144 THE MAEQITETTE IKON-BE AKING DISTEICT. 



all less ci-ystalliiie tliaii the three formations of the Lower series. These are: 

 (1) A lower slate, passing locally into a quartzite or conglomerate, (2) an 

 iron-bearing formation, and (3) an upper slate member. Where the lowest 

 member of this series rests immediately npon the Lower Huronian, the 

 underlying member may be any one of the three older formations, and 

 the character of the overlying conglomerate varies accordingly. 



Volcauics, as distinguished from intrusive ernptive rocks, occur inter- 

 laminated with the Upper Huronian sediments. "In the Michigamme 

 iron district is an extensive area of greenstones, greenstone-conglomerates, 

 agglomerates, and surface lava-flows, many of which are amygdaloidal" 

 (p. 121). 



At the end of Upper Huronian deposition the land was again raised 

 above the sea, and after the rocks had been folded, gently as a rule, but 

 intensely locally, the atmospheric agents once more l^egan their work of 

 cutting them down. The land was then again submerged, and after some 

 time (during which elsewhei'e the Keweenawan rocks were formed) the 

 Potsdam sandstone was laid down npon them. 



Smyth, H. L. The quartzite tongue at Ecpublic, Michigau. Jour, of GeoL, 

 Vol. II, 1894, pases 080-(!91. 



The subji'ct of discussion in this paper is the origin of the quartzite 

 tongue mapped by Brooks as penetrating the iron-bearing formation at 

 Republic, on the western side of the eastern heel of the horseshoe. It is 

 this quartzite that was stated by Wadsworth in 1880 to be an eruptive 

 greisen, and was later (in 1892) determined to be a quartzite whose position 

 between two portions of the iron-bearing formation was explained by 

 supposing that the rocks on the different sides of the "tongue" were of 

 different ages — that on the western side belonging with the Holyoke forma- 

 tion, and the larger eastern mass of the same rocks to the Republic series. 

 An imconformity was shown to exist by Wadsworth between the quartzite 

 and the eastern jaspilites and ores. 



Smyth explains the phenomenon as du(^ to a fault along tlie contact 

 plane between the quartzite and the iron formation, which is also a plane 



