Plate XXIV. — Aotinolite Slates from the Iron-Bearing Member of the 

 Penokee Series and Cherty Iron Carbonates from the Vermilion 

 Series. 



Fig. L Actinolite-maguetite-scliist. Specimen 9.558, slide 3191. From Peuokee gap; NW. i of Sec. 

 14, T. 44 N., R. 3 W., "Wisconsin. In polarized light, x 2.5. Magnetite and quartz are the 

 most abundant constituents, but minute needles of aetiuolite are seen radiating from the 

 magnetite areas. The rock is a regularly banded one, and while this is not strongly marked 

 in the thin section its distinct lamination is apparent. The rhombic form of many of the 

 magnetite areas is seen. Those probably represent sections of octahedra. (See p. 218.) 



Fig. 2. Actinolitie slate. Specimen '.KiSO, slide 3167. From the SE. i of Sec. 20, T. 44 N., R. 5 \V., 

 Wisconsin. In pol.arized light, x 2.5. This is an actinolitie slate in which the con- 

 cretionary arrangeuumt of the various constituents is shown. The close a,ssociatiou of 

 aetiuolite aud magnetite is apparent, the minute aetiuolite needles frequently radiating 

 from the magnetite particles. Tlic .ai'tinolite is manifestly prior to the quartz in crystal- 

 lization, as the individuals nt the latter are penetrated in every direction. by the actincdite 

 needles. This figure is particularl,\- uoticeabh". on account of the very I'oarsely crystal- 

 line character of the ([uartzose b.ackgronnd. It runs between the concretions in such a 

 manner .as to make it evi<lent that it must have crystallized subsequent to their formation. 

 We have here, then, an illustration of the complete rearrangement of the silica originally 

 present and the inobable introduction of a good de.al of silica from an extraneous source. 

 (Seep. 216.) 



Fig. 3. Cherty iron carbonate. Specimen 8726, slide 3367. From Vermilion lake, Minnesota. In 

 (Trdinary light, X 25. This shows very finely the alternation of b.ands of chert and iron 

 carbonate. The darker belts are nearly pure siderite, and the lighter ones almost jiure 

 chert. The cherty belts are seen to contain minute rhombohedra of carbonate, and they 

 cnt across the carbonate belts in such a manner as to imply a rearrangement of silica 

 originally present and the introduction of additional silica, or botli. The figure is here 

 introduced in order fhat the cherty carbonates of other districts may be compared with 

 those from the Penokee series. (See p. 260.) 



Fig. 4. The same, in polarized light, X 50. It is seen fhat the cherty background is completely <Tys- 

 tallized; that is, contains no Amorphous material and is made up wholly of quartz. 

 The intricate manner in which the quartz anastomoses, cutting the iron carbonate, shows 

 conclusively that to some extent it is later than the siderite, while it has probably been 

 extfcnsivcly rearranged, (See p. 260.) 



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