Plate XXV.— Sideritic Slates from the Animikie Series. 



Fir,. 1. Cherty iron carbouate. .Speciiueu 1U172, slide 3422. From Dawson's road, Port Arthur, 

 Canada; Animikie series. In ordin:iry light, X fiO. The figure well illustrates the 

 appearance of one of the richer Animikie cherty carhonates. Throughout most of tlic sec- 

 tion iron carbonate is the cliief constituent, and the manner in which the individu.ils are 

 packed together is well shown ; when surrounded by chert they show ])ertect rliomliohedral 

 forms. Alteration of iron carbonate to ir(m oxide has begun. (8ee i>. 2t!4.) 



Fig. 2. Sideritic chert. Specimen tilSS, slide 1173. From north shore of Gunflint lake, T. fi.5 N., K. 3 

 W., Minnesota; Animikie series. In ordinary light, x 25. The Hgure illustrates tlu' for- 

 mation of iron oxides, pseudomorplious after siderite. A background of chert contains 

 numerous small roundish and rhombolicdral areas of siderite and iron oxide. Between tlie 

 little altered and wliolly .altered siderite a complete gradation is seen. (See p. 264.) 



Fio. 3. Actinolite-siderite-slafe. Specimen 10579, slide 5188. From the east side of north arm of 

 Gunriint lake, Minnesota; Animikie series. In ordinary light, X 25. This section is 

 from one of the typical Animikie slates. The thinly laminated character of the roek is 

 well shown. Its background consists in about equal proportions of actinolite and siderite, 

 mingled with a little chert. The dark colored material comprises all three of the oxides of 

 iron. (See p. 263.) 



Fig. 4. The same, in pol.arized light, X 25. The figure is from another part of the section, wliicli 

 shows the termination of a thick layer of nearly pure chert. Such lozenge-shaped eberly 

 areas within the Animikie shites are very common and are frequently of Large size. In the 

 cherty background are seen rhomliie outlines of siderite. The light colored border between 

 the slaty laminje and the chert is mostly actinolite. This relation is suggestive that the 

 actinolite has resulted I'.oni a reaction between the silica ami siderite. The lamina' of the 

 slate are seen to curve about the chert area. A short distance fnmi this nodule the 1am- 

 in« are parallel, as repre.sented in the previous figure. (See pp. 263, 265.) 

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