126 . C. B. Van Hise — Iron Ores of Michigan. 



that in order to produce the ore, two things must have occurred: 

 first, the concentration of iron oxide in the places where are 

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

 these places. 



Time at ivhich concentration occurred. — The final concen- 

 tration of the Lower Marquette ores occurring at the contact 

 of the Upper and Lower Marquette series must have taken 

 place later than Upper Marquette time. This is indicated by 

 the fact that while these ores are so frequently found at the 

 contact plane the fragments of the overlying conglomerate are 

 almost wholly of the lean chert or jasper, or magnetite-actino- 

 lite schist, and include few of the rich hard ore. which if 

 present ought to have yielded fragments. This not only shows 

 that the final concentration had not occurred, but that the 

 cherty carbonate (if the assumption be correct that the ore- 

 formation was originally of this material) at the surface had 

 been decomposed before Upper Marquette time. These con- 

 glomerates therefore give us an indication of the early char- 

 acter of a part of the ore-bearing formation from which the 

 ore-bodies were later derived. The position of these ore- 

 bodies at the contact plane of the Upper and Lower Mar- 

 quette series is also evidence that they have been here con- 

 centrated subsequent to Upper Marquette time ; for it is 

 exceedingly improbable that erosion so generally stopped at a 

 horizon rich in ore, a material which is softer than jasper and 

 would therefore be more rapidly cut out. Further, this con- 

 tact is not at a certain plane of the ore-bearing formation, but 

 here is at a higher horizon and there at a lower one. Bearing 

 in the same direction, although perhaps not strongly, is the 

 occasional welding of the quartzite-conglomerate and ore-bear- 

 ing formation already mentioned. 



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

 the diorites and dike-rocks give evidence that the concentra- 

 tion of this ore has occurred subsequently to the intrusion of 

 these rocks. It is certain that some of these eruptives are 

 intrusives later than the Upper Marquette series, since they 

 cut across the overlying quartzite. Others of them appear to 

 have yielded fragments to the Upper Marquette series and 

 therefore antedate these rocks. • Finally if the ore bodies had 

 become concentrated before the Upper Marquette folding and 

 erosion, their invariable positions above the impervious forma- 

 tions would be inexplicable. The folding would perhaps have 

 as often left them below as above these formations. Taking 

 all the facts together, it is highly probable that the concentra- 

 tion of all the ores occurred during and later than the folding 

 and erosion subsequent to Upper Marquette time. 



Manner of ore- concentration. — Surface waters bearing oxy- 

 gen passing downward through the Upper Marquette series or 



