316 THE MARQUETTE IRON-BEAEING DISTRIOT. 



Lake (Atlas Sheet XXVII). The broad eastern area is a gently westward- 

 jilnnging syncline with minor folds. In the eastern arm, which swings to 

 the south in sec. 2, T. 47 N., R. 26 W. (Atlas Sheet XXXIV), the dips are 

 always away from the Ajibik quartzite and imder the Negaunee formation. 

 Following the main belt from sec. 3, T. 47 N., R. 26 W., into sees. 4, 5, 8, 

 and 9 (Atlas Sheet XXXI), the slates, upon the whole, constitute a great 

 anticlinal dome. There the folding is complex. The pressure was more 

 severe in a north-south than in an east-west direction, so that on the north- 

 ern side of the area the dips are, in general, to the south, and upon the 

 southern side to the north. This, however, is by no means a simple fold, 

 but an anticlinorium with a large number of minor rolls with east-west axes. 

 The north-south major cross fold causes these minor plications to plunge 

 under the iron formation to the west, and the contact line between the for- 

 mations curves outward and in^^'ard in a lumiber of reentrants and salients. 

 The salients correspond to anticlines in the slates, the reentrants to synclines. 

 The same irregularity is probable upon the east side of the area, but here 

 a swamp prevents a close delimitation of the Siamo slate and the Negaunee 

 formation. Following the belt to the southwest, the southwestern termi- 

 nation of the fold occurs in sec. 20 (Atlas Sheet XXXII), where the iron 

 formation appears in a semicircular belt al)out the plunging anticline. 

 The Siamo slate, thus plunging- beneath the iron formation, i-eappears as 

 an anticlinal dome in sees. 19 and 30, T. 47 N., R. 26 W. On a smaller 

 scale, the phenomena of folding are here the same as in the large exposures 

 of this formation to the northeast. 



petroctRAphical character. 



Macroscopicai. — The Siaiuo slatc varies from a coarse-grained feldspathic 

 graywacke approaching a quartzite, through typical, massive graywacke, to 

 a very fine grained, slaty rock. The slate and fine-grained graywackes are 

 more abundant than the coarse, feldspathic graywackes. 



The finer-grained phases are very general!}- aff"ected by a slaty cleav- 

 age, which in places approximately corresponds with the bedding, but which 

 also at other places cuts across the bedding at various angles. As explained 

 on a previous page, the slate in many places is folded into a series of minor, 



