INTEKESTING LOCxVLlTIES OF NEGAUNEE FOKMATIOX. 391 



crystals large enough to be distiiii't tu the naked eye. Many hands are 

 composed almost wholly of griinerite and magnetite, and these are inter- 

 laminated with those in which the quartz is eqi;ally abundant W'ith the other 

 minerals, or becomes predominant, serving as a matrix for them. 



In thin section these griinerite -magnetite -schists are seen to repre- 

 sent the extreme phase of metamorphism of the rocks of the Negaunee 

 formation. The quartz is more coarsely crystallized than in anv of the 

 previously described rocks, the grains averaging from 0.2 to 0.4 nun. in 

 diameter, and in one of the coarser varieties averaging about 1 mm. The 

 amphibole is of two kinds, the ordinary white, slightly pleoehroic griiner- 

 ite, and a green amphibole. The griinerite occurs in the ordinarv blades 

 and crystals, but in some cases has a very uniformly parallel arrangement 

 of its fibers, which is rather unusual, the ordinary varieties in other parts 

 of the Marquette district having a radiating, sheaf- like arrangement. 

 This may indicate that dynamic metamorphism was more severe in the 

 area of the Magnetic mine than elsewhere. The colored amphibole has a 

 tendency to occur in idiomorphic crystals. It gives beautiful pleochroism: 

 C is light pea-green, b is dark greenish-yellow, a is light transparent yellow. 

 The absorption formula is b > C > 3- In one case the angle C;C was 

 found to be 16°. In the same slides different bands are composed pre- 

 dominantly of each of the varieties of amphibole, but also in these and in 

 other slides there are beautiful and complicated intergrowths of the two, as 

 at Michigamme. In some cases the green variety constitutes the outer 

 bands of the crystals ; in other cases the reverse is true. In some instances 

 nuclei of the colored amphibole are entirely surrounded by the fibrous 

 griinerite, as though it were an added growth, and in other instances the 

 ojjposite occvirs. The magnetite in its occurrence is the same as in the 

 previously described griinerite-magnetite-schists. In the gradation phases to 

 the Ajibik quartzite, biotite, chlorite, and garnet are abundantly associated 

 with the green amphibole. 



th:e iron-ore deposits. 



THE ORE HORIZONS. 



The ore deposits may be divided, according to position, into three 

 classes, (1) those at the bottom of the iron-bearing formation, (2) those 



