124 TOE MAUyrETTE lltOX HKAItING DISTIHCT. 



Ill the iiorrli halt' of the Eastern area tlie Uxyers of the scliists — 



are alternately of a darker and lighter shade of green, which gives these particular 

 greenstones their characteristic striped appearance. In these banded rocks * * * 

 intrusions of comparatively little altered acid and basic matter are abundant. These 

 are for the most part conformable to the bedding of the schists, and embrace 

 granites, gneisses, schistose prophyries, diorites, and diabases. Whenever, in tliese 

 undoubtedly eruptive rocks, a schistose structure is apparent, this is conformable to 

 the bedding of the banded greenstone-schists. 



Tlie southern portion of the area * * * is occupied by niucli more massive 

 and homogeneous greenstones of a nearly uniform light-green color and an almost 

 aphanltic structure. These are characterized by their division into oval or lenticular 

 areas which interlace and which are separated by a finely schistose material of much 

 finer grain. This peculiar parting, * * * at iirst glance, resembles the spheroidal 

 weathering of many eruptive rocks. There is, however, better reason for regarding 

 it as of mechanical origin. * * * Intrusive rocks are rarer than in the banded 

 greenstones of the northern portion of this area. (P. 137.) 



A large number of thin sections of these schists are carefully described. 

 Some of the descriptions will be referred to in the body of this monograph. 

 At this point it will l)e necessary simply to (piote the author's conclusions. 



With reference to the rocks of the Nortliern area he writes (p. 158): 



* * * The structure of these greenstone schists is such as to necessitate a 

 belief in the original nature of their stratification ; while, on the other Land, their 

 chemical as well as their mineralogical composition renders it impossible to .separate 

 them from the massive and highly altered greenstones (uralite, diabases, etc.) with 

 which tliey are more intimately associated. Their parallel banding indicates a 

 fragmental, but their chemical and their mineral composition indicate an igneous 

 origin. The only satisfactory reconciliation of these opposite sets of characters is to 

 be found in that group of rocks intermediate between sediments and lavas, known as 

 volcanic tuffs. 



In the opinion of the writer, then, the banded greenstone-schists of the Northern 

 Marquette area are to be regarded as consolidated and highly metamorphosed diabase 

 tuffs. These are intimately associated with numerous contemporaneous flows of 

 diabase and quartz-porphyry, together witii tuft's of the latter rock; while all have 

 been broken through by much younger dikes, both basic and acidic. 



The rocks of the Southern area are not banded. "They are, for the 

 most part, massive, pale green in color, and apparently homogeneous in 

 structure." 



