BASEMENT COMPLEX OF STUEGEOK KIVEE TONGUE. 467 



have once been plagioclase phenociysts may indicate that the orig-inal rocks 

 were in the form of lavas. 



The principal difference between the hornblende-schists and the 

 greenstone-schists seems to be in the nature of the amphibole in the two 

 rocks and in the presence of quartz and newly formed plagioclase in the 

 first named. The materials of the greenstone-schist were derived from the 

 alteration of those of the original rock, as were also those of the hornblende- 

 schist, but the former now consist mainly of the direct products of this 

 alteration, whereas in the latter the substances now existing have been 

 worked over and entirely recrystallized. 



THE BIOTITE-SCHISTS. 



Mica-schists are not common in the Sturgeon River tongue. They 

 constitute by no means so large a part of the Basement Complex in this dis- 

 trict as they do in the other portions of the Lake Superior region that have 

 been studied. Indeed, only a few ledges of this rock have been observed 

 in the country between the Sturgeon River and the Felch Mountain sedi- 

 mentary tongues, and most of these are along the southern edge of a green- 

 stone knob 300 to 400 paces north of the southeast corner of sec. 17, 

 T. 42 N., R. 28 W. 



The mass of this knob is a dark horublende-schist. On the south side 

 of the top of the knob this rock is in contact with a very evenly banded or 

 streaked rock of a general dark-gray color. In the hand specimen it resem- 

 bles very closely a fine-grained banded augen-gneiss. Near its contact 

 with the hornblende-schist the rock is ajjparently porphyritic, with pheno- 

 crysts of feldspar from 1 to 15 mm. in length, and an occasional one of 

 quartz scattered throvigh a matrix composed of narrow alternating bands of 

 almost black and light-gray material. On cross fractures of the rock 

 the phenocrysts are seen to be drawn out in the direction of the bands. 

 Cleavage takes place very readily along the planes of the banding, yielding 

 surfaces covered with tiny scales of black biotite. A little farther from the 

 contact the light-colored bands are thicker and more distinct. At first 

 glance they appear to be uniformly thick for long distances, but a more 

 careful inspection shows that they wedge out rapidly and are replaced by 

 other bands of the same character. The dark bands are not thicker than 



