THE IltON-BEARING MEMBER, 227 



l»articiilar iuteiest as bearinjj;- upon the question as to whether such an excessively 

 fine grained silica as that seen here is of the same origin with the more completely 

 crystalline material more generally characteristic of the various ferruginous schists. 

 The appearance of tliis section would seem to make it very probable that some of 

 the tine grained silica is of a secondary origin and not in the nature of an original 

 ileposition. 



-8. Magiietitic siderite, from a lower middle horizon. Specimen 9191 (slide 

 299r)); from 1850 N., 325 W., Sec. 6, T. 15 X., K. 2 E., Wisconsin. 



A dark gray, aplianitic, very heavy rock, possessed of a very thin lamination, 

 tlie laminai being alternately light gray and dai'k gray. Some of the darker c^ilored 

 lamina' contain enough magnetite to make them attractable by the magnet, but the 

 liigli sijecific gravity is plainly uot wholly due to the presence of this magnetite in the 

 darker colored bands. Sp. gr., 3*50. Composition: Silica, 15-62 ; alumina, 4'27 ; ferric 

 oxide, 8-14; ferrous oxide, 32'85; niaiiganous oxide, 5-06; calcium oxide, 0-81; magne- 

 sium oxide, 2-66; carbon dioxide, 30-32; water, 0-68 = 100-11. 



The thin section is from the predominating grayish part of the rock — none of 

 the black magnetitic bands being represented in it — which seems to be composed 

 almost entirely of a gray felted mass of siderite individuals. These are known to be 

 siderite from analysis. Mingled with this siderite is a small proportion of an exceed- 

 ingly minutely crystalline and even amorphous silica, and probably clay; and scat- 

 tered sparsely through it are minute crystals and irregularly outlined particles of 

 magnetite. Both in the hand specimen and in the thin section the rock presents 

 every appearance of being in an unaltered original condition. (PI. xxi, Fig. 3.) 



From the section on the West Brmich of the Montreal river. 



29. Ferruginous chert or Hint, from a low horizon. Specimen 9018 (slide 2886); 

 from 100 N., 1165 W., Sec. 27, T. 16 N., li. 2 E., Wisconsin. 



The rock is a compact chert or flint of a dark brown color, mottled irregularly 

 with white spots. Sp. gr. 2-65. 



The thin section is made u]) mainly of a silica, which ranges from very minutely 

 crystalline through a chalcedonic phase to an entirely amorphous material. Staining 

 this background of silica, in irregular patches and ijarticularly in strongly marked 

 ccmcretionary areas, is a pale broAvn hydrated oxide. The concretionary areas are 

 most striking, being composed of concentric bands of the iron oxide, which lie in 

 ap])earance as mere stains in the siliceous background. These concretionary areas 

 are closely similar to those described above as characterizing the thin sections of Ij 17, 

 21, 22, 24, 26, with this diflerence, however, that instead of a black magnetite or a 

 nearly oparpxe hematite, the iron oxide concerned is now a pale brown hydrated kind. 

 Whatever may be the origin of these peculiar concretionary areas, it is certainly the 

 same in this case as in the case of the other rocks referred to. The siliceous back- 



