PETROGRAPHICAL CHARACTER OF GOODRICH QUARTZITE. 415 



but even in such cases these quartzes were often granuhited into jasper-hke 

 material. 



By disappearance of the sihceous element and increase of the second- 

 ary hematite and magnetite the recomposed rocks pass into magnetic or 

 specular ore. Macroscopically these ores often show a peculiar gray color, 

 and in thin section they are usually easily separated from the ores of the 

 Negaunee formation l)y the presence of brilliantly polarizing flakes of 

 muscovite and of occasional particles of fragmental quartz. 



The conglomerates, recomposed jaspers, or recomposed ores, by a les- 

 sening of the amount of chert, jasper, and iron oxide, grade upward into the 

 quartzites. In the purer phases these quartzites consist mainh' of well- 

 rounded, simple fragments of quartz, many of which are enlarged, Init 

 with these are usually complex particles of chert and jasper. The (piartz 

 grains generally show strong pressure effects, such as undulatory extinction 

 or fracturing in a complex manner. This fracturing is in certain cases in a 

 rectangular system corresponding to the shearing planes. In other phases 

 there is an abundant matrix composed of finely crystalline quartz, with 

 sericite, biotite, and chlorite, in which the large fragmental grains of quartz 

 are set. By an introduction of feldspar the quartzites pass into feldspathic 

 quartzites, and from these to the graywackes of the Michigamme formation. 

 In the less pure quartzites, sericite, biotite, and chlorite frequently developed 

 abundantly from the clayey background. 



In the more mashed phases of the quartzite, and particularh' in the 

 Republic trough, the rocks are micaceous quartz-schists. Feldspar is plenti- 

 fully associated with the quartz in a number of cases. Usually the grains 

 of quartz still show a roundish appearance, but they interlock intricatelv, 

 and show no evidence of original cores. In the finer-grained, most mashed 

 kinds the background is a finely granular, interlocking mass of quartz. 

 Between and wrapping around the larger grains abundant muscovite is 

 found. In some of the rocks muscovite is a chief constituent, and these 

 may be called muscovite -schists. Muscovite -biotite -schists and biotite- 

 schists are also associated with the quartz-schists. These are, in nearly 

 all aspects, like the more metamorphosed rocks of the Michigannne forma- 

 tion. In a few places feldspar is a chief constituent of the background, 



