PETKOGEAPHICAL CHAEACTEE OF ^EGAUifEE FOEMATION. 367 



areas of eherty silica, small crystals of magnetite, and needles of actinolite 

 or griinerite. The silica is rarely partly amorphous, l)eing in minute opa- 

 line cboplets, but is more commonly completely individualized quartz, the 

 grains varying in the different slides from 0.01 to 0.03 nun. in diameter. 

 The siderite is in closely packed, small rhombohedra. Upon the weathered 

 surfaces the siderite is entirely oxidized, being changed into hematite or 

 limonite, with pseudomorphous forms. In this iron oxide is contained 

 eherty silica, identical with that in the unaltered part of the rocks. 

 Between the two there is a transition zone, in which are seen the various 

 stages of alteration from the unchanged siderite to the secondary hematite. 

 In one of the finest instances the transition band is broad, and there are 

 seen many rhombohedra of siderite surrounded by bands of beautiful, 

 blood-red, translucent hematite. These borders vary from mere films to 

 those so broad that but a minute speck of the siderite remains. If the 

 oxidized portion of the slide were seen by itself it would be regarded as a 

 ferruginous slate, with which it is in every respect identical ; but in this case 

 it can not be doubted that the siderite is the original source of the liema- 

 tite. Where the siderite is less abundant and the chert more plentiful, the 

 rhombohedra of siderite are set in a matrix of chert, wliich may consist 

 wholly of individualized quartz, Init which sometimes a|)parently contains 

 some opaline silica. Oftentimes bands consisting largely of silica alternate 

 with bands consisting largely of siderite. In the less pure phases, near 

 the base of the Negaunee formation, the eherty siderite in some cases 

 alternates with strata of an impure clayey rock, apjjroaching the Siamo 

 ferruginous clay -slates; in other cases, mingled with the siderite itself 

 is fragmental material, including both quartz and feldspar, and tiieir 

 alteration products. Not infrequently within these semifragmental rocks, 

 along cracks and joints, all transitions between the impure siderite and a 

 ferriferous or eherty slate, partly fragmental and partly nonfragmental, 

 may be seen. 



Where the sideritic slates are altered, not by weathering, but by deep- 

 seated metasomatic action, there develop abundant magnetite and a light 

 amphibole, nonpleochroic in thin section, which will be called griinerite. 

 There is thus produced a magnetite-griinerite-siderite-slate, intermediate 

 between the sideritic slates and tlie typical magnetite-griinerite-schists. It 



