336 THE MAEQUETTE IKON-BEAEING DISTKICT. 



THICKNESS. 



The average oi'iginal thickness of the Negaunee iron formation may 

 have been greater than its present maximum thickness, for we have no 

 means of ascertaining what part of it and of overlying formations was 

 removed by erosion. 



If subordinate foldings are not considered, the interstratified eruptives 

 are neglected, and the maximum breadth of outcrop is multiplied by the sine 

 of the angle of dip, tliis gives a thickness of about 1,500 feet; but the sub- 

 ordinate folding and eruptives certainly reduce this thickness somewhat — 

 probably as much as one-third. In the broad area of Ishpeming and 

 Negaunee it is impossible to determine the thickness, for nowhere have 

 diamond drills penetrated the underlying Siamo slate. The folding is 

 here so complicated that an accurate estimate of the thickness can not be 

 given, even of the part of the formation which is exposed and explored. It 

 is, however, certain that the thickness is considerable, and it may be more 

 than 1,000 feet. From what has been said in reference to erosion it is evi- 

 dent that the formation varies from its maximum thickness to disajjpearance. 



PETROGRAPHICAL CHARACTER. 



Macroscopicai. — Petrograpliically the iron-bearing formation comprises sid- 

 eritic slates, which may be griineritic, magnetitic, hematitic, or limonitic; 

 griinerite-magnetite-schists; ferruginous slates; ferruginous cherts; jas- 

 pilite; and iron ores. The ferruginous cherts and jaspilite are frequently 

 brecciated; the other kinds less frequently. 



The sideritic slates are most abundantly found in the valleys between 

 the greenstone masses in the large area south of Ishpeming and Negaxmee, 

 although they occur at other localities. These rocks are regularly lami- 

 nated, fine-grained, and when unaltered are of a dull-gray color (PL XVII). 

 The purest phases of them are approximately cherty iron carbonate, as 

 shown by two analyses made by Greorge Steiger in the laboratory of 

 the Smwey. 



