388 THE MAKQUETTE IKON-BEAKING DISTEICT. 



consists ill part of griiuerite and in part of yreen lioniblende. The cle3v- 

 age rnns from one to the other, showing- that both are parts of the same 

 crystal indi^^dual. The two are discriminated by the color and pleochro- 

 isni of the green hornblende, and Ijy a slight difference in the extinction. 

 The gTiinerite-rocks and the grunerite-magnetite-rocks are associated with 

 a small quantity of grunerite-magnetite-schist. In the passage to the latter 

 rock the quartz first appears as small oval areas, and tinally as distinct 

 bands. The quartz grains are jienetrated through and through by the 

 griiuerite blades. They include numerous crystals of magnetite, and, 

 except garnet, therefore appear to be the last mineral to develop. At the 

 bottom of the formation in the southeastern part of the area mica and 

 quai-tz appear, and the griinerite-magnetite-schists grade into ferruginous 

 mica-slates belonging to the upper part of the Ajibik quartzite, or more 

 jjroliably the equivalent in age of the Siamo slate. These transition 

 varieties are frequently garnetiferous. 



In thin section the jaspilites of Mount Humboldt are similar to those 

 of Michigamme. A small amount of pleochroic amphibole is present, as 

 at tliat locality, and in one case this has partly altered to chlorite. The 

 slides show remarkably well the ditference between the original sheared 

 hematite and the secondary magnetite. The former, in reflected light, 

 mav be seen in a series of extremely close microscopic folds, the laminae 

 of which are often broken at the' more acute bends. The crystals of mag- 

 netite take a nearly perfect polish and give brilliant reflections. These 

 are found to be largely concentrated at the places of fracturing and at 

 the turns of the folds. As is well known, these are places where spaces 

 are natm-ally formed by the folding process. So in;irked is the difference 

 between the reflecting power of the original sheared hematite and the 

 mag-netite that the two may be discriminated with the naked eye in section 

 or on the polished surface. The slides of the mashed breccias, looked at 

 with the naked eye, very closely resemble the mashed conglomerates of 

 the overlying Ishpeming formation. The broken fragments of jasper are 

 flattened in a common direction, the diffei'ent areas overlap, and the rock 

 ha-5 a very strongly conglomeratic appearance. However, when examined 

 under the microscope, the fragmental quartz, almost invarialily present in 

 .the true Ishpeming conglomerates, is entirely absent. 



