OUTLINE OF THIS MONOGRAPH. XXIII 



large way is folded into a jjreat westwavd-iiluuging syncliue, ami upon this major fold are superim- 

 posed folds of a higher order. The western boundary of the Wewe formation was the western limit 

 of the seashore at the time of the deposition of this formation, and thus basal conglomerates are here 

 found, which in all respects are similar to those of the Jlesnard formation, being composed chiefly in 

 each case of detritus derived from the immediately subjacent rocks of the Archean. As the result of 

 the folding, the slates show very generally a cleavage or lissility, but in many places where they are 

 coarse or brittle they are fractured through and through, or even transformed into reibungsbreccias. 

 The formation grades into the Kona dolomite below and into the Ajibik quartzite above. 



The Ajibik quartzite, from 700 to 900 feet thick, is mainly a metamorphosed sandstone. The time of 

 the Ajibik (luartzite marks a rapid advance of the Lower Marquette sea, and therefore the formation 

 extends to the western end of the district. In the eastern part of the area the Ajibik quartzite grades 

 down into the Wowe slate, but for the major portion of the district it rests uneonformably upon the 

 Basement Complex. At many localities contacts and basal conglomerates are known. In some places 

 the motamorphism has l)een so severe as to transform the Basement Complex and the Ajibik quartzite 

 into crystalline schists, with parallel structures. The quartzite grades above either into the Siamo 

 slate or into the Negaunee iron formation. In different parts of the district, depending upon various 

 conditions, the original sandstone has been transformed into quartzite, cherty quartzite, ferruginous 

 quartzite, ferruginous cherty quartzite, quartz-rock, and quartzite-breccia. Some of the dynamic 

 breccias so closely resemble ordinary conglomerate as to deserve the name pseudo-conglomerate. 



The Siamo slate, from 600 to 1,200 feet thick, is chiefly an altered mudstone, although locally it 

 was a grit or sand-rock, which has subsequently been changed to gray wacke or quartzite. The larger 

 area of the formation is confined to the eastern part of the district, although a belt runs near the 

 north side of the Marquette series to the west end of the district. The major folding is similar to 

 that of the other formations. Superimposed upon the larger folds are secondary folds, and these at 

 various places are monoclinal. The formation is very generally affected by a cleavage or lissility, and 

 in the case of the monoclinal folds the cleavage is inclined in the same direction as the axial jilanes 

 of the folds. The Siamo slate grades into the Ajibik (£uartzit(< below and into the Negaunee iron 

 formation above. 



The Negaunee formation, from 1,000 to 1,500 feet thick, is nonfragmental, heavily ferruginous 

 throughout, and contains the greater iron-ore deposits of the district. It is therefore called the iron- 

 bearing formation. Large quantities of intrusive greenstones are associated with the formation, the 

 masses of which vary in magnitude from great bosses 2 miles or more long and a half mile wide to 

 small dikes. The largest area of the Negaunee formation is in the east-central part of the district. 

 From this area two belts extend west to the western end of the district. Upon the whole, the forma- 

 tion is soft, and occupies lowlands between the more resistant greenstones and the Ajibik quartzite. 

 The formation is underlain by the Siamo slate or Ajibik quartzite, into which it grades, and is over- 

 lain uncomformably by the Upper Marquette series. Petrographically the formation comprises sideritic 

 slate, which may be griineritic, magnetitic, hematitic, or limonitic ; griinerite-magnetite-schist ; ferru- 

 ginous slate; ferruginous chert; jaspilite; and iron ore. The ferruginous chert and jaspilite arc fre- 

 quently brecciated ; the other kinds less frequently. The sideritic slate is the original form from which 

 the other varieties of rock have developed. The griinerite-magnetite-schists were formed by partial 

 recrystallization of the silica, by oxidation of the iron oxide in part to magnetite, by the union of a 

 part of the silica and iron protoxide, producing griinerite, and with the loss of carbon dioxide. The 

 ferruginous slates are the direct result of the decomposition of the iron carbonate and the peroxidation 

 of the iron with partial or complete recrystallization of the silica. The ferruginous cherts differ from 

 the ferruginous slates in that the iron oxide and the chert are largely concentrated into alternate 

 bands. The jaspilites differ from the ferruginous cherts in that each of the quartz grains of the chert 



