PETEOGRAPHICAL CHARACTER OF :\IICHIGAMME FORMATION. 449 



In proportion us the t'eldspar is decomposed and the quartz is granu- 

 lated, the rocks approach the mica-schists, an intermediate phase being 

 represented by the mica-skites. These still show evidence of their frag- 

 mental origin, occasional fraginental grains of quartz being seen, some of 

 which are enlarged. Many of these fraginental grains are easily separable 

 from the newly developed quartz, showing as they do undulatory extinction 

 or fracturing. The quartz grains show an imperfect arrangement, with their 

 longer diameters in a common direction. The folia of biotite also have a 

 parallel arrangement. In a further stage much of the quartz has been 

 granulated, and the feldspar is largely replaced by secondary mica and 

 quartz. 



These mica-slates on the one hand grade into the ordinary slates and 

 graywackes step by step, and on the other hand, by greater alteration, they 

 pass into mica-schists. Where the process of metamorphism is complete, 

 the fragmental quartz grains are wholly granulated by the mashing, which 

 has kneaded the rock throughout. In many slides each folium moved 

 differentially in reference to those above and below it. The fragmental 

 feldspar is wholly changed into quartz, mica, and chlorite. The folia of 

 new mica developed with their longer axes in a common direction. In pro- 

 portion as the deformation is greater, sericite and muscovite become prom- 

 inent with the biotite. Thus in place of the fragmental rock a completely 

 crystalline mica-schist is produced. 



The details of the processes of development of these schists will not 

 be here described, but they are similar to those given for the development 

 of the mica-schists in the Penokee series.' There has, however, been the 

 difference explained above, that mashing has played a nmch more important 

 part in the case of the mica-schists of the Marquette district. As a con- 

 sequence, some of the schists are strongly foliated. In the crystalline 

 schists a large amount of garnet, staurolite, andalusite, and chloritoid has 

 developed. These minerals include large quantities of the prior quartz. 

 They show no evidence of strain, and they are believed to have developed 



' The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. Van 

 Hise: Mon. U. S. Geol. Survey, Vol. XIX, 1892, pp. 332-343. 

 MON XXVIII 29 



