CHAPTER Vri. 



By C. R. Van Hise. 



THE ERUPTIVE S. 



Strurtiiral rplatioiis. General character of the rock. Comparison of Penokee greenstones with 

 greenstones of the .Southern Complex anil Keweenaw series. Microscopical character of the 

 diabases. Eruiitives in thi' Irou-l)earing member. Summary. 



Structural relations. — The eruptives of the Penokee series are structur- 

 ally of two classes. Some are sheets, while others are dikes. The field 

 relations tA' many ledges, however, are not well enoug-h exposed to show 

 to which class tliey Ijclong. The exposures in T. 44 N., R. 5 W., Wis- 

 consin, are large, and there seems to be every indication short of dem- 

 onstration that these rocks are really interleaved with tlie sediments 

 of the Iron-bearing member. In the east })art of T. 47 N., R. 46 W., 

 Michigan, and the west part of T. 47 N., R. 45 W., Michignn, the iron- 

 bearing belt has an unusual widtli. Here the eruptives are particularly 

 abundant, as shown by natural exposure and by test pitting, and the rela- 

 tions of the greenstones and the iron-bearing- rocks are such as to indicate 

 that to some extent the greenstones are interleaved, Ijut tliere is not suffi- 

 cient evidence of this to enable one to make the assertion without tpialifica- 

 tions. There i.s no absolute proof whether these interleaved greenstones 

 are contemporaneous x-oleanic outflows which interrupted the deposition of 

 the iron belt, or subsequent intrusions. All the evidence at hand is of a 

 lithological character, and it points to subsequent intrusions rather than to 

 contemi»oraneons Hows. The rocks are medium grained, holocrystalline, 

 nonainygdaloidal, and do not contain minerals in two generations ; proper- 



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