Plate XXIII. — Ferruginous Cherts and Actinolite Slates from the Iron- 



BEABiNG Member. 



Fig. 1. Ferruginous chert. Siieciineii 9081, slide 4206. From the SE. i of Sec. 24, T. 45 N., R. 1 W., 

 Wisconsin. In ordinary liglit, X 25. The iherty backgronnd contains areas vhich are in 

 part roughly oval or ronudisb, but are more largely exceedingly irregular. These areas 

 are composed of ijuartz and iron oxide, the latter being mostly hematite, the remainder 

 magnetite. The regular areas suggest concretions, the history of which has been given. 

 The irregular areas resemble fragments, but are probaljlj of chemical and dynamic origin and 

 have formed within the rock itself. Tiiey doubtless represent original iron carbimate areas. 

 This carbonate has hugely changed to oxide, but has also to some extent been leached out, 

 thus leaving cavities. After or before the completion of tliis process silica has entered and 

 tilled the cracks and cavities. The result of this oxidation, solution, and siliciiication, com- 

 bined with movement, has been to put in the place of well defined areas of iron carbonate 

 the exceedingly irregular forms presented by the figure. (See p. 223). 



Fig. 2. The same, in polarized liglit. The relations just mentioned are here again observed. It is 

 further seen that the background, instead of being finely spotty and j)erhaps i)artly amor- 

 phous, as in the |)revious plate, is comjdetcly crystalline. That there has been an exten- 

 sive rearrangement and entrance of silica, is shown, even more plainly than in the previous 

 figure. (See p. 223.) 



Fig. 3. Actinolitie schist. Specimen 955.5, slide 3190. From Peuokee gap; N\V. ^ of Sec. 14, T. 44 N., 

 R. 3 W., Wiscousiu. In ordiiiary light, X 25. A quartzose background contains very numer- 

 ous minute needles of actinolite and many particles of magnetite, the latter being roughly 

 concentrated into bands, one of which with a part of another is shown in the figure. The 

 figure represents a typical rock of this kind. (See pp. 218-219). 



Fig. 4. The same, in polarized light. The completely crystalline character of the (juartzose back- 

 ground appears, and the intricate manner in which this material is cut by needles of actino- 

 lite is perceived by comiiaring this figure with the preceding. (See pp. 218-219). 

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