RfiSUME OF LITERATURE. 89 



Maequettian system — Continued. 

 Tower group — Continued. 



(Thej^ pass eastward into schists prevailingly porphja-ellitic.) 

 Stuntz conglomerate, porodyte and porphyrel, 20 feet. 

 Graywacke group. 2,. 500 feet. 

 Gra3'wacke and hornfels. 



Grajrwacke with indications of fine mica and hornblende. ("Nascent mica 

 schists.") 



Laurentian system. 89,500 feet. 



Vermilion group. Over 1,500 feet. 



Crystalline schists — micaceous, hornblendic, dioritic, granulitic. 

 Gneissic group. Over 88,000 feet. 



Chlorite-gneiss. (Not universally developed.) 

 Saganaga, White Iron, and Basswood gneisses. 

 Thus the crystalline schists and gneisses fall entirely within the Laurentian 

 system. There are no Huronian gneisses in Minnesota. We find nothing of "older" 

 and "newer" gneisses. We find no "clay slates" beneath the horizon of the 

 crj'stalline schists. But I can not deny the existence of a difi^erent state of things in 

 other regions. To me it seems probable, however, that a comparatively undisturbed 

 region like northeastern Minnesota must approach near to a noi'mal exhibit of the 

 real succession of the Archean rocks. 



Irving, R. D. On the classification of the early Cambrian and pre-Cambrian for- 

 mations: Seventh Ann. Rept. U. S. Geol. Survey, 1888, pp. 365-i51:, with 22 plates 

 and maps. 



Irving, in 1888, discusses the classification of the early Cambrian and 

 pre-Cambrian formations, and particularly those of the Northwestern States. 

 The relations of the Animikie, Penokee, Marquette, Menominee, and Vermil- 

 ion Lake iron-bearing series to the underlpng and overlying series are again 

 fully discussed. The Keweenawan is held to overlie the Huronian every- 

 virhere by a very considerable unconformity. At the base of the Keweenawan 

 is a great mass of gabbro, which extends from Duluth northeast to the inter- 

 national boundary, more than 100 miles, and at its maximum is more than 20 

 miles wide. This basal gabbro is now in contact with one member of the 

 Animikie, and now with another, while in other places it is in contact with the 

 lower crystalline schists or granite. In the Huronian are placed the original 

 Huronian, the Iron-bearing series of Michigan and Wisconsin, the Black 

 River Falls iron-bearing series, the Animikie series, the St. Louis and 

 Mississippi slate series, the Vermilion Lake iron-bearing series, the Baraboo 



