PETROGRAPHICAL CHARACTKK OF MICHIGAMME FORMATION. 451 



oxide, exhibits a reddisli-bi-owii color. Fnmi tliese ferriferous (•url)onates 

 there have developed ferrug-inous slate, ferruginous chert, jasper, griinerite- 

 magnetite-schist, and iron ore, the processes and results being ic^entical 

 with similar rocks from similar materials in the Negaunee formation. (See 

 pp. 336-37r).) The description of these processes will therefore not be here 

 repeated. 



Certain minor differences separate these rocks from tliose of the 

 Negaunee formation — the griineritic rocks are finer- grained, the iron 

 oxide is largely limouite, and in all phases of them carbonaceous material 

 is abundant. 



The amount of pure nonfragmental material is subordinate, but because 

 iron ore develops from it, it is not unimportant. There is in the Michiganune 

 formation a much larger quantity of material intermediate between clastic 

 and nonclastic sediments. In some places the fragmental and nonfragmental 

 material is largely concentrated in alternate bands. In other places the two 

 are intermingled. Where least altered, these intermediate rocks may have a 

 background consisting of siderite and cherty quartz, with some ferrite, which 

 contains numerous well-rounded fragmental grains of quartz and feldspar. 

 As the metamorphosing processes set in, the siderite goes through the same 

 set of transformations as where it is alone, and the same is true of the frag- 

 mental material, so that there results a great variety of rocks. Where the 

 processes are chiefly metasomatic the siderite changes to ferrite, and ferru- 

 ginous graywackes, ferruginous slates, cherty graywackes, and cherty slates 

 are produced. In a common variety a ferrite background contains the 

 clastic constituents. If at the same time the feldspar alters to l^iotite and 

 chlorite, the slates are biotitic and chloritic. Where the dynamic effects are 

 stronger, griinerite and magnetite develop from the siderite, the secondary 

 mica has a parallel arrangement of its folia, and the quartzes are arranged 

 with their longer axes in a common direction, or are granulated, so that there 

 result hematitic and magnetitic mica-schists, griineritic mica-schists, etc. At 

 different j)laces there are all gradations from the least to the most metamor- 

 phosed varieties, and from those which originally consisted wholly of 

 nonfragmental material to those which consisted wholly of fragmental 

 material. In the first case the peculiar rocks of the iron formation were 



