518 THE MARQUETTE IRON-BEARING DISTRICT. 



/ SECTION II.- THE POST-CIiABKSBITRG GREENSTONES. 



The freshest of the igneous rocks of the Marquette district are th' e 

 which constitute the independent dikes that have ah-eady been raentioi'^d 

 as being in all probability the youngest dikes in the entire area, because 

 they cut inditlerently the larger dikes of altered diabase and all the foriiia- 

 tions of both the Upper and the Lower Marquette series, and because they 

 are only locally schistose. 



Most of these )'ounger rocks exist as well-deiined dikes with sharply 

 marked walls, although in several instances tliey occur also as boss masses. 

 They are found as dikes in the Basement Complex on Light-House Point 

 (Atlas Sheet XXXVIII); in the Lower Marquette series, cutting the 

 quartzites and marbles southwest of Lake Mary, in T. 47 N., R 2.5 W. 

 (Atlas Sheet XXXVII); in the iron-bearing formation at many of the 

 mines; and in the Upper Marquette series in the Ishpeming and Miclii- 

 gamme formations, especially in the western portion of the area mapped. 

 They are even more abundant still farther west, in the slates between 

 Michigamme and L'Anse, beyond the limits of the map. 



The boss masses of these younger greenstones are rare so far as the 

 area luider discussion is concerned. The most typical occurrence is that 

 of the knob in sees. 35 and 36, T. 48 N., R. 30 W. (Atlas Sheet VIII), on 

 the shore of Lake Michigamme. 



PETROGRAPHICAL CHARACTER. 



Petrographically these greenstones are aphanitic to medium-grained, 

 dark-gray or black basic rocks, in which secondary products may be abun- 

 dant, but in which the original structures are well preserved. As has already 

 been indicated, these rocks are scarcely ever schistose, in which respect they 

 are sharply contrasted with the greenstones of pre-Clarksburg age. They 

 are massive rocks that resemble strongly some of the dike and sheet rocks 

 of the Keweenawan series on Keweenaw Point, and hence they have been 

 thought by Lane to be the lower portions of the Keweenawan eruptives, in 

 the same way that the pre-Clarksburg intrusives are regarded as the lower 

 portions- of the Clarksburg greenstones. 



