402 THE MARQUETTE IRON-BEARING DISTRICT. 



Therefore the plane of unconformity bet\yeeu the Upper Marquette and 

 Lower Marquette series must have been a great horizon for downward- 

 flowing waters. 



It has been seen that the whole of the iron-bearing formation was 

 pi'obably originally a lean, cherty carbonate of iron, with perhaps some 

 calcium and magnesium, and that from this rock the ferruginous cherts and 

 jaspers developed. If we now go no further back than the feiTuginous 

 cherts and jaspers, in order to produce the ore two things must have 

 occiu'red: first, the further concentration of iron oxide in the places 

 where the ore bodies are found; and second, the removal of silica from 

 these places. 



The final concentration of the ores occurring at the contact of the 

 Upper Marquette and Lower Marquette series must have taken place later 

 than Upper Marquette time. This is indicated by the fact that the uncon- 

 formable formations are welded together by the iron ore at many places. 

 The relations of the ore bodies within the ore formation to the greenstone 

 masses and dikes give evidence that the concentration of this ore occurred 

 subsequently to the intrusion of these rocks. It is certain that some of these 

 igneous rocks were intruded during or later than Upper JMarquette time, 

 since they cut across the Goodi'ich quartzite. Others of them appear to 

 have yielded fragments to the Upper Marquette series, and therefore ante- 

 date these rocks. Finally, if the ore bodies were concentrated before the 

 Upper Marquette folding and erosion their invariable positions above 

 the impervious formations would be inexplicable. The folding would 

 perhaps have left them as often below as above these formations. Taking 

 all the facts together, it is highly probable that the final concentration 

 of all the ores occurred during- and later than the folding and erosion subse- 

 quent to Upper Marquette time. 



Surface waters bearing oxygen, passing downward through the Upper 

 Marquette series or tlie iron-bearing formation of the Lower Marquette 

 series, would decompose the iron carbonates with which they came in con- 

 tact and thus become carbonated. These carbonated waters would then 

 be capable of taking other iron carbonates into solution. What })roportion 

 of the oriffiual iron carbonate still remained in the ore-bearingf formation 



