PETEOGEAPHJCAL CHARACTER OF MICHIGAMME FORMATION. 447 



R. 29 W. (Atlas Sheets XII and XV ) These ores difier from the soft ores 

 of the Negaunee formation in that tlie iron oxide is largely limouite and 

 in that the associated rocks contain much carbonaceous and graphitic 

 material. The ferruginous phases are particularly prevalent just above 

 the Clarksburg volcanics and the Ishpeming formation — i. e., a short 

 distance above the formations which are immediately subjacent to tlie 

 Michigamme formation. All of the pits may and probably do belong to 

 the same horizon. If this be true, the central belt is near the crest of 

 an anticline which rises high enough to bring this low ferruginous horizon 

 of the formation to the surface. The foot-wall of the ore bodies is, so far 

 as observed, the impervious fragraental Michigamme slate. The ores and 

 peculiar associated rocks therefore appear to be in bunches or in lenses in 

 the carbonaceous slates, strongly suggesting that the abundant organic 

 material had to do with the deposition of the iron compounds. 



The ferruginous and uonferruginous slates and graywackes, bv an 

 increase in metamorphism, pass into the mica-schists and mica-gneisses. 

 At Lake Michigamme, it has been said, mica-schist is alnindantly devel- 

 oped in its typical form. This mica-schist, while a completely crystalline 

 rock having well-developed schistosity, still shows in places, when closely 

 examined, the original beddnig and an alternation of coarse and fine 

 material such as occurs in the slates and graywackes to the east. The 

 schistosity varies from parallel to perpendicular to the bedding, usually 

 being at some intermediate angle Where the schists are completely crys- 

 talline, garnet, staurolite, chloritoid, and andalusite are often plentifully 

 present. 



In the most coarsely crystalline kind the i-ock is in places veined 

 throughout with a granitic-looking material, and feldspar has abundantly 

 developed within the rock, forming a gneiss. The gneiss is pegmatized 

 through and through, as though the material, either as a magma or in the 

 form of a water solution, had penetrated the joints, the partings ])arallel to 

 the laminae, and also the interspaces between the constituent particles, and 

 had in these places produced quartz and feldspar. A close examination 

 shows that many of the apparently granitic veins are but the coarser beds 

 strongly pegmatized. The pegmatized areas grade into the ordinary mica- 



