236 THE PENOKEE IRON-BEAKING SERIES. 



52. Black chert, from the summit of the Irnn-bearinn- member. Specimen 7534 

 (slide 19G8), from 500 N., 1075 W., 7535 (slide 1900), trom 535 N., 1050 W., Sec. 12, 

 T. 47 N., R. 4(> W., Michigan. 



The tliin sectious are concretionary, brecciated, and ferruginous cherts, analo- 

 gous to a number above described. Slide 1008 shows a predominating groundmass of 

 exceedingly finely crystalline and amorphous silica, in which are strewn small parti- 

 cles <if ([uartz which are certainly fragmental. The fragmental grains of quartz stand 

 out in the background in a wonderfully distinct way. They varj^ iVoni well rounded 

 to angular; some of them are distinctly enlarged. This section illustrates well the great 

 dittereuee in appearance between fragmental quartz and the nonfragmental quartz of 

 the iron formation. Here, as in previously described rocks, as an ujjper horizon, is a 

 mingling of chemical and mechanical sedimentation; the begiuning of the transition to 

 the upi)er fragmental mend)er of the series. There are also present magnetite, hema- 

 tite, and iron carbonate, all of which are arranged in a senucouci-etionary fashion, and 

 ill such a way as to suggest the derivation of the whole from an original carbonate. 

 Slide 1009 dift'ers from 1908 only in containing large areas of what seems to be a 

 secondary calcite. 



From the section on the Miner d; Wells property. 



53. Cherty iron carbonate, from near the base of the Iron-bearing member. 

 Specimens 12885 (slides 5507), 12880 (slide 5508); See. 13, T, 47 N., R. 46 W., Michigan. 



An aphanitic rock, showing a very thin and for the most part regular 

 lamination, though in certain layers these lamina* are somewhat bent. The laminae 

 range in tliickncss from that of a sheet of i)ai)er to as much as a ([uarter or half an 

 inch. Tliey range in cohtr from black through various shades of brownish gray and 

 greenish gray to a very light gray. All save the black bands show a very earthy, 

 compact look, and the whole appears at first sight as that of some banded or earthy 

 limestone; but the high specific gravity of the rock proves at once the presence of 

 nuicli iron. The surfaces of some of the black lamiiue glisten brightly as though con- 

 taining a carbonaceous or graphitic material. Composition of 12885: silica, 40-01; 

 titanic oxide, 0-12; ahimina, 0-83; iron sesquioxide, l-.'55; iron protoxide, 26'CO; nmn- 

 ganous oxide, 2-09; calcium oxide, 0-C3; magnesium oxide, 2-80; carbon dioxide, 

 17-72; i)hosi)horic acid, 0-07; iron sulphide, 0-11; water at red heat, 1-71=99-50. 



In the tliin sections the light colored bands ai'e seen to consist of an almost solid 

 aggregate of minute rhombohi'dra of iron carbonate, whose outlines are particularly 

 well observed on the borders of the bands wlune separated slightly from the rest ot 

 the nuxss by the silica, which, while constituting the main constituent of the darker 

 (■olored bands, penetrates the siderite in irregular tongues and seams. This silica is 

 exceedingly finely crystalline and i>erhai)s in i)art amorjilious. Mingled with it in the 

 <larker colored bands are films of cldorite, detached rhonibohedra of iron carbonate, 

 and dark colored scams lying i)arallel to the general lamination of the rock, but non- 



