Plate XXVIT. — Formation of Concretions in Iron-Bearing Member. 



Fie. 1. Sideritic chert. Specimen 12508, slide 5522. From See. 16, T. 47 N., R. 46 W., Micliigan. In 

 ordinary light, x 25. A cherty background contains numerous roundish and rbomUo- 

 hedral areas of siderite. This siderite has begun to alter to oi)a(|ue black oxide of iron. 

 In places in the section this alteration has gone far, and roundish forms are produced 

 which are imitative of the shape taken by the original carbonate, although to a ronsiderable 

 extent this carlxmate has been replaced in its alteration by silica. (See p. 232.) 



Fig. 2. Sideritic chert. Specimen 7622, slide 2072. From the Montreal river, between Wisconsin 

 and Michigan. In ordinary light, X 25. A background of chert (contains numerous 

 rhombohedra of siderite of greatly varying sizes. The siderite has altered to a consider- 

 able extent to oxide of iron, and illustrates the formation of concretions as in the previous 

 figire, but the stage of growth is more advanced. In one case a nearly solid oval area of 

 oxide of iron has been produced by the alteration of one of siderite. In several other 

 areas the oxide of iron forms a ring about the siderite inclosed. In still others the oxide 

 of iron, while it is scattered somewhat irregularly through the areas, as a whole retains the 

 form of the original siderite areas, although tliesjuice ouce occupied by the siderite is taken 

 in part by silica. In the large complex coucreliiui the series of alteration thus illustrated 

 has been comj)leted. It contains at present no siderite, but consists of a series of concen- 

 tric rings of iron oxide which have a siliceous l)ackground. This coucretiou is cut bv a 

 vein of silica, as are also three other areas on one sidc^ of the figure. These veinlets have 

 clearly formed after the development of the concretiojiary areas. (See p)). .30-231). 



Fig. 3. Another part of the same se<tion, X 25. The figure again illustrates the formation of con- 

 cretions of iron oxide from original siderite areas. This concretionary arrangement of 

 the iron oxide is shown in nearly all of the siderite individuals, hut is shown in a particu- 

 larly fine m.anner by the large area in the middle of the figure. Enough siderite remains 

 so that its rhombcdjedral cleavage is nicely shown. Upon the outer part of the area is a 

 tolerably continuous ring of black oxide of iron, beyond which areothi'r im])erfect larger 

 rings of red an<l black oxides of iron. The alteration in tbi- interior of the area has to 

 some extent followed the cleavage lines, an<l we thus liave an explanatiiui of the irregular 

 forms in the iuteriius of the comretious of tlie ])revi<)us figures. It is to be further noted 

 that the forms of the rings in the concretions <lo not conform to those of the original sider- 

 ite area, but form regular ovals. (See pp. 230-231.) 



Fig. 4. Ferruginous chert. Si>e(^imen 9009, slide 2765. From the same <'X))osure as Figs. 2 and 3. In 

 ordinary light, X 25. This figure illustrates the result of an almost complete oxidation 

 of the siderite areas, only a trace of that mineral reuiaiuiug. The whole space that it 

 once pcuupied is taken by ri-ddish brown hematite and a small amount of silica. The 

 greenish areas are prol>aldy ililorite. (See pp. 228-230.) 



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