Plate XXIX. — Fkom the Iron Formation of the Animikie Series and 



FROM Lawrence County, Ohio. 



Fig. 1. Concretionary chert. Specimen 10.577, .slide 4907. From north side Gunflint lake; Animikie 

 series. In ordinary light, X 60. The photographs, Figs. 1 and 2, PI. xxvi, are repro- 

 ductions from the same section astliis figure. It is drawn, however, to show more exactly 

 the relations of the various oxides of iron to the cherty background. In tlie lirst phice 

 veins of (diert intricately intersect the oxides of iron in such a manner as to show that the 

 silica entered after the iron oxides had formed. The magnetite in the concretions is some- 

 times in nearly continuous bands, but in other parts is mingled intimately with the hem- 

 atite. If the large area takes the place of one of iron carbonate, at diti'erent times the 

 circumstances were favorable for the jiruduction of each of the iron oxides in preponderat- 

 ing quantity. (See pp. 265-266. ) 



Fig. 2. Actinolitic .slate. Siiecimen 7012, slide 2081. From the SW. i of Sec. 23, T. 6."i N., K. 4 W., 

 Minnesota; Animikie series. In ordinary light, x 25. The relations of the magnetite, 

 actinolite, and i|uartz are here nic(dy shown. The nnignetite is plainly the lirst mineral to 

 crystallize, fre((uently having crystal outlines, and being included within bcitli the Diher 

 minerals. The close association of actinoliie and magnetite is again illustrated, the twci 

 c-ombiued having usually roundish or oval forms. The silica cuts the section in such a 

 manner as to give it in places vein-like forms, which must have develoiied suliseipienUy 

 to the formation of the magnetite and actinolite. (See p. 266.) 



Fig. 3. Actinolitic and sideritic slate. .Spe<'imen 10.580, slide 51S9. From the Gnnllint beds; Animi- 

 kie series. In ordinary light, X 60. Tlie section illustrates a pha.se of tlu- iron fonnatlon 

 in which all of the minerals, (|unrtz, siderite, magetite, and actinolite are together. The 

 colorless i)art is the cherty background, which is completely, although tinely crystallized. 

 The siderite is represented by the vague rhombohedral areas, which include numerous 

 minute ])article& of gray and black material, presumably oxides of iron. This mineral is 

 the nio.st abundant (me in the section. The magnetite occurs as usual with its characteristic 

 crystal (Uitlines, and in places is included in such a maniuT in the siderite as to sug- 

 gest its formation from that mineral. The actinolite is scattered here and there in minute 

 blades and needles, being particularly abundant upon one side of the lignre. (See p. 265.) 



Fig. 4. Cherty inni carboimte. Specinuui 9814, slide 3880. From Lawrence county, Ohio. In ordi- 

 nary light, X 25. The i)liotogra]ihic Figs. 1 ami 2, PI. XXI, are from this section. The 

 beginning of the alteration of siderite to limonite and hematite, and also the beginning of 

 the formation of <'oncretions, are here better seen. The background is tinely crystalline 

 and ajiiorphous silica, the nature of which is better shown by the ligures referred to. This 

 cherty iron carbonate from Ohio is remarkably like that from the Penokee series shown 

 in PI. xxvii. Figs. 1 and 3, the (uily difference being that the alteration of the siderite is 

 of a ditierent character, in one ca.se black oxide of iron being produced, and in the other 

 red and luowu oxides. (See p. 247.) 

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