C. R. Van Hise — Iron Ores of Michigan. 129 



Now the micaceous ore from the large deposits, as tirst sug- 

 gested by Prof. Pumpelly, gives the same evidence of shearing. 

 When it is remembered that in the folding of thick forma- 

 tions accommodations and re-adjustments must occur, it is nat- 

 ural to suppose that this re-adjustment has more largely taken 

 place at the contact between the Upper and Lower Marquette 

 series than at any other one horizon, for this is emphatically 

 the plane of weakness. Thus would be explained the finely 

 laminated micaceous variety of ore. It is not impossible that 

 the heat and pressure caused by the shearing along the contact 

 plane would be sufficient to change soft ore into micaceous 

 hematite, but to the writer it appears more probable that these 

 ores represent sheared specular hematite and magnetite. Since 

 the final concentrations of the ores occurred during and subse- 

 quent to the folding of the series, it is necessary to believe 

 that this shearing was a contemporaneous process, or else that 

 after it occurred residual silica was replaced by iron oxide and 

 that this metasomatic change did not affect the prior lamina- 

 tion of the original ore. Doubtless both explanations are 

 applicable in varying degrees at different places. 



That it is easy to reduce hematite to magnetite is well 

 known, and it is probable that the production of the granular 

 infiltrated variety of this ore is due to the reducing character 

 of some of the solutions which have passed down along the 

 great contact plane of percolation where the magnetites are 

 extensively found. This reducing power could readily be 

 imparted by organic acids. That some kind of reducing agent 

 has been present is indicated by the veins of pyrite which are 

 frequently associated with the magnetic ores. 



It is supposed that the magnetite of the magnetite-actinolite 

 schists is due to the direct oxidation of an original carbonate of 

 iron. This is known to be true of the Penokee and Animikie 

 magnetites and also of the magnetite of the Lower Marquette 

 series in the deeper workings of one of the mines. When iron 

 carbonate is decomposed in the presence of an excess of oxy- 

 gen the sesquioxide is of course formed ; but if it is supposed 

 that an insufficient amount of oxygen was present, as is quite 

 probable, from three molecules of iron carbonate would be 

 produced one molecule of magnetite and three of carbon diox- 

 ide, if only a single atom of oxygen was available. 



Ores of the Upper Marquette series. — The ore-deposits of 

 the Upper Marquette series, so far as studied, show that the 

 original material was an iron carbonate and that the ores are 

 secondary concentrates resting upon impervious formations. 

 Sometimes this impervious formation is a black slate ; at other 

 times it is an intrusive basic eruptive ; and at still others it is 

 a surface volcanic. As a consequence of folding, or by the 

 junction of two of these basal formations, and of concentra- 



