the igneous 

 and the sedimentary rocks and from these solutions limonite may 

 be deposited. According to F. W. Clarke, organic acids and 

 humus assist in the solution of ferrous compounds and furnish to 

 swamp waters the material from which bog iron ores are formed. 

 The atmospheric oxidation of siderite affords the irridescent 

 films of ferric hydroxide so often seen on the surface of stagnant 

 swamp waters. 



FIG. 102. Old limonite pit, Ivanhoe, Virginia, showing pinnacled sur- 

 face of limestone which overlies the ore-bearing clay. The level surface 

 before mining began is seen on either side of excavation. (By permission 

 of the Macmillan Company, from Ries' Economic Geology.) 



According to N. S. Shaler, bog iron ores are far more abundant 

 around the margin of swamps and often wanting at the centers. 

 The deposition of the ore is not always as slow a process as is 

 sometimes supposed. A. Geikie cites the accumulation of such 

 ores to -a thickness of several inches in 26 years in some of the 

 Swedish lakes. The oxidation of ferrous sulphate solutions may 

 also yield limonite. Acorcding to R. A. F. Penrose, limonite 

 may be derived from glauconite through some process of altera- 

 tion. It may form a replacement deposit in limestones, and even 

 ferriferous limestones themselves may yield residuary limonite. 

 (See Fig. 102.) 



