INTERESTING LOCALITIES OF THE SI A MO SLATE. 325 



and graywacke, both furrugiiious and nonferrug-inous, from the finest- 

 grained jjhases to coarse rocks whicli approach a (juartzite. The uppermost 

 horizons, by interhimination or gradation, pass into or are overlain by the 

 typical rocks of the Negaunee formation (PL X, fig. 2). The 8iamo forma- 

 tion here constitutes the foot-wall of the iron-ore deposits. In some places 

 it is a ferruginous quartzite, but at most places it is a slate, the alternate 

 beds of which are ferruginous and nonferruginous. The.se have a southern 

 dip at an angle of aljout 45°. These beds are cut at many ijlaces by a cleav- 

 age which at various points passes into fissility, and which dips at a steeper 

 angle to the south, and hence cuts diagonally across the bedding. It 

 appears probable, therefore, that the ferruginous layers were originally of a 

 different character from the nonferruginous ones. The central parts of the 

 formation show that the slate is folded in a series of minor rolls. There 

 is a uniform secondary structure, with a high dip to the south, which 

 corresponds in a general way to one set of legs of the series of folds. 



As examined in thin section, the basal ferruginous chert is again found 

 to contain a great deal of fragmental quartz, and also a large amount of 

 siderite, out of Avhich the hematite plainly developed. Many of the larger 

 areas of sidei"ite are surrounded by zones of hematite. The hematite 

 decreases in amount, and the siderite increases, in passing- inward. In 

 other cases through the siderite everywhere are crystals of hematite and 

 magnetite. In the upper part of the formation the Negaunee ferruginous 

 chert in some cases appears somewhat suddenly upon the ferruginous Siamo 

 slate; and in other cases there are interlaminations of the two, and it is 

 apparent in these latter that the ferruginous chert is secondary material, 

 as it does not follow the fragmental layers closely, but cuts across them 

 minutely and irregularly. One of the slides from the central part of the 

 belt shows beautifully the development of the finer-grained rock into a 

 mica-slate, the fissility of which is very uniformly parallel, and which comes 

 abruptly against a graywacke layer at an angle of about 30°. Here the 

 planes of fissility die out or become extremely irregular, but reappear 

 upon the other side of the naiTow graywacke band. In some of the 

 graywacke belts irregular fragments of slate are found which are })lainly 



