Plate XX.— FERRUGINOUS CHERT. 



Fig. 1. Ferruginous chert from Taylor mine sec. 9, T. 49 N., R. 33 W. This specimen illustrates a 

 somewhat different stage of alteration from PI. XIX. The silica has been almost perfectly 

 concentrated into bands. The same is true of iron oxides. Movement has fractured the 

 siliceous bands, and along these the removal of silica has begun. If nearly all of the silica 

 were replaced by hematite, iron ore would be formed. lu fig. 1 of PI. XIX the reverse 

 process is seen — that is, the solution of ore and the deposition of silica. It is a general 

 law of the Lake Superior region that the solution of silica and the deposition of iron oxides 

 occur at places where abundant percolating waters are concentrated. It will be shown 

 later (see Pis. XXVIII and XXIX) that these favorable conditions are just above impervious 

 formations which occur in pitching troughs. This figure, from a specimen obtained from 

 the Upper Marquette series outside of the district mapped, is here inserted for comparison 

 with the ferruginous cherts of the Negaunee formation. Natural size. 



Fig. 2. Ferruginous chert from south of Jackson mine, sec. 1, T. 47 N., R. 27 W. (Atlas Sheet XXVIII). 

 The iron oxide and chert were largely concentrated into bands before the last folding. At 

 the time of the folding radial cracks were formed, especially in the chert layers, due to the 

 position of the rock on the crown of an anticline. Along these cracks the silica has to 

 some extent been leached out and iron oxide introduced. One light-colored area of chert 

 appears to be a secondary infiltration, but it was apparently present before the last folding, 

 as it is fractured the same as the other layers. Natural size. 

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