EELATJONS OF NEGAUNEE FORMATION TO EltUl'TlVES. 329 



XXXI). It is called tlu- ir»)u-l)eariny t'orimitiou because withiu it occur 

 many of the Man[uette imu-ore deposits. 



RELATIONS TO ERUPTIVES. 



Vast quantities of greenstone are associated with tlie iron-bearing 

 formation. This greenstone includes both intrusives and extrusives, the 

 former being much the more abundant. The intrusive rocks are diabases 

 and their altered equivalents. The most conspicuous of these intrusives 

 are in the form of bosses, varying from those of small size to those 2 

 miles or inore long and a half mile wide. The bosses are of exceed- 

 ingly irregular shapes, and from them radiate numerous dikes, varying 

 from small ones to those many feet in diameter. These dikes usually 

 do not outcro]), Init mining shows that they frequently connect one Vxtss 

 with another, and thus unite into one mass several apparently detached 

 areas of greenstone. In ni.any cases the greenstone intruded the sedi- 

 mentary series in a laccolitic fashion, so that the iron formation has a 

 quaquaversal dip about the greenstone masses (PI. XI). In some places 

 fragments of the Negaunee formation are included in the intrusives 

 (PI. XII.) In other places the greenstone breaks across the iron formation, 

 and at these the latter beds may dip against the gi-eenstone, although in 

 many cases the dip of Negaunee beds may be locally modified (figs. 17, 

 24, and 25). 



The intrusives particularly affect the iron formation, the bosses of this 

 rock found in the underlying and overlying formations being relatively few 

 and of small size. This is illustrated by the fact that a map including- 

 the greenstone areas about Ishpeming and Negaunee would approximately 

 cover the distribution of the iron-bearing formation. Large and abundant 

 masses of intrusives are also found in the central-eastern arm of the iron 

 formation, are very conspicuous in the masses of griinerite-magnetite-schist 

 constituting Slount Humboldt (fig. 27), and are abundant in the great out- 

 crops of iron formation at Republic and at Michigamme. At the latter place 

 fragments of the Negaunee formation are included within the intrusives 

 (PI. XII). While this general relation is very marked, the greenstone not 

 infrequently penetrates the superior formation (PI. XXX), and is also found 

 in the inferior formation. A possiljle explanation of this relation between 



