Plate XVII.— BANDED, CHERTY SIDERITE. 



Fig. 1. Clierty siderite from sec. 19, T. il N., R. 27 W. (Atlas Sheet XXVI). This is one of the purest 

 cherty siderites found in the Marquette district. The gray material consists almost wholly 

 of very finely crystalline and opaline silica and of siderite. The hluish-gray layers contain 

 some silica, the greenish layers some siderite. On the weathered surface the siderite is 

 entirely decomposed and in place of it is hematite and limonite. The beginning of the 

 same kind of alteration may be seen to affect some of the .siderite belts quite to the center 

 of the specimen. As examined in thin section the secondary limonite is found to be in 

 pseudomorplious areas after the siderite. Between the unaltered siderite and that which is 

 completely decomiiosed there is every gradation, different granules showing all stages of 

 the trausformatiou. Natural size. 



Fig. 2. Cherty siderite from the Peuoliee district, sec. 13, T. 47 N., R. 46 W. (See PI. XXI, fig. 4, 

 Mon. IT. S. Geol. Survey, Vol. XIX.) The original cherty siderite of the Peuokee district 

 is represented perfectly by the grayish-green material. Its very clo.se similarity to that of 

 the Marquette siderite represented in the previous figure is noticeable. The beginning 

 of the transformation of the siderite to limonite and hematite is beautifully shown. The 

 transitions between the two are clearer than in the previous figure. The processes of 

 change begin along the bedding planes and along intersecting veins. These two together 

 raalce two sets of nearly right-augle planes, which doubtless are shearing planes. The 

 veins are entirely filled with limonite and hematite, and therefore are minute layers of ore. 

 The changes along the bedding illustrate the beginning of the process which results in the 

 formation of the iron-ore deposits. It is noticeable that, as a result of the alterations, 

 the original banding of the rock is emphasized, although the emphasizing bands are not 

 so regular as the original sedimentary laminoe. This emphasizing of the original banding 

 of the iron-bearing rocks by metasomatic changes is a general law for the iron formations of 

 the entile Lake Superior region. Natural size. 

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