PETEOGKAPHICAL CHARACTER OF NEGAUNEE FORMATION. 361 



The anah'sc's on \)nge 33.S show that the rocks are composed mainly of 

 impure g-rihicrite, iiiag-netite, and ((uartz. The analyses indicate that the 

 amphibole is intermediate between griinorite and actinolite, but upon the 

 whole is nmch nearer the former than tlie latter. 



By oxidation of the iron carbonate the sideritic slates pass into the 

 ferruginous dates, the iron oxide being hematite or limonite, or both. 

 These rocks, in regularity of lamination and in structure, are similar to the 

 sideritic slates, differing from them mainly in tlie fact that the iron is present 

 in another combination (PI. XIX). In the different ledges may be seen 

 every possible stage of change from the sideritic slates to the ferruginous 

 slates. The only necessary change is a loss of carbon dioxide and per- 

 oxidation of tlie iron. In PI. XVII the beginning of the process is beauti- 

 fully shown. On weathered surfaces, along veins, and along some of 

 the bedding planes the transformation is complete. Between this trans- 

 formed material and the original rock there is complete gradation. 

 PI. XIX illustrates different ferruginous slates in which the siderite is 

 partly or wholly decomposed. 



1l\iq ferri(f/i>/0Hs cherts are rocks consisting mainly of alternating layers 

 of chert and iron oxide, although in the iron-oxide bands cliert is contained, 

 and also in the chert Ijands- iron oxide is found (Pis. XX-XXII). This 

 iron oxide is mainly hematite, but both limonite and magnetite are some- 

 times present. Rarely magnetite is the predominant oxide of iron (PI. 

 XXIII). In such cases the silica is usually coarsely crystallized. In the 

 field the ferruginous slates are found to grade step by step into the ferru- 

 ginous cherts, and it is manifest that they were produced from them by a 

 reari-angemeut of the iron oxide and silica, with a possible introduction of 

 extraneous silica and iron oxide. The rocks are folded in a complicated 

 fashion, as a result of wliich the layers present an extremely contorted 

 appearance. The folded layers frequently show minor faulting. On account 

 of the exceedingly brittle character of these rocks, they are very often 

 broken through and through, and soinetimes they pass into reibungs- 

 breccias. Sometimes the shearing of the fragments over one another has 

 been so severe as to produce a conglomeratic aspect. The ferruginous 

 cherts are particularly abundant at the middle and lower parts of the iron- 

 bearing formation, just above or in contact with the greenstone masses. 



