452 THE MARQUETTE IKON-BEARING DISTRICT. 



produced; iu the second case the mica-schists and mica -gneisses were 

 formed. Between one extreme and the other there is every gradation. ' 



RELATIONS TO THE UNDERLYING FORMATION. 



It has ah-eady been said that the Michigamme formation grades slowly 

 down into the Goodrich quartzite or into the Bijiki schist, and that therefore 

 the line of separation between them is more or less arbitrary. The relations 

 to the Clarksburg formation are considered on pages 461-463. 



THICKNESS. 



The thickness of the formation must be considerable, as it covers a 

 wide area, Init it is impossible, on account of the subordinate folding to 

 which it has been subjected and the extensive development of slaty cleavage 

 or fissility which cuts across the bedding, to make even an approximately 

 accurate estimate. It is possible that within tlie area described its thickness 

 is not more than 1,000 to 2,000 feet, but it may be much more. 



INTERESTING LOCALITIES. 

 Spurr, Michigamme, and Champion area. Beginning at tllC UOrtllWCSt part of tllC 



area (Atlas Sheets V, VIII, and XII), about three-foui-ths of a mile south of 

 the Spurr mine are open pits of soft limonite iu the slate. Exposures occur 

 in the valley separating the two belts of the Bijiki schist along the north 

 side of Lake Michigamme at only one or two localities. These are on the 

 north side of East Point. Passing to the eastward, north of Champion, 

 in sees. 29, 30, 31, and 32, there are very numerous exposures of the less 

 altered kinds of the ]\Iichigamme formation. The larger and better exposed 

 area is a rough elevated plateau north of the Bijiki schist. The rocks of 

 the area comprise fine-grained, black, carbonaceous, graphitic, and pyri- 

 tiferous slates; coarser-grained slates of the same varieties; ordinary 

 black slates; fine-grained, dark-colored graywackes; coarse-grained gray- 

 wackes; occasionally rocks which approach a quartzite; and, at one place, 

 a conglomerate. Between the different varieties of rock there are all sorts 

 of gi-adations and interlamiuations. 



The conglomerate contains small pebbles of chert and quartz, and 

 larger pebbles of what appears to be dense black slate. On the exposed 

 surface these weather out, gi^^ng the rock a pitted appearance. The finer- 



