230 THE PENOKEE IRON-BEAKING SERIES. 



altered areas of carbonate tliere is contained more or less of the finely divided silica 

 of the matrix, and in snch cases we appear to have an intermediate phase between the 

 unaltered carbonate and those cases above described as characterizing other sections 

 where simple rings of iron- oxide are within a siliceous baclcground. In other words, 

 the iron carbonate remaining after formation of rings of oxide has been more or less 

 removed and silica deposited in its place. As has been noted above, these concen- 

 tric areas of iron oxide and iron oxide and silica are found in every stage of forma- 

 tion until the iron carbonate disappears. (PI. xxvii, Pig. 4.) 



34. Ferruginous and sideritic chert, from a liigh horizon, interstratified with 33. 

 Specimen 7G'22 (slide 2072); from IGO N., 100 W., Sec. 21, T. 47 N., R. 47 W., Michigan. 

 The rock is from another layer of the ledge from which 33 comes. It has a most 

 striking and peculiar character, being grayish, niottled with black, and suggests even 

 to the naked eye a fragmental texture, blackish angular fragments being imbedded 

 in a grayish cherty mass. 



In tliin section, a minutely crystalline to amorphous silica fmins a groundmass 

 which occupies a relatively small portion of the whole area. Contained in this ground- 

 mass are small crystals and aggregates of crystals of siderite, oval or sjjhcrical 

 concretions of mingled flinty silica and iron oxides, i>articles of a blackish, probably 

 carbonaceous, material, with or without accompanying sich^rite, and also irrcgnhuly 

 outlined and even sharply angular areas or fragments composed of a mixture of Hinty 

 silica, iron oxide, and carbonaceous material with or without siderite. Tlie iron car- 

 bonate, except that it is more often fresh, appears as in 33. Where altered, its alter- 

 ations are the same as in tliose sections, the peculiar beautiful concretionary forms 

 there described occurring here also and are evidently of the same origin. The more 

 irregular outlines, and especially the angular fragment-like areas that appear in 

 this groundmass, are entirely similar in composition and structure 'o the material 

 wliich has been described as composing tlic bhu'kcr bands in 33. These are at times 

 plainly fragments, as may be seen from their angular outhnes, sliarp definifion from 

 the matrix of the rock and from the fact that the lamination lines abruptly terminate 

 at the extremities of the areas. The lines of lamination in these fragments, when 

 they are perceptible, are never parallel tor any two fragments. 



It seems evident that the history of the rock exposed where Nos. 9007, 9008, 9009 

 and 7622 were obtained has been about as follows: It was first a stratiform car- 

 bonate of iron, including apparently more or less carbonaceous matter, as such strati- 

 form carbonates so generally do, and perhaps more or less of silica. P>y a process of 

 subsequent silicification, accompanied by oxidation, the structures now apparent were 

 produced. At times tlie substitution of the silica and the solution and oxidation of 

 the carbonate went on so as not seriously to break u]) the continuity of the original 

 layers; but in other cases the rock liad l>e<'ome sliattered and the silica entered 

 into the minute cracks and interstices of tlie lock, and this shattering was 



