482 THE CRYSTAL FALLS lEON-BBAEING DISTRICT. 



Marquette range. It is a very fine grained rock composed of very small 

 splinters of quartz, flakes of sericite, and a fevv of chlorite. 



THE IGNEOUS ROCKS. 



The igneous rocks associated with the sedimentary beds in the Sturgeon 

 River tongue are all greenstones in composition. Many of them are unques- 

 tionably intrusive; a few may be tuffaceous. 



The intrusive greenstones do not differ essentially from those cutting 

 the Basement Complex. Some of them are in the form of small bosses. 

 Others are clearly dikes, though for the most part these dikes follow the 

 bedding of the sedimentary rocks. Still others may be intrusive sheets. 

 The rocks regarded as possibly tuffaceous are distinctly banded. Some are 

 made up of alternate bands of dark and light shades. The darker bands 

 consist principally of a schistose greenstone, and the lighter ones principally 

 of arkose or granitic sandstone. These rocks are well bedded, apparently 

 constituting a definite portion of the conglomerate series near its lower 

 horizon.^ 



THE INTRUSIVE GEEENSTONBS. 



The intrusive greenstones are usually fairly massive rocks, with a dark 

 bluish-green color and a moderately fine grained texture. On their edges 

 the}' often pass into schistose phases, presenting the structure and appear- 

 ance of chlorite-schists. A very typical schist of this character occurs on 

 the southern edge of the g-reat greenstone mass 1,525 to 1,600 paces north 

 and 300 to 400 west of the southeast corner of sec. 18, T. 42 N., R. 28 W. 

 In the hand specimen the rock appears to be a well-characterized chlorite- 

 schist, spangled with plates of a light-colored inuscovite measuring 1.5 to 

 2 mm. in diameter. 



The intrusive character of some of the greenstones is clearly shown by 

 the fact that thej^ occur immediately on the strike of the conglomerate bands, 

 and often cutting across them, as is the case at 300 paces east of the north- 

 west corner of sec. 17, T. 42 N., R. 28 W. (see PI. LII), and at 400 paces 

 south, 100 west, of this same corner. 



PETROGRAPHICAL DESCRIPTION. 



The greenstones intrusive in the Algonkian sediments are not essen- 

 tially different from those cutting the members of the Basement Complex. 



' See Van Hise's Notebook 184, pp. 21-23. 



