EXPOSURES OF THE NEGAUNEE FORMATION. 331 



a considerable width in sees. 20 and 21, T. 47 X., H. 27 W., hnt farther 

 west, as a consequence of the inter-Marquette erosion, it occupies but a 

 naiTOW zone until Humboldt is reached, and it is not even certain that for 

 a part of this distance the entire formation is not cut out. However this 

 may be, in sec. 18, T. 47 N., R. 28 W., the formation reappears with a 

 considerable width, and has a breadth of half a mile south of Humboldt. 

 West of Humljoldt for some distance the formation ma}^ be entirely cut 

 off l)y the Upper Marquette transgression, but exposures reappear at 

 Champion. From Champion to the eastern side of the Republic tongue 

 the formation is cut out. At the soiitheast end of the Republic tongue it 

 swino's to the south, west, and northwest, to the western side of the trough, 

 being again cut out at intervals. Thus in the Republic tongue the two 

 belts are In a syncline which is independent of the main Marquette area. 

 West of Republic is another similar tongue. 



From the main area the northern belt extends west from Ishpeming, 

 with frequent exposures, to sec. G, T. 47 N., R. 27 W. West of this place 

 it is known only l)y occasional outcrops to near Michigamme. At ^Miclii- 

 gamme and Spurr the iron formation has a considerable width, and from 

 the latter place it extends to the west for an undetermined distance. 

 It is wholly possible that in the area between Michigamme and see. 4, 

 T. 47 N., R. 28 W., the Upper Marquette transgression entirely cut out the 

 Negaunee formation for a greater or less part of the distance, l)ut in the 

 absence of evidence of this it is mapped as continuous. 



As has been seen, throughout much of the extent of the Negaunee 

 formation there are abundant masses of intrusives, and these, rather than 

 the iron-bearing formation itself, usually give the prominent topographic 

 features. In the liroad Ishpemiug-Negaunee area this is particularly 

 the case, nearly all of the bluffs being composed of greenstone, the iron 

 formation occupying the valleys between the numerous greenstone knobs 

 and ridges (Pis. XIII and XIV). For much of this part of the district 

 the 1,400-foot contour is approximately the boundary line between the 

 greenstone and the iron formation. However, where the Negaunee forma- 

 tion is a jasper or a grunerite-maguetite-schist, it is likely to be liard and 

 resistant, and so to make important topographic features. Large outcrops 



