INTERESTING LOCALITIES OF NEGAUNEE FOKMATION. 381 



griinerite - mag-iietite - slates, gTiinerite - inag-netite - schists, ferrugimms slate, 

 and occasionally ferruginous chert. The widespread distribution of the 

 griineritic and magnetitic phases of the formation, in connection with these 

 greenstones, at once suggests that the intrusive rocks are the cause of the 

 development of these varieties of the Negaunee formation from tlu; sideriric 

 slates, rather than the ferruginous cherts and jaspers. How this alteration 

 occun-ed has already been explained in the general description of the 

 griinerite-magnetite-schists (pp. 35!;)-36I, 3()S-36;)). Tlie presence of much 

 residual siderite in this area is doubtless partly explained, at least, by the 

 protective influence of the greenstones, and ])ossibh' also by the relatively 

 imjjervious character of the secondary griinerite-magnetite-schist as com- 

 pared with the broken ferruginous cherts and jaspers. 



In thin section all the varieties of rocks described under the general 

 description (pp. 358-375) as cherty siderites, magnetitic, griineritic, and 

 sideritic slates, magnetitic and griineritic schists, and ferruginous slates, are 

 found, with all of their transition varieties. To give a description here 

 would be substantially to repeat that alreadv given, and a few onlv of the 

 peculiar features will be mentioned. 



It is in this area that the very finely crystalline and apparently partly 

 amorphous forms of silica are found. In some cases the siliceous back- 

 ground seems to be almost nonpolarizing-. In a more advanced stage of 

 alteration, minute opaline ilniplets or granules, averaging perhaps 0.01 mm. 

 in diameter, and surrounded by films of iron oxide, constitute the back- 

 ground. These di'oplets or granules are rather characteristic of tlie early 

 stages of the rearrangement of the silica. The silica is, however, ordinarily 

 completely individualized, and occurs either in granules similar to the drop- 

 lets or in ordinary chert, the grains averaging in some sections as much as 

 0.03 mm. in diameter. The hematite in the ferruginous slates, even where 

 the siderite has wholly disappeared, has a decided tendency to occur in 

 rhombohedra. At the lower horizons of the formation, where fragmental 

 material begins to appear, certain peculiar varieties are found. In some 

 cases there are seen large feldspathic areas, which appear to be partly altered 

 into or replaced by the magnetite, grihierite, and quartz. Small, distinctly 

 fragmental grains of quartz are plentiful. Not infrequently the quartz 



