Plate XIX.— GRUNERITIC MAGNETITE-SCHIST AND FERRUGINOUS SLATE. 



Fig. 1. Griineritic magnetite-schist from Republic mine. Tlie lighter-colored bands are strongly 

 quartzose. The darker bands are heavily ferruginous, but contain a great deal of quartz. 

 The iron oxide is largely magnetite, but with this is much hematite. The griinerite is 

 scattered throughout the rock, but is more prevalent in the heavily ferruginous bands. 

 In its regular banding the rock is very similar to the original cherty sideritic slates 

 represented by PI. XVII, fig. 2. 



Fig. 2. Ferruginous slate from sec. 7, T. 47 N., R. 26 W. (Atlas Sheet XXXI). The bluish-gray bands 

 are largely chert, but in them iron oxide is contained. The reddish-brown bands are largely 

 limouite and hematite, but contain much chert. This rock is evidently exactly what would 

 bo produced by the complete oxidation of the cherty siderite shown in fig. 1, PI. XVII. 

 The chert bands of the two are almost absolutely of the same color and composition. In 

 place of the siderite bands of the latter are the limonite and hematite bands. The change 

 emphasizes the structure as indicated in the description of fig. 2, PI. XVII; Also, as in that 

 figure, the ferruginous layers are not so regular as the original siderite layers. In the 

 rearrangement the iron-bearing solutions have penetrated to a greater or less degree into 

 the cherty layers. At a number of places the rock was fractured across the layers. At 

 such places the iron oxide has been leached out to some extent, and the belts of chert 

 connect different layers of that material. Last of all, along one vein secondary iron oxide 

 has formed. Natural size. 



Fig. 3. Ferruginous slate or jasper from sec. 7,T. 47 N., R. 26 \V. (Atlas sheet XXXI). This figure rep- 

 resents a somewhat more advanced stage of alteration. The iron oxide is largely concen- 

 trated in the red and black bands and the silica is largely concentrated in the yellowish-red 

 layers. The illustration might perhaps as well have been placed with the jaspers as with 

 the ferruginous .slates. It is, in fact, a transition variety. If the chert were somewhat 

 more stained with brilliant-red hematite it would be called jasper. 



The specimen beautifully illustrates deformation in the zone of combined fracture and 

 flowage. The rigid cherty layer is fractured and faulted. The fault is normal. The more 

 plastic ferruginous layers accommodated themselves to the changed position of the siliceous 

 layer by flexure. The specimen looks as though black hematite material had flowed in 

 between the broken siliceous bands, like dough. The specimen illustrates in miniature how 

 a fault may pass into a flexure either above or below. Natural size. 

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