450 THE MARQUETTE IKON BEARING DISTRICT. 



in the (juiesceut stage after dyuamic action had ceased, but -while the heat 

 still produced hot solutions which bore abundant mineral material. 



By the formation of the secondary feldspar, probably by the same proc- 

 ess both within and between the grains, quai*tz- mica -feldspai'- rocks or 

 mica-gneisses have developed. These mica-gneisses have an interlocking, 

 granitic-appearing background, composed of quartz and feldspar in about 

 equal abundance. That the rock was originally fragmental is indicated 

 only by occasional roundish grains of quartz and feldspar, but it is always 

 difficult to determine cextainly what pai-t of the quartz and feldspar is 

 original and what pai"t a secondary development. Both the original and 

 secondary feldspars are stained with limonite and ai-e decomposed to a 

 greater or less degree into chlorite, biotite, and quartz. Biotite is the 

 predominant micaceous mineral, but muscovite is present, and chlorite is 

 abundant. Magnetite is also present in numerous crystals, and a small 

 amount of hornblende is foimd. The veins cutting the gneisses are com- 

 posed mainly of iron-stained feldspar, with, however, much chlorite and 

 quartz. This feldspar, which is beyond all question secondary, is identical 

 in its appeai'ance with that contained throughout the rock. In the mica- 

 gneiss are curious black concretionai-y-looking areas, which in thin section 

 are seen to be essentially the same as the remainder of the rock, except 

 that they contain numei'ous large crystals of hornblende and much zoisite. 

 Each of the hornblende individuals includes many of the other mineral 

 particles, and in their development they appear to be analogous to the 

 staurolite and garnet. 



The occuiTence of these mica-gneisses within the Michigamme forma- 

 tion is of great interest as pro^'ing the development of this kind of I'ock 

 from a clastic. In almost every I'espect the coarsest of these mica-gneisses 

 are similar to many mica-gneisses of the Basement Complex. The only 

 difference between the two is that in the Michigamme formation these crys- 

 talline forms may be traced by gi-adations to phases in which the fi-agmental 

 characters are clearly apparent. 



The pm-est and least-altered phase of the ferruginous rocks is sideritic 

 slate. This is a fine-grained gray rock, composed almost wholly of siderite, 

 which upon the weathei'ed surface, where the carbonate passes into iron 



