THE IRON-BEAKIN(i MEMBElt. 235 



The rocks are dark red eberts or jaspers, wbicli for the most i)art lack, liowever, 

 the aphaiiitic tliiity app<'arance of true jas](er. They are banded ipiite rejiidarly with 

 Hgbter colored seams. 



The thill section .ilSS shows a backsrouiid of exceedingly tiuely crystalline, min- 

 gled probably with amorphous silica, through which are scattered a few small appar- 

 ently tVagmental particles of the same material and numerous minute stains of brown 

 hydrated iron oxide which occasionally present rhoniljic outlines. The thin section 

 3184 differs IVom •US.'i simply in containing much larger and even jiredominnting 

 quantities of iron oxide, much of which is hematite. Rhombic outlines to the parti- 

 cles of iron oxide are frerpiently to be seen. The iron oxide is aggregated especially 

 into certain bands, so as to give the section a laminated appearance. 



49. A ferruginous chert, from an upper middle horizon. Speiamen 9.">0.'} (slide 

 318.5), from 1775 N., 1925 W., Sec. 13, T. 47 N., R. 4G ,W., Mi(;higan. 



A light gray, evenly laminated earthy rock, banded with seams of dark red 

 hematite. 



The tliiu section is a ferruginous chert very closely resembling 3183 in 48. 



50. Iron carbonate or siderite rock, from an upper horizon. Siiecimens 9481 

 (slide 3177), 9482 (slide .3178), 9483 (slide 3348). All from 250 N., 1000 W., Sec. 12, T. 

 47 N., R. 40 W., Michigan. 



An aphanitic, diirk gray, earthy looking, stratiform rock, banded in some ]««■- 

 tions with nearly black bands, which are at times bi-oken and the detached portions 

 imbedded in a lighter colored material. Sji. gr., from 3-22 to 3-40. 



The thin sections are composed mainly of a felted mass of iron carbonate, which 

 in some of the darker bauds is mingled with a dark colored material in tine particles, 

 presumably of a carbonaceous nature. Certain bands, as seen in section .3177, 

 contain a finely divided silica which has a distorted lamination, the carbonate l)eing 

 sei^arated into more or less detached areas. At the same time these bands show 

 large sized patcbes of a greenish chlorite. The carbonate is also altered to a consider- 

 able extent to iron oxide, the latter being largely arranged in rings. This is another 

 good illustration of the formation of concretions. 



51. Ferruginous cliert, from an upper horizon. Specimen 9485 (slide 3179); 295 

 N., 980 W., Sec. 12, T. 47 N., R. 40 W., Michigan. 



A very dark colored, nearly black aphanitic; rock, banded irregularly witli ill 

 defined dark red seams. 



The; thin section is evidently from one of the dark reddish brown .seams. It 

 presents a confused aklniixture of chlorite. In-own iron oxide, and magnetit«, the latter 

 mineral Ijeing in distinctly outlined crystals, all in ;i minutely crystalline and amor- 

 phous siliceous background. Many of the brown particles also present rhombic out- 

 lines, and are taken to have arisen from an oxidation of the <'arbonate. Siderite is 

 not, liowever. recognizal)le in tlie section, but may be present plentifully in the dark 

 colored earthy looking bands mm nlioned ;ihove. 



