368 THE JOURNAL OF GEOLOGY. 



regions of the Appalachian Valley, where there are often found, 

 in different places along the same belt, deposits of iron ore 

 and deposits of manganese ore in positions similar with relation 

 to the enclosing rocks. 



These conditions of moving water might also cause the occur- 

 rence of the two ores in interstratified layers, as is sometimes the 

 case. Such a condition would result if iron were deposited in a 

 certain place at one time, and if, later, on account of some 

 increased facility for oxidation, iron was deposited before it 

 reached that place, and the manganese, being less easily precipi- 

 tated, were carried on and laid down upon the first deposit of iron. 



Suppose the metalliferous solutions to be confined in a shal- 

 low basin, or, at least, to pass through it so slowly that they be- 

 come thoroughly oxidized. Under such conditions the deposition 

 of iron and manganese would go on continuously, and so nearly 

 on the same spot that a comparatively homogeneous manganifer- 

 ous iron ore would be formed. If the supply of metalliferous 

 solutions were not continuous, but were intermittent, as is some- 

 times the case in local basins, such as coastal lagoons, which are 

 often dependent for their supply of water on the changes of 

 season and the sudden fluctuations of weather, then interstratified 

 layers of iron and manganese ore might be produced. The iron,, 

 becoming oxidized on the surface, sinks to the bottom, possibly 

 in some cases to be converted there to the simple carbonate by 

 organic matter. Further oxidation precipitates hydrous oxide or 

 carbonate of manganese on top of the iron. A renewed supply of 

 surface waters brings more solutions of iron and manganese, or 

 else the evaporation of the water in the closed basin concentrates 

 the materials which have not yet been precipitated. In either 

 case there is a further alternate deposition of the two ores.- 



Another process of separation of iron and manganese in na- 

 ture might take place by the formation of sulphide of iron. It 

 has already been shown that iron is sometimes deposited as sul- 

 phide and later oxidized in the same manner as the carbonate. 



^ In some cases these iron and manganese deposits are undoubtedly formed by 

 the replacement of limestone or other rocks, as is further discussed on pages — to — , 



