okant.] GEOLOGY OF MINNESOTA. 305 



ley, where, however, the three divisions described above are not recog- 

 nizable. But, as a rule, west and southwest from Duluth these rocks 

 arc hidden by the drift. 



The Algonkian (or Taconic of the .Minnesota survey) is seen in its 

 greatest development in the country lying just north of Lake Superior. 

 It is separated into an upper and lower division — the Iluronian and 

 Keweenawan. 



The Ruronian in Minnesota is represented by dark carbonaceous 

 slates and ferruginous quartzites. A belt of these rocks stretches 

 from the extreme eastern end of the State west and southwest to the 

 vicinity of Duluth and to the Mississippi river. They lie just south 

 of the Archcaii complex and rest OIL its upturned edges. The dip is 

 at low angles toward Lake Superior. After going - a few miles north- 

 west of Duluth the Iluronian is seldom seen until reaching Pokegama 

 Falls on the Upper Mississippi, and west of this these rocks are com- 

 pletely covered by glacial deposits. The Huronian in Minnesota is 

 included under what has been described as the Animikie on the north 

 side of Lake Superior. 



The Keweenawan consists chiefly of interbedded sheets of interme- 

 diate and basic eruptives, with a few elastics. These rocks occupy the 

 territory south of the strike of the Huronian and north of Lake Su- 

 perior. They are seen in great development in the vicinity of Duluth 

 and all along the lake shore to Pigeon Point. 29 The principal rock 

 type is a very coarse-grained, gray gabbro, which has a geographical 

 extent of 1,000 square miles (260,000 hectares). It is used somewhat 

 for a building stone, and is well known as "the Duluth granite." 



The Paleozoic.— To this belong the rocks of the entire southeastern 

 and southern portions of the State. They are comparatively little 

 exposed, except along the river courses. 



The Cambrian extends along the Mississippi river and its tributaries 

 from St. Paul to the southern limit of the State. It is especially well 

 exposed in the bluffs along the Mississippi. The eastern part of the 

 State, north of St. Paul and south of Duluth. is also occupied largely by 

 Oambrian strata. A belt alsoextends from the last area sonthwestward 

 along the southern borderof the Archcan to the extreme southwestern 

 corner of the State. The rocks of this formation are mostly thick beds 

 of magnesian limestone and of sandstone, but in the area lying in the 

 southwestern corner of Minnesota a:, extensive quartxite occurs; asso- 

 ciated with this is the famous Indian '• pipestone" or catlinite. 



The Silurian is seen overlying conformably I he Cambrian in many 

 places along the Mississippi river. It is represented onlv bv a com- 

 paratively small thickness of strata, which are confined almost en- 

 tirely to tin- Lower Silurian. Its chief member, the Trenton limestone, 

 is seen m its best development along the Mississippi in the vicinity of 

 loi ok 20 



