37(5 THE MARQUETTE IRON-BEARINd DISTRICT. 



jasper layers are belts of specular micaceous liematite, the laminte of which 

 show slickeusides, indicating that readjustment has occurred between them. 

 The jaspilite varies upward into a banded rock consisting of alternate 

 layers of pure, white, finely crystalline quartz and dark bands composed 

 of hematite and magnetite (PI. XXIII). Intermediate layers show the 

 transition between the rocks having bands of white and of red quartz. 

 The grains of the jasper and white quartz belts are larger than those of the 

 ordinary varieties of jasper, and are to a large degree crystal-faced, as 

 shown by the innumerable reflecting facets when held in the sun. At the 

 top of the formation is a thin belt of ore, making up a part of the ore body 

 of the Michigamme mine. The remainder belongs with the Ishpeming for- 

 mation. The bands ( »f white chert and red j asper have frequently a lenticular 

 character. The rocks are often folded and fractured in a minor way. The 

 cracks are filled with secondary magnetite, and more rarely griinerite. In 

 some places the folding was so severe as to make genuine breccias. At one 

 place, a short distance east of the Spun- mine, the inter-Marquette erosion 

 cut away all of the jasper, and here the griinerite-magnetite-schist is 

 at the top of the formation. A minor fold here occurs, so that in a single 

 exposure the strike may be seen to vary from an east-west direction to a 

 northwest and finally to a north direction. North of the Spurr mine 

 minor corruoations are seen, which give local northern dips in the general 

 southward-dipping formation. 



In thin section much of the Michigamme and Spurr jaspilite shows a 

 concretionary arrangement of the iron oxide, many of the concretions being 

 made up of a large number of concentric rows of hematite and magnetite 

 particles. While much of the hematite is in small particles or areas in 

 these concretions, in many of them are large crystals, which look like later 

 infiltrations. The quartz is nuich more coai'sely crystallized than the quartz 

 of the formation in the main area about Ishpeming and Negaunee and to 

 the south and east of these towns, the average grains being fr&m 0.10 to 

 0.15 mm. in diameter. Each of these quartz grains contains a large number 

 of the smaller crystals and flecks of hematite. Also included in these 

 quartzes are numerous long, minute, curved needles of rutile. The grii- 

 nerite of the jaspers has usually a distinct pleoclu'oism, giving yellow and 



