490 THE MARQUETTE IRON-BEAEmG DISTRICT. 



and ores, sometimes being parallel to the bedding of" these rocks and some- 

 times cutting across it. In the underground working some of these dikes 

 may be traced continuously into the large greenstone bluff south of the 

 mine, and others into that east of the pit. A few of the dikes still preserve 

 their diabasic structure, but most of them have become "chlorite-schists" or 

 " soapstones." In composition they are identical with the peripheral schis- 

 tose portions of large knobs. The disturbance created in the bedding of 

 the sediments contiguous to many of the boss masses, moreover, is of such 

 a nature that it admits of but one interpretation, viz, that the knob green- 

 stones are irruptive into the Marquette, not as interleaved sheets, but as 

 true bosses, in some places with laccolitic features. In the majority of 

 cases the bedded rocks dip away from the contacts, and at much higher 

 angles near the greenstones than at a distance from them. Besides, wherever 

 small areas of the bedded rocks lie between the arms of an irregularly out- 

 lined knob the beds are usually bent into little folds, with axes pitching 

 away from the greenstone. Further, a glance at the detailed maps of the 

 area around Ishpeming and Negaunee (Atlas Sheets XXVIII, XXIX, and 

 XXXI) will show that the exposures of the greenstones, in this area at 

 any rate, are so in-egularly distributed throughout the iron-bearing forma- 

 tion as to indicate that the crystalline rocks are bosses and not interleaved 

 sheets. An accurate mapping of the greenstones, wherever undertaken, 

 effectually disposes of this latter idea, even in the absence of the intrusive 

 phenomena described above. It would seem that the intrusive relation of 

 the knob gi*eenstones to the bedded rocks is settled beyond reasonable 

 doubt. 



The greenstone knobs are very miich more abundant in the iron- 

 bearing formation than in any other, though they are by no means limited 

 to it. They occur in all the formations of the Lower Marquette and in the 

 lower members of the Upper Manjuette, but their immber in these other 

 formations is inconsiderable and their size small. 



PETROGRAPHICAL CHARACTER. 



The material of the eastern knobs differs in no essential respects from 

 much of the basic material intrusive in the Basement Complex. All the rocks 

 comprising them are altered diabases, sometimes coarse-grained, sometimes 



