OIMGIX OF IRON ORES OF NEGAUNEE FORMATION. 405 



Upper Marquette aud 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. The specular hematite may 

 have been soft ore, for it is not impossible that shearing along- the contact 

 plane, with the heat developed, was sufficient to cause this transformation. 



A close examination of the slate ores sjaows that they are composed of 

 two parts, one of which was mashed, the other being granular or crystal- 

 outlined hematite and magnetite. The latter material fills the cracks left 

 as a result of the mashing, perhaps occupies the place of residual silica, 

 and welds the micaceous leaves together. Thus this granular ore was 

 certainly deposited after the folding. How much was introduced during 

 the folding' it is impossible to say, for this part can not be sejjarated from 

 that present before the folding. 



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 passed down 

 along the great contact plane of percolation, where the magnetites are exten- 

 sively found. Reducing power could readily be imparted by organic acids, 

 and that some kind of reducing agent was present is indicated by the veins 

 of pyrite which are frequently associated with the magnetic ores. 



The magnetite of the gTiinerite-magnetite-schist has been seen to be 

 partly due to an imperfect oxidation of the original iron carbonate. It is, 

 however, doubtful whether any considerable quantity of the magnetite 

 of the greater number of AA-oi'ked ore bodies is directly of this derivation, 

 although some of the lesser magnetite deposits appear to be an enriched 

 giainerite-magnetite-schist. In these cases there is no particular difficulty 

 in accounting for the larger part of the magnetite, but the same difficulty 

 exists in explaining the imperfect oxidation of the infiltrated material as in 

 the other instances. 



PEOSPECTINa. 



In considering the advisability of prospecting in any particular locality 

 the foregoing conclusions as to the relations of the iron ores may be of 

 assistance. These may be briefly summarized as follows: The iron ores 

 are always confined to the iron-bearing formation. They always rest 



