Plate XXI.— Sideritic Rocks, from the Iron-Bearing Member and from 



Lawrence County, Ohio. 



Fig. 1. Sideritic chert. Specimen 9814, slide 3880. "Limestone ore." Lawrence county, Ohio. 

 In ordinary lijllit, X 2.5. The .section has a cherty background, which contains large oval 

 and roughly rhonibohedral areas of iron ciirhonate. It is introduced liere in order to coin- 

 jiare this carbimate with those of the Penokee and Auimikie districts. 



Fill. 2. The same, in jxilarized light. The tine mosaic character of the <'herty background is here 

 exhibited. The outlines of the carlxuiate areas become less distinct than in the previous 

 ligure. 



Fig. 3. Sideritic slate. Specimen 9191, slide 29S6. From the NE. i Sec. 6, T. 45 N., R. 2 E., Wis- 

 consin. In ordinary light, X 60. The section is cut transversely to the lamination of 

 the rock. The hand s])eciineu is very thinly and regularly laminated. The laminae are 

 seen to be somewhat irregular. The obscure dark and liglit portions are both iron car- 

 bonate, which differ chieHy in the inclusions which tliey contain. The minute white 

 particles are chert and amorphous silica. (See p. 237.) 



Fig. 4. Sideritic and ferruginous chert. Specimen 9474, slide 3082. From Sec. 13, T. 47 N., R. 46 

 W., Michigan. In ordinary light, x 60. The section shows the alteration of iron carbon- 

 ate into iron oxide. The b.ackground is chert. Upon one si<le of the tigure are rhombo- 

 hedra of little altered siderite. Upon the other are black p.seudoniorphou8 areas which 

 are composed of somewhat hydrated hematite. Between the two are gradation phases. 

 (See p. 237.) 

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