572 THE MAKQUETTE IROX-BEAEING DISTRICT. 



Marquette series, but occur within all the formations of the district. A 

 few dikes are later than any of the Marquette sedimentary rocks. The 

 fact that the intrusives are of far greater abundance in the broken and 

 fractured Negaunee formation than in the other formations suggests that 

 the cracks and crevices here produced by the folding gave avenues of 

 access which were taken advantage of by the igneous rocks to wedge them- 

 selves in between the rocks of the iron-bearing- members, to force them 

 aside, and thus to form great dikes and bosses of igneous material. Often- 

 times they break directly across the bedding (fig. 25); sometimes they 

 produce a subordinate folding (fig. 18); but even in this latter case the 

 material usually breaks across the bedding to a greater or less degree. 

 In many instances there is a quaquaversal arrangement of the formations 

 about the intrusive igneous masses, which suggests that the igneous material 

 has been intruded along the bedding of the formation, thus forming essen- 

 tially laccolites or batholites. At Michigamme and Humboldt the Siamo 

 slate and the griinerite-magnetite-schist may be seen doming' some of the 

 smaller of the laccolites. (PI. XI, and figs. 24 and 25.) Subsequent erosion 

 has removed the capping iron formation from many of these larg-er domes 

 and left the greenstone masses in the forms of bosses, the iron formation 

 dipping away from them upon all sides, just as do the sedimentary forma- 

 tions from the Henry Mountain laccolites. The major portions of the 

 gi-eenstones were once diabases, but are now epidiabases. The rather fresh 

 diabase dikes in the disti'ict may be contemporaneous with the igneous 

 I'ocks of the Keweenawan period. 



DEXfDATIOX. 



From the foregoing paragi'aphs it is e^-ident that the rocks of the 

 Marquette district were folded into mountain masses. The highest parts of 

 the mountains were probably near the great north-south anticline through 

 Marquette, and the mass next in importance was probably at the western 

 anticline at Lake Michigamme. These major heights must have been 

 connected by numerous cross ridges, con-esponding to the close east-west 

 folds. Dm-ing- and subsequent to the fokhng these mountains were cut 

 down to an approximate plain, so that the district is at the present time 



