370 THE MARQUETTE IRON-BEARING DISTRICT. 



obtain. The manner in which this phase ot" rock develojied from the 

 sideritic slate has already been indicated. 



The ferruginous cherts (Pis. XX-XXIII) differ from the ferruginous 

 slates chiefly in that the silica has been more extensively rearranged. As a 

 consequence of this the chert and iron oxides are more or less concentrated 

 in alternating bands, instead of being uniformly mingled in a mass, as in 

 the ferruginous slates. However, the chert bands are never free from the 

 iron oxide, nor are the iron-oxide bands ever free from the chert. Between 

 almost pure iron oxide and almost pure chert bauds there are all grada- 

 tions. The silica of the chert is usually completely individualized, but 

 in different sections varies from partly amorphous through finely crys- 

 talline to rather coarsely crystalline. The quartz which does not show 

 evidence of much rearrangement is very like in size of granules and in 

 appearance to the quartz of the sideritic slates, but that in veins and filled 

 areas is much more coarsely crystalline. In arrangement the particles of 

 iron oxide appear to be wholly independent of the quartz. There is no 

 apparent concentration of the iron oxides between the quai"tz grains, but 

 they occur concentrated in laminje or as separate flecks included in the 

 grains of quartz, just as though they were all in their present positions 

 before the silica began to crystallize. In the ferruginous cherts which 

 are near the ore bodies cavities are very common, due to the solution of 

 the quartz. These cavities have often been subseqviently partly or wholly 

 filled by hematite. In all these particulars these ferruginous cherts are 

 similar to those from the Penokee and Animikie districts, but they differ 

 from them in not showing extensively the somewhat remarkable concre- 

 tionarv structure characteristic of tliose disti'icts, although in a few places 

 this is well developed. 



The Marquette ferruginous cherts have been subjected to profound 

 dynamic action, and the brittle rock has become shattered through and 

 through, producing innumerable cracks and fissures, and not infrequently 

 reibungsbreccias (Pis. XXI-XXIII). Within the spaces thus produced sec- 

 ondary hematite and magnetite in well-defined crystals have formed. By the 

 crystal outlines the secondary iron oxide can frequently be discriminated 

 from that present before the mashing occurred. The metamorphosing 



