THE PROTEROZOIC ERA. 



187 



originally composed of ferruginous and non-ferruginous muds and grits, now 

 altered into slates, graywackes, schists, and gneisses. The predominant rocks of 

 the Clarksburg formation were volcanic flows, tuffs, ashes, and breccias, now 

 represented by greenstones. Interbedded with the greenstones are some gray- 

 wacke, conglomerate, slate, etc. The formation does not lie above the Michi- 

 gamme formation in stratigraphic sequence. It grades below into the Ishpem- 

 ing formation at some points, and into the Michigamme at others; while above, 

 it sometimes grades into the latter. In origin, therefore, it was contempor- 

 aneous with parts of the Michigamme formation. The aggregate thickness 

 of the Upper Marquette series is not accurately known, but is probably more 

 than 5000 feet, and perhaps as much as 10,000 feet. Figs. 70-72 show the 

 relations of these to older formations. 



Fig. 70. — Section showing the relations of Huronian and Animikean, in a portion 

 of the Marquette (N Mich.) region, where eruptive rocks exceed the sedimentaries. 

 H, Huronian; A, Animikean; Ae, eruptive diabase. Length of section, 3 miles. 

 (Van Hise, Bayley, Smyth, U. S. Geol. Surv.) 



The deformation and metamorphism of the Animikean rocks of this region 

 at the close of this period was great. They now occur in a great east-west syn- 

 clinorium, and the alterations of the rocks were such as to show that the beds 

 now accessible were in the zone of combined flowage and fracture, and there- 

 fore far below the surface, when they were deformed and metamorphosed. In 

 spite of the great changes which these beds have suffered, they are less meta- 

 morphic than those of the Huronian system (Lower Marquette) of the same 

 region. 



In the Menominee region, 1 the Animikean (or Upper Menominee) series includes 

 (1) the Vulcan formation and (2) the Hanbury slate. The former, with a fer- 

 ruginous basal conglomerate, rests unconformably on older rocks, and is made 

 up of quart zites, ferruginous quartzose slates, jaspilites, and iron ores. It has 

 a thickness of 600 to 700 feet. The Hanbury formation is primarily slaty, but 

 includes some iron ore and ferruginous chert, and has a thickness of perhaps 

 2000 to 3000 feet. The series is closely folded and much metamorphosed. The 

 distinction between it and the Huronian is much less than in the Marquette 

 region. They are separated mainly because of the basal conglomerate at the 

 bottom of the series. 



The Animikean system is represented in the Crystal Falls district of Michigan 2 

 by the Michigamme formation, unconformable on the Huronian. Slates, mica 



1 Van Hise and Bayley, Menominee folio, U. S. Geol. Surv. 



2 Van Hise, 21st Ann. Rept., U. S. Geol.. Surv. ; and Van Hise, and Clements, Smyth, 

 and Bayley, Mono. XXXVI, U. S. Geol. Surv. 



