xviir OUTLINE OF THIS TAPER. 



Chapter V treats of the Iroubearing member. This pei'sistent formation, aver- 

 aging about 800 feet iu thiclciiess, rests upon the vitreous (piartzite of the Upper-slate 

 member. It now consists of three main types of rock, cherty irou carbonates, ferru- 

 ginous slates and cherts, and actinolitic and maguetitic sMtes. The lirst of these is 

 the original type of rock and from it, by means of chemical changes, the second and 

 third types, as well as the ore bodies, have been produced. Tbeore bodies are found 

 in the lowest horizons of the formations and are secondary concentrations. They 

 occur in V-shaped troughs, one side of the V's being the upper quartzite of the Quartz- 

 slate, and the other diabase dikes. In the Auimikie series, on the opposite side of 

 lake Superior, is an iron-bearing formation which has the same types of rock, derived 

 from the same original form as in the Penokee series. 



Chapter VI treats of the Upper-slate member. This formation rests upon the 

 Iron-bearing member. It has a maximum thickness of 12,000 feet, and has an extent 

 east and west for many miles, although it is not so extensive as the Iron-bearing and 

 Quartz-slate members. The formation is of clastic origin and consists mainly of gray- 

 wackes and graywacke-slate. It is now locally altered by metasomatic changes so 

 that it has become a crystalline mica-schist. 



Chapter VII treats of the eruptives. The Penokee eruptives are diabases, which 

 structurally are of two classes, dikes cutting the formations, and interbedded sheets 

 which are probably intrusives of the same age as the dikes. The eruptives are fresh 

 in the slate members, but are much or completely altered in the Iron-bearing member, 

 showing that environment, not age, is the important factor iu the preservation of 

 these rocks. 



Chapter VIII treats of the Eastern area. In the eastern part of the district, as a 

 result of contemporaneous volcanic action, the Penokee succession is disturbed, and 

 associated with the ordinary detrital rocks are surface basic, volcanic flows, and also 

 greenstone-conglomerates and agglomerates. Consequent upon this volcanic disturb- 

 ance the regular alternation of clastic and nonclastic memliers of the Penokee suc- 

 cession is much msdifiecl, so that the numb(>r and order of the formations here found 

 iliffer fi'om those in the remainder of the district. 



Chapter IX treats of the general geology of the region. While the outcrop of the 

 members of the Penokee series as a whole are gently curved, sharp flexures and faults 

 are not common. One fault occurs at P>ad 'river, another at Potato river, and pcr- 

 hajjs one in the Eastern area. The base of the Quartz-slate member contains frag- 

 ments derived from the Cherty limestone member, showing that between these forma- 

 tions there was an erosion interval. How great the time gaj) represented by this is 

 there is no means of judging, except that the chert of the limestone was certainly in 

 its i)resent condition at the time of the deposition of the (Juartz-slate. The Quartz- 

 slate, Iron-bearing and Upper-slate members form a conformable succession. Between 

 tlje Penokee serieii inojier and the Southern Coni])lex there is a very great uncon- 



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