328 THE MARQUETTE IRON-BEARING DISTRICT. 



great blocks of the jasper were broken off and are contained in the gray- 

 wacke. At one phice minor faulting occurred, and as a result of this the 

 lamination of the jasper abuts directly against that of the graywacke. 

 These phenomena at first sight led to the belief that there is here an 

 unconformable contact between the jasper and the graywacke, and that the 

 whole series is overturned; but a closer study showed this conclusion to be 

 erroneous, and that the phenomena are explained, as above, by interstratifi- 

 cation of the graywacke and jasper, by the isoclinal folding, brecciation, 

 and minor faulting. As would be exj^ected, there results an extremely 

 irregular boundary line between the two formations, which is placed so 

 that upon the whole the jasper is predominant upon one side and the 

 quartzite upon the other. 



The last important set of exposures of the Siamo formation is in sees. 

 19 and 30 of the same atlas sheet. Here the rock is a coarse graywacke, 

 which at times contains very large grains of quartz and feldspar, becoming 

 almost conglomeratic in narrow belts. These are, however, minutely inter- 

 stratified with finer material. In the northeastern part of the area, near 

 the top of the formation, the usual intevlaminations of the graywacke and 

 iron-bearing formation occur. 



As examined in thin section, these rocks again beautifully exhibit all 

 stages of the alteration of the fragmental feldspar into biotite, chlorite, 

 sericite, and quartz. In many cases in an intermediate stage we have these 

 different minerals interlocking, and residual feldspar also present. Such a 

 section, viewed witli a high power, looks like a crystalline schist, but with 

 a lower power the fragmental character of the whole is at once apparent, 

 and so many transition phases are found between such areas and those in 

 which the feldspar is but slightly affected as to show conclusively that the 

 complex areas were originally feldspar. 



SECTION VI.— THE IfEGAITNEE FORMATION. 



The Lower Marquette iron-bearing formation is given the distinctive 

 name Negaunee because in the town of that name and to the southward 

 are extensive typical exposures of the formation (Atlas Sheets XXVIII and 



