Plate XVIII.— MAGNETITE-GRUNERITE-SCHISTS. 



Magnetite-griinerite-schist from Republic mine (Atlas Sheet XI). This is one of the coarsest 

 varieties of the griinerite-magnetitc-schists. In place of the siderite of Plate XVII we have 

 griinerite and to some extent limonite, hematite, and magnetite. The griinerite is caused 

 by the dccompoaition of siderite into iron protoxide and carbon dioxide and the union of 

 the former with silica. The iron oxides, and especially the magnetite associated with the 

 griinerite, are in part the direct results of the oxidation of the original siderite. Some of 

 the limonite and hematite are due to the decomposition of the griinerite. Finally, a part 

 of each of the iron oxiiles is a secondary concentration. This is shown by their appearance 

 in veins cutting the bedding. Natural size. 



Sideritic magnetite-griinerite-schist from sec. 13, T. 47 N., R. 27 W. (Atlas Sheet XXVIII). 

 The different bands consist mainly of griinerite, hematite, magnetite, and quartz, in 

 varying proportions. The darker-colored bands contain much of the iron oxide. In the 

 lighter bands griinerite is abundant. In all of the layers there is a sufficient amount of 

 residual siderite to show that from this mineral and silica the griinerite formed, and from 

 it, with partial or complete oxidation, the magnetite and hematite developed. The most 

 of the hematite is of the specular variety, but in places blood-red flecks of hematite may 

 be seen, and parts of the specimens are stained by limonite. This is doubtless the result 

 of weathering. Natural size. 



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