Plate XXVI. — Ferruginous Cherts and Iron Carbonates from the 



Animikie Series. 



Fig. 1. Concretionary chert. Sjiecimen 10577, slide 4997. From north side Guntiint hike; ,\nimikie 

 series. In ordinary light, X 25. The complex concretionary structure so characteristic 

 of the ferruginous cherts is here heautifully shown. The concretions arc closely i>acked 

 together, the amount of material between them being relatively .small. This interstitial 

 substance is ehietly chert, but it contains numerous rhombohedra of siderite. The oxides 

 of iron are mostly limonite and hematite, but with them is minified some magnetite. The 

 arrangement of these iron oxides with reference to one another is usually somewhat irregu- 

 lar, but occasionally the magnetite and hematite are in alteruate layers. (See p. 264.) 



Fig. 2. The same, in jiohirized light, X 60. Another part of the same section is here shown, the 

 eulargeuient being greater. One of the concretions is seen to be compound; that is, the 

 larger belts of iron oxides inclose two smaller concretions.. Within the concretions the 

 quartz, like the iron oxide, is seen to have a banded arrangement. The liackground is 

 very finely crystalline, but the silica is mostly or wholly individualized. (See p. 264.) 



Fig. .S. Ferruginous eln'rt. Specimen 10.576, slide 5186. From the Guntiint beds. In ordinary 

 light, X 25. The .section illustrates the sharp alterations which sometimes occur between 

 line grained evenly laminated chcrty carbonate and ferruginous chert, with a well devel- 

 oped concretionary structure. The fine grained part is composed of exceedingly crystalline 

 and amorphous silica, and of siderite in minute rhombohedra. In the concretionary part 

 of the section no carbonate remains. AVithin the concretions the quartz is somewhat 

 coarsely crystallized. (See p. 264.) 



Fig. 4. Sideritic chert. Specimen 6138, slide 1173. Also from the Gundint beds. In i)olarized light, 

 X 25. The semiamorplious and chalcedonic phases of silica which are found in the fer- 

 ruginous rocks are nicely shown. The durUer ccdored part of the figure consists chiefly of 

 siderite and inm oxide, pseudomorphous after it. (See p. 264.) 



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