INTERESTING LOCALITIES OF NEGAUXEE FORMxVTION. 379 



no transition horizon. At other places there are minor interlaminations of 

 the two. The ferruginous cliert of the iron formation has a very regular 

 strike and dip, being remarkably free from the minor folding which is so 

 prominent in the iron formation to the southward. 



As examined in thin section, the ferruginous slates and cherts of this 

 locality differ from those of other places only in that the lower horizons 

 show a certain amount of fragmental quartz mingled with, or in layers 

 interbedded with, the nonfragmental material. This clastic quartz is often 

 enlarged. Also mica is occasionally seen. 



Negaunee-ishpeming area. — Soutli of tlie Tcal Lalvc raugc are uumerous exj)OS- 

 ures adjacent to the mines of Negaunee-Ishpeming and vicinity (Atlas 

 Sheets XXV and XXVIII). Here, as has been explained, the iron forma- 

 tion occupies the lower lands, usually those below the 1,400-foot contour 

 (Pis. XIII and XIV). The exposures are in a series of bay-like areas, which 

 open out to the Avest, but are surrounded and overto})ped to the north, east, 

 and south by amphitheaters of greenstone (PI. XIII). In these bays are 

 found some of the great mines of the area, such as the Cleveland Cliffs, 

 Lake Superior, Lake Angeline, and Salisbury. At or close to the contact 

 with the Groodrich quartzite tlie rock is always typical banded ore and jasper 

 or jaspilite (Pis. XVI, XXIV-XXVII), and at the lower horizons it is the 

 typical ferruginous chert (Pis. XX-XXXII). Between the two there are 

 often gradations, but often also they are separated bv a dike of altered 

 g-reenstone. Mining has shown that the masses of greenstone not only 

 border but underlie the bays of iron formation, being, however, deeper 

 below the surface in passing west, thus making westward-plunging basins 

 of greenstone in which the Negaunee formation material rests (PI. XIII). 

 At the bottoms of these basins are the great ore deposits of the district. 



Thus in this area are found the largest ore dej^osits and the most 

 numerous A'arieties of the ferruginous chert and jasper. The strike of the 

 formation is generally east and west, corresponding to the close north-south 

 folds; but as the folding is highly complex, this probably being in part due 

 to the intrusive greenstones, strikes in all directions may l)e found. The 

 ferruginons chert and jasper are most intricately crumpled, and are broken 

 and faulted in a minor way. The brilliant appearance of the crumpled 



