THE IGNEOUS KOGKS. 489 



hi oh nd£»'e some 2i miles in length and not more than an eighth of a mile iu 

 width. Other assemblages of knobs cover irregular but nearly equidimen- 

 sional areas. There can be detected no striking differentiation of their 

 parts. They are nearly nnifornj in composition and structure throughout, 

 and hence thev have features that all)- them with l)oss masses. In one 

 instance a laccolitic character is plainly discernible iu a mass of greenstone 

 that has raised into a dome the griinerite-schists which cover it. The place 

 in question is in sec. 12, T. 47 N., R. 29 ^V. (Atlas Sheet XVI), where the 

 relations between the greenstone and schists are as indicated iu fig. 17, 

 p. 330. (See also PI. XL) 



Geographically considered, the knobs are not found east of the center 

 of R. 26 W. From this point west to the extreme limit of the mapped area 

 they occur iu greater or fewer numbers, being most abundant in the 

 Ishpeming-Negaunee mining area and in that north of Michigamme Lake 

 and along Michigamme River. In this western area they form long, naiTow 

 rido-es rather than irregularly shaped knobs. Moreover, the rocks in these 

 rido-es have a composition different from that of the rocks forming the 

 knobs farther east. They seem to have suffered dynamic metamorphism 

 to a greater degree than the eastern rocks, while the latter have suffered 

 more severely from the effects of weathering. (See also pp. 32M-330.) 



THE BOSSES. 



THE EASTERN KNOBS. 



RELATION.S TO MARQUETTE SEDIMENTS. 



The relations of the eastern greenstones to the rocks witli which they 

 are associated prove conclusively that they are intrusive in them, and are 

 neither interleaved flows, as they have so frequently been stated to be, nor 

 areas of the Basement Complex from which the Marquette beds have been 

 eroded, as was supposed by N. H. Winchell. That they are intrusive is 

 shown by the peripheral dikes extending from some of them into the sur- 

 rounding sedimentaries and by the nature of the disturbances created in the 

 bedding of the intruded rocks near the contacts with the greenstones. At 

 the south of the hard-ore open pit of the Lake Superior mine in Ishpeming 

 (Atlas Sheet XXVIII) a number of small dikes may be seen in the jaspers 



