THE IRON-BEARING MEMBER. 237 



coutinuoiis. Tliese cousi.st. mainly of cliloiitf, but may in'obably also iiicliide soint- 

 carbonaceous material. Similar films appear also in the light colored band.s. The 

 sei^tion is cut by small veins numing in various directions, which are filled as often 

 with siderite as quartz, and sometimes a single vein contains both minerals; also 

 occasionally chlorite is contained. 



5i. Cherty iron carbonates, from middle horizons. Specimens 9473 (slide 3135), 

 9473 (slide 3081), 9-474 (slide 3082); 9475 (slide 3083), 9476 (slide 3084), 9477 (slide 

 30S.>); all from 1400 N., 180 W.; 9479 (slide 317.")), 9480 (slide 3176) ; both from 1.31'.^) 

 N., 180 W.; also 7548 (slide 2061), 7549 (.slide 2062); from 1450 to 1500 N., 15(» W. 

 All in Sec. 13, T. 47 N., R. 46 W., Michigan. 



These specimens represent a large precipitous exposure of a ferruginous slaty 

 rock in the bed and on the south side of the outlet of Sunday lake. In fhe main 

 the exposures show a dark colored platy look with an earthy fracture and the general 

 appearance of an earthy carbonate. Interbedded with these earthy i^ortious, which 

 are themselves finely laminated, are nearly black flinty seams. These again show a 

 fine banding of lighter and dai'ker shades. In certain portions there are very often 

 black seams, which upon their surfaces show a graphitic luster. The exposure is in 

 general heavily stained with red and brown iron oxides, the fresh fractures showing 

 these oxides often arranged along certain of the lamina?, which they have at times 

 entirely replaced, but in other cases along irregular cracks. Some of these streaks 

 of iron oxides reach as much as a foot in width, in which case they form a moderately 

 rich hematite iron ore with a porous texture and slight metallic luster. Some more 

 minute cracks have been filled with brilliantly lustei-ed specular iron. The sp. gr. of 

 the more compact and less siliceous portions varies from 3 to 3-50. The chemical 

 composition of 9472, which is little altered and includes a number of minute silicified 

 bands, is: silica, 28-86; titanic oxide, 0*20; alumina, 1*29; iron sesquioxide, 1"01; iron 

 protoxide, 37*37; manganous oxide, 0'97; calcium oxide, 0-74; magnesium oxide, 

 3-(i4; water, 0-68; carbon dioxide, 25-21; phosphoric acid, trace; organic matter, 

 undetermined = 99-97. 



The thin sections from these specimens show a rock closely resembling tliat of 

 43. The sections differ from one another only in relative proportions of minerals 

 contained, and in that some of them contain a litfle chlorite. The relation of the 

 .silica, iron carbonati', and of the blackish sui)po.sedly carbonaceous seams are all as 

 in 43. Some of the sections include portions of the hennitite seams, in which case one 

 side of each section shows the unaltered carbonat^^ rhombolicdra, the middle of the 

 section showing these rhombohedra partly changed to hematite, while on the other 

 side of the slide they are completely replaced by the hematite. (PI. xxi. Fig. 4.) 



From the exposureii hi Sees. 7 and IS, T. 17 N., K. lo W., Michigan. 



55. (!herty iron carbonate, from a low horizon. Specimen 12543 (.slide .5.3.36); 

 from 1600 N., 1075 \V., Sec. 18, T. 47 N., R. 45 W., Michigan. 



