238 THE PENOKEE IRON-BEAEmG SERIES. 



A rock closely similar to 1288G in 53. Sp.gr., 3*29. Composition: silica, 3(v73; 

 titanic oxide, 0'19; alumina, ()'38; iron sesquioxide, 0-98; iron protoxide, .■>4-74; man 

 ganous oxide, 0-52; calcium oxide, 0--18; magnesium oxide, 2-74; carbon dioxide, 

 22-44; phosphoric acid, 0-009; iron sulphide, 042; water at 105°, (••12; water above 

 1-05O, 1-40 = 100-84. 



The thin section sliows bands of an almost compact siderite with very sliglit 

 admixture of silica, which very gradually grade into and alternate with others com 

 posed mainly of a very minutely crystalline and amorphous silica. In tlie latter are 

 included numerous rhombohedra of iron carbonate. The difterences noted between 

 this rock and 53 lie in the somewhat larger size of the siderite rhombohedra. the 

 presence of only a small proportion of chlorite, and in exhibiting the transition 

 between siderite and iron oxide. The section is cut by veins, which are tilled with 

 quartz, siderite, and actinolite. The quartz and siderite are in larger individuals here 

 than in the remainder of the section. 



56. Clierty iron carbonates, from middle horizons. Specimens 7520 (slide 1962), 

 from N., 1730 W.; 7527 (slide 2297), from N., 1000 W.; 7.528 (slide 2059), from N., 

 735 W., Sec. 7, T. 47 N., K. 45 W., Michigan. 



The thin sections all contain a considerable quantity of iron carbonate. In 

 slide 2059 it is the chief constituent, in 1962 it about equals in (piautity the flinty 

 background, while in 2297 it is subordinate in (juantity. Slide 1962 is essentially like 

 .53. The alteration of the iron carbonate in 2059 has resulted in the formation of 

 al)undant hematite and numerous small crystals of magnetite. In 2297 the alteration 

 of the carbonate and the introduction of silica has resulted in forming numerous 

 beautiful concretions, most of which contain several concentric belts of iron oxide. 

 In quite a large number of these concretions iron carbonate varies in (luautity from 

 little to a chief constituent. 



57. Ferruginous chert and jasper, from a middle horizon. Specimen 12665 

 (slide 5389), from 200 N., 1475 W., Sec. 7, T. 47 N., K. 45 W., Michigan. 



A flue grained, dark purplish rock carrying vaguely outlined bright red spots 

 which have the appearam-.e of jasper. 



The thin section is one of the brecciated mixtures of chert and silica, diftering 

 from a number above described only in having the iron oxide mainly bright red 

 hematite instead of magnetite or the brown oxide. A greeuisli chloritic material is 

 lu'csent in a number of the concretionary areas. The silica ranges from very finely 

 crystalline to amorphous. Remnants of iron carbonate are a])i)arent in some of the 

 concretionary areas. The very large proi)ortion of iron peroxide present in this 

 section disguises its structure sonu^what, but enough is jierceiitible to make it evident 

 that we have here to do with a phase of one of the brecciated rocks. 



58. Magnetic cherty iron carbonates, from high horizons. Specimens 7530 

 (slide 1964), from 84(» N., 500 W.; 7524 (slide 2290), from 700 N., 100 W., See. 7, T. 47 

 N., R. 45 W., Michigan. 



