INTERESTING LOCALITIES OF NEGAUNEE FOltMATION. 377 



o-reenish colors. As usual, the yrihierite has a tcudenc)- to be associated 

 with the magnetite and hematite. In one case an opaque crystal of hem- 

 atite or magnetite was found to l)e surrounded b}' Ijlades of griinerite, each 

 blade being parallel to one of the sides of the crystal. Where the con- 

 cretionary jasper is mashed, the concretions have an oval form, the longer 

 axes beino- in a common direction. In the liandetl fei-ruginous rock con- 

 taining white quartz layers (PI. XXIII) the quartz grains contain very little 

 oxide of iron. AVh\- this material is absent here and present in the jasper 

 is not apparent. The grains of quartz in both the red and white siliceous 

 layers in many slides have crystal ovitlines, appearing in thin section as 

 closely fitting polygonal areas. 



By an increase of magnetite and griinerite and a decrease ot hematite 

 the jaspilites pass into typical griinerite-magnetite-schists. Each quartz 

 o-rain includes hematite and magnetite crystals, and often blades of 

 griinerite. A concretionary arrangement of iron oxide occurs in the 

 transition phases. Pleochroic hornblende is absent. In the most strongly 

 o-riineritic rock, which is prevalent near the base of the formation, the 

 quartz almost disappears, and there is a background composed of inter- 

 locking blades of griinerite which include a large amount of magnetite, 

 and thus they become griinerite -magnetite -rocks. The iron ores are 

 magnetites. 



Boston and Dexter areas. — East of tlic Micliigamme mlue the exposures of the 

 Negaunee formation are rare for nearly 15 miles. However, in the SW. \ 

 sec. 32, T. 48 N., R. 28 W., is the Boston mine (Atlas Sheet XVIII). In 

 sees. 3 and 4, T. 47 N., R. 28 W., are a number of exposures of the forma- 

 tion (Atlas Sheets XIX and XXII), and north of the center of sec. 3 is the 

 Dexter mine. On the line between sees. 3 and 4 is the contact between 

 the Negaunee formation and the Groodrich quartzite. The unconformity 

 between the two formations is here not marked. The two. are slightly 

 overturned, so that the quartzite appears to lie under the Negaunee jasper. 

 A short distance to the northeast is a hill composed largely of the Negaunee 

 formation, but at the foot of its northern slope is found the Siamo slate, so 

 that at this place we have the Negaunee formation accurately delimited 

 above and below. 



