476 THE MARQUETTE lEON-BEARIXG DISTRICT. 



the g-reen schists of the Moiia Ibnuatiou in the Basement Complex. The 

 uhimate products in the two cases are practically identical, except that the 

 Mona schists are, as a rule, more weathered than the Clarksburg- rocks. The 

 final, most crystalline phases of the Mona rocks were described by Williams 

 as schistose "diorites." In this volume they are regarded as the most 

 highly metamorphosed phases of tuifs. Rocks analogous to these exist 

 also in the Clarksburg series. They are very lustrous, foliated rocks, 

 resembling in their hand specimens typical hornblende-schists. The only 

 differences noted between these schists and the amphibolic sediments and 

 tuffs described above are in the greater schistosity of the former and in 

 the greater abundance of biotite and amphibole in them. 



In the most typical of the hornblende-schists a small quantity of 

 quartz is present, with the rounded outlines of clastic grains. The horn- 

 blende is idiomorphic in cross-section, and is often actinolitic in habit. In 

 a few specimens Avell-defined crystals of actinolite are surrounded by 

 zones of chloritoid, witli tlie deep bluish-green pleochrolsm of this mineral. 

 In others a light-green amphibole is surrounded bv hornblende with the 

 properties of uralite. In tiie majority of specimens, however, only one 

 amphibole occurs, and this is usually the uralitic variety Moreover, it 

 is often cellular, including many grains of magnetite, and in some cases 

 even grains of sedimentary quartz. In origin this hornblende is evidently 

 secondary, and the schists themselves are consequently secondary rocks. 

 The)" are Interbanded with schistose sediments in which the sedimentary 

 structure can still be made out. This latter fact, together with the micro- 

 scopic structure of the schists, makes it seem very probable that the 

 amphibole-schists are highly foliated, recrystallized phases of mixed sedi- 

 ments and tuffs in which the tuffaceous material predominates over that of 

 sedimentary origin. 



If this view of the origin of the schists is correct, these rocks throw 

 considerable light on the origin of the hornblende-schists of the Mona 

 series, and add considerable weight to the statement made in a former 

 chapter to the effect that these rocks are mashed tuffs. 



THE BRKCCIAS AND CONGLOMERATES. 



In the discussion of the Clarksburg formation the breccias and con- 

 glomerates should be distributed between the sediments, gradation varieties, 



