1198 A TREATISE ON METAMORPHISM. 



descending waters bearing oxygen and following the impervious basement, 

 and as the oxygen was not in sufficient quantity to oxidize the iron to the 

 ferric oxide it was thrown down as magnetite. 



It has been pointed out (pp. 845-846), that magnetite may be pro- 

 duced in two other ways — by the reaction of siderite and pyrite upon 

 each other, or by the partial oxidation of pyrite or pyrrhotite. In the 

 Cornwall deposits" sulphur is a constant associate in small percentage 

 except at the surface, and this fact suggests the possibility that the reactions 

 between iron carbonate and iron sulphide or between iron sulphide and 

 oxygen may have been also instrumental in the production of the magnetite ; 

 but the amount of residual sulphur is so small that, the theory of the 

 derivation of the deposit in large measure from pyrite has scanty support. 



To what extent the principles worked out in reference to the Lake 

 Superior ores are applicable to the iron-ore deposits of other parts of the 

 world it is yet too earty to say, but, as just suggested, the descriptions of 

 the Cornwall deposit show that it has many features in common with the 

 ores of the Lake Superior region. Many of the Silurian deposits of the 

 Appalachian region are at contacts of limestones with shale or slate. In 

 some places these latter are certainly in such positions as to furnish imper- 

 vious basements, and therefore the ores present many features analogous to 

 those of the Lake Superior region. But a much more extended study of the 

 iron-ore deposits of other regions must be made before any generalization 

 can be made with reference to them. 



MANGANESE ORES. 



The manganese deposits, like the iron-ore deposits, are very largely 

 secondary segregations by descending waters. Indeed, iron and manganese 

 are very closely associated. Many of the iron-ore deposits are manganifer- 

 ous, and some of them sufficiently so locally that they also become ores of 

 manganese. For instance, in the Gogebic district of Michigan a small 

 amount of the iron ore was sufficiently manganiferous to be sold as a man- 

 ganese ore rather than an iron ore. Manganese ores are always associated 

 with iron oxide. The amount of iron is variable, running from a small 

 percentage to an amount considerably in excess of the manganese. 



« Lesley, J. P., and d'lnvilliers, E. V., Report on the Cornwall iron-ore mines, Lebanon County. 

 Ann. Kept. Geol. Survey Pennsylvania ior 18S5, pp. 533-534. 



