THE IRON-BEARING MEMBER. 223 



none of the concretionary appearance about them that characterizes the apparent frag- 

 ments of 17. Bxatniiied with a liijilicr power in tlic pnhirized light, some of these 

 areas are found to be made u]) mainly of quartz, whicli for the most part is quite like 

 that of the interspaces, tiiougli tlic latter is occasionally coarser than is seen within the 

 apparent fragments. The black |)arfich^s alluded to ap))car to be wholly iron oxide 

 and in the main magnetite, which iiiiiicral, however, oc(Mirs also in (|iiite large sized 

 crystals, arranged in bands, as already stated, bordering the api»arent fragments, but 

 also sometimes scattered through them, and again in clusters within the interstitial 

 quartz. (PI. xxviii, Fig 2.) 



From the Pcnoker rnnge in Sec. 3i, T. 45 N., R. J W., Wiscomin. 



19. Magnetitic quartz-schist. Specimen 9081 (slide 4:i06); from 140 N., W., 

 Sec. 24, T. 4.5 X., R. 1 W.. Wisconsin. 



The rock is a very highly nnignetitlc, fine grained, dark gray schist, in which 

 the niagnitier reveals only quartz and magnetite, certain irreguiar bands being much 

 more highly magnetitic than the rest of the rock. 



The thin section of this rock is very closely like those of the Tylers fork rocks, 

 particularly 17, presenting the singular concretionary and apparent fr.igmental 

 appearance. In the case of the present rock there is mort; of the red and brown 

 oxides of iron than of the magnetite. In many places the iron carbonate is found in 

 the concretions. Here at times the iron carbonate extends beyond the outlines of the 

 concretions and ap|»arent fragmenfal areas in such a manner as to make it jirobable 

 that the iron carbonate was the original mati-rial, and that the miiieials contained 

 in it — iron oxide, actinolite, and f[uartz, making up concretionary and a])parently 

 fragmental areas — -have formed from in part and re])laced in part the original iron 

 carbonate. In places the rh«nubic shajics of the brown and led iron oxides ni;ike it 

 evident that these are the direct restilts of oxidationof siderite individuals. ( IM. XXlii, 

 Figs. 1 and 2.) 



From the Potato rirer section. 



20. Magnetitic quartz-schist. Specimen 9104 (slide 4214); from l(t50 X., 250 

 W., Sec. 19, T. 45 X., R. I E., Wiscbnsin. 



A fine grained, nearly white, cherty rock, dotted willi magnetite jcntieles. and 

 irregularly banded with thin seams of the same mineral. Sp. gr., 3'21. 



The thin section of this rock resembles those of the actinolitic anci magnetitic 

 quartz-schists of Penokec gaj) (11 and 12), particularly those in which there is the 

 minimum of parallelism in arrangement of the several ingredients. Ihe arrangement 

 of the quartz individiuds and the bunchiiu-ss of the magnetite and ai-tinolite suggest 

 a vague concretionary arrangement. The actinolite is scattered about through the 



