304 GEOLOGICAL EXCUE8ION TO THE ROCKY MOUNTAINS. 



Below is given a brief account of those formations found within the 

 limits of the State. 



Group. 



System. 



Cenozoie Pleistooene . 



Mesozoic CretaoeouB 



Series. 



Eteoenl Glacial. 



Paleozoic . 



Aluonkian (Taconic) 

 Archean 



Devonian | Corniferous. 



Hudson River. 

 Silurian \ « ;.il. na. 



Trenton. 



st. Peter. 



Shakopee. 



Biohmond. 



Lower Maiincsian. 



Jordan. 



St. Lawrence. 



St. Croix. 



Keweenaw an (NipJgOn) ! 



Hurouian Animikie. 



i Keewatin. 



I. auren tian / s Vermilion. 



I ( Ini'iss, etc. 



Cambrian (Ozark series) 



77/6' Archean is made up of semicrystalline and crystalline schists, 

 gneisses and granites, and basic eruptives. The semicrystalline schists 

 are mostly sericitic and argillaceous in character and, together with a 

 large number of graywackes and greenstone schists, compose the Kee- 

 watin series. In this series are found extensive beds of very pure 

 hematite, which supply an excellent ore for steel-making. 28 The crys- 

 stalline schists are micaceous and hornblendic and lie below the less 

 crystalline; they belong to the Vermilion scries, as described in the 

 reports of the Minnesota survey. Still lower are gneisses and granite. 

 The entire Archean has been broken through by ancient eruptives, 

 mostly granites and quartz-porphyries, and by more recent diabases. 

 The Archean has its greatest development in the region along the 

 international boundary uorth and east of Vermilion Lake. Here the 

 strata strike in a general east-northeast direction, and the dip varies 

 only a few degrees from the vertical. 



Keweenaw 



Pig. 11. — General section from Vermilion lake to Duluth. 



This formation extends southwestwardly through the State, out- 

 crops being occasionally seen, especially in the upper Minnesota val- 



