INTERESTING LOCALITIES OF THE ISHPEMING FOIIMATION. 423 



which are largely composed of siderite, and out of this siderite the grii- 

 nerite and magnetite may be seen developing. Tlie description of the 

 griinerite-iuagnetite-rocks and griinerite-magnetite-sehists given in the gen- 

 eral description (pp. 417-419) applies to the 8purr and .Michigamme area. 

 Thev are in all respects like similar rocks from the Negaunee formation, 

 with the exception that the griinerite is in coarser blades and crystals, and 

 that frequently there are numerous minute Ijlack particles tliroughout tiie 

 rock which have a carbonaceous appearance, but Avhich may be ilecks of 

 iron oxide. The quartz of the griinerite-magnetite-sehists is very similar 

 to that of the Negaunee formation of the Michigamme area. Upon the 

 whole, however, it is somewhat more finely crystalline, the grains aver- 

 aging, in the ditferent sections, from 0.03 to 0.1 mm. in diameter. In 

 certain slides the nonpleochroic griinerite and the pleochroic ami)hibole are 

 intergrown, exactly as they are in the Negaunee formation in this area. 

 Where the rocks are exposed to weathering the griinerite is more or less 

 altered into biotite and chlorite, and where these minerals occur there is 

 also seen brilliant blood-red hematite, which has doubtless developed l)y 

 oxidation from the magnetite. Not infrequently the griineintic and mag- 

 netitic rocks are garnetiferous. The garnets include the various other 

 minerals, and it is apparently the last mineral to develo}). 



Lake Michigamme area. — Upou the anticliual ridge borderiug the nortli side 

 of Lake Michigamme and constituting the headlands of this body of water 

 are numerous expo.sures of typical Bijiki schist (Atlas Sheets V and VIII). 

 In fact, it is from the exposures adjacent to the mouth of the Bijiki River 

 that this formation is given its name. The rocks are here griinerite- 

 mao-netite-rocks and griiuei'ite-magnetite-scliists, in all respects like those 

 constituting the ridge north of the railroad track between Michigamme and 

 Spurr. In thin section, also, thej^ are identical. The ordinary hornblende 

 is the nonpleochroic griinerite, but at the northwest point of the westward- 

 projecting headland in sec. 28 there are beautiful intergrowths of the non- 

 pleoclu-oic and pleochroic amphiboles, the latter giving blue, green, and 

 yellow colors. It is possible that the phenomena are due to combinations, 

 of varying proportions, of the actinolite and griinerite molecules. 



East of Lake Michigamme (Atlas Sheet XII) this anticline is repre- 

 sented by a prominent ridge of the Bijiki formation, running from the 



