Plate XXII.— Feeruginous Cherts from the lEON-BEARiNa Member. 



Fig. 1. Coucretionary chert. Specimen 9048, slide 2886, From the SE. i Sec. 27, T. 46 N., R. 2 E., 

 Wisconsin. In ordinary light, X 25. In a cherty background are beautiful concretions, 

 wliich are composed of concentric rings of iron oxiile and chert. One concretion particu- 

 larly is very fine, showing many closely packed concentric rings. Silica is seen Ijreaking 

 across these rings In a lew places. (.See pp. 227-228.) 



Fig. 2. The same, in polarized light. Here the cherty background appears as a fine mosaic. The 

 quartz in and about the concretions is more coarsely crystalline than the average of that in 

 the matrix. One concretiim has as a nucleus comparatively closely crystalline (juartz. This 

 variation in the character of the silica is suggestive that the fine spotty silica is perhajis 

 original. The concietions, as shown by subsequent i>lates, have been produced from iron 

 carbonate, and as the iron oxide formed from the carl)Ouate, the remaining space was occu- 

 , pied by silica, which crystallized in larger particles than the supposed original silica. (See 

 pp. 227-228. ) 



Fig. 3. Brecciated chert. Specimen 7622, slide 2072. From the Montreal river, between Michigan 

 and Wisconsin. In ])olarized light, x 25. The background is again cherty, and contains 

 within it many small, rather perfect rhombohedia of siderite, which have altered to _a 

 greater or less degree to iron oxide. Contained in this groundmass are irregular areas 

 which do not have a concretionary structure, but ai>pear to be true fragments in this 

 matrix. One of the areas is severed in every direction by numerous ramifying veinlets of 

 silica. The same thing is to a less degree noticeable of others of these areas. That this 

 cutting silica is secondary can hardly be doubted. (See pp. 230-231.) 



Fig. 4. Ferruginous and brecciated chert. From the same locality as the last. In polarized light, 

 X 25. The background is cliert, as in the ])revious figure. In this background are fmind 

 perfect concretions and l)recciated areas. One of the latter shows plainly its fragmental 

 character. It is built up of lamina^ which are apiiroximately parallel and have evidently 

 been broken fidiii a n^gnlarly laminated rock and here deposited. In this figure arid the 

 • previous one we have clearly a mingling of fragmental and nonfragmental material, the 

 fragmental jjortion of which is ]iossibly derived from the immediately underlying iron- 

 bearing beds. (See pp. 230-231.) 



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