INTERESTING LOCALITIES OF THE SIAMO SLATE. 327 



When examined in thin section, the purest phases of the fernig-inons 

 chert are fonnd to contain a certain amount of phiinly fragmental cpiartz. 

 Siderite is also found. This suggests, as at the Nonpareil mine and 

 Siamo Hills, that this mineral was a partial soiirce, at least, of the ii-on 

 oxides. 



In the NW. ^ sec. 2 and in sec. 3, T. 47 X., R. 2(1 ^V. (Atlas Sheet 

 XXXTV), there are large exposures of typical slate and graywacke. These 

 show minor rolls and transition phases into the Negaunee formation. 



West half of T. 47 N., R. 26 w. — The most extcnsivc area of the exposures of the 

 formaticm occurs in sees. 4, 5, 8, and !l, T. 47 N., R. 26 W. (Atlas Sheet 

 XXXI). Here, as on the Siamo Hills, are nearly all j)hases of the 

 formation. On the irregular west side of the area the transition pliases 

 between the Siamo slate and the Negaunee formation are well shown. In 

 going east from the Buffalo mine embayment, one sees at various places in 

 the slates and graywackes unusually ferrug-inous jjhases and often fer- 

 ruginous chert. These are probably hear the top of the formation. As 

 examined in thin section, the rocks of this area are less modified than those 

 of the Siamo Hills area, and therefore show particularly Avell the feldspar 

 decomposition into chlorite, biotite, and muscovite, especially the first. 



About the center and north of the center of sec. 20 of the same town- 

 ship (Atlas Sheet XXXII) are other large exposures of the Sifimo formation. 

 These are chiefly of the feldspathic graywacke phases, which approach a 

 quartzite. On the west side of the exposures the interlaminations and 

 gradations between the Siamo slate and the Negaunee formation are par- 

 ticularly well seen. In some cases there are numerous interlaminations of 

 the fragmental and nonfragmental rock before the pure iron formation 

 material is I'eached. The relations are still further complicated b}" the fact 

 that both the graywacke and the jasper are rolled into a series of minor 

 isoclinal, westward-dipping folds, and these subordinate folds are not simple 

 but are each composed of several anticlines and synclines of the third 

 order. Moreover, the axes of these folds are inclined. Thus, the apj^arent 

 number of interstratifi cations of gray wacke and jasper is far greater than is 

 really the case, as the same beds reappear at the surface several times. In 

 places the folding went to such an extreme as to brecciate the rocks, so that 



