BASEMENT COMPLEX OF STURGEON EIVEE TONGUE. 469 



Here and there in the midst of this fine-grained groundmass are noticed 

 lenticular and long narrow aggregates composed of grains of plagioclase 

 that are much lai'ger than the grains of this mineral occurring in the sur- 

 rounding matrix. They look as though they might be the crushed remains 

 of what were originally plagioclase phenocrysts. 



Thus the microscopic study of these rocks tends to confirm the results 

 of their field study. They were probably porphyritic lavas or intercalated 

 flows that have suffered alteration as the result of intense pressure and 

 movement. Their present composition suggests that they were originally 

 quartz-porphyries or perhaps andesitic porphyrites. Whatever their original 

 nature, their origin is difterent from that of the biotite-schists of the Mar- 

 quette district.^ 



THE INTRUSIVE ROCKS. 



The intrusives in the schists and gneissoid granites of the Basement 

 Complex are granites, identical with the gneissoid granites above described, 

 and greenstones. The former cut only the schists. They are probably 

 apophyses from the larger granite masses. The greenstones cut the schists 

 and the granites. They are similar in all respects to the gTeenstones in the 

 sedimentary series, and thus are the youngest rocks in the district, with 

 the exception of the horizontal Paleozoic sandstones and limestones that 

 cap some of the higher hills. 



The greenstones are all more or less altered diabases. In some the 

 ophitic structure may be detected, but in most of them no traces of their 

 original constituents nor of their structure remain. Nearly all are more or 

 less schistose. The only evidence that the most schistose phases were once 

 massive igneous rocks is in their composition and their occurrence in dike- 

 like fissures. As the schistosity of these greenstones increases, the amount 

 of their alteration also increases; there is a greater abundance of horn- 

 blende present in them and a greater quantity of quartz, until in the most 

 schistose phases the rocks are now typical hornblende-schists. 



One of the best examples of these greenstones occurs in the series of 

 ledges extending in nearly a straight line for 6 miles from the southern 

 portion of sec. 13, T. 42 N., R. '29 W., to the northeast corner of sec. 14, 

 T. 42 N., E,. 28 W. Except in its eastern ledges the rock constitutes bold, 



•Mon. U. S. Geol. Survey, Vol. XXVIII, pp. 200-203. 



