STRUCTURAL GEOLOGY. 13 



The Lower Koewatin.] 



Nomenclature. The following summary sketch shows the succession and the 

 structure of the Archean as made out by the field examinations and corroborated by 

 the petrographies! studies. We find no use for the terms Laurentian and Coutchi- 

 ching as stratigraphic divisions of the Archean, as they seem to be represented, the 

 former by metamorphic and igneous rocks of the Archean, and hence of irregular 

 stratigraphic occurrence, and the latter by metamorphic conditions which are also 

 of different and uncertain horizons. We use the term Keewatin as applicable to all 

 the clastic rocks here put in the Archean in the state of Minnesota, that term having 

 been employed by Lawson for the region of the Lake of the Woods in his Canadian 

 report published in 1886.* At the present time these rocks are all included by the 

 Canadian survey, whether crystalline or fragmental, under the term Huronian, and 

 that term would be employed here were it not for the objections brought forward by 

 Lawson, chief of which is that the Huronian of the typical locality, as defined by 

 Logan in his final description, f includes also the Animikie and excludes all mica 

 schists and gneisses. To this may be added the fact that the original definition of 

 the term Huronian embraced under that term also the Keweenawau.J By the United 

 States Geological Survey the Archean, so far as it shows evidence of clastic origin, is 

 included in the Algonkian. The crystalline rocks are divided between those which 

 are palpably intrusive in the Algonkian elastics and those which are near the bottom, 

 the latter only being put into the Archean. The term Algonkian might be used, but 

 if due regard be paid to the law of priority it should give place to Keewatin. We 

 divide the Keewatin into two non-conformable parts, separated by a great strati- 

 graphic break. There may be a basal conglomerate at the contact of the fragmentals 

 of the Lower Keewatin on the massive greenstones below, at least in some places, 

 comparable to that which has been described on the south side of lake Superior, but 

 it has not been well identified in Minnesota. Several conglomeratic contacts on the 

 Lower Keewatin have been seen, but in all cases they have proved to be formed by 

 the Upper Keewatin on the Lower. 



The Lower Kcnrnthi. The oldest rocks known in the state consist of greenstones. 

 They constitute what has been designated, in some of the annual reports, the 

 Kfucixliiirtn. They are both massive and fragmental. As massives they seem to 

 grade into some overlying greenstone beds which manifest clastic characters, the 

 transition rarely having been seen to be that of an erosion interval marked by a basal 

 conglomerate. The only place at which a conglomeratic structure has been identified 

 at this horizon is that described in the Lake county chapter (volume iv, page 292), 

 where a greenstone conglomerate 105 feet thick lies on the massive greenstone. The 



* Geological Survey of Canada, New Series, vol. i, 1886. 

 + Geology of Canada, 1863, pp. 50-57. 



IKxijuixse li't'iilniiiijvi- i/n fiiiniiln jjniir xi'rrir ,~i I' in li-llii/i'im' ilr la carte ffeulniiii/in- <! ilc la rnHn-limi (/<* inini-rauit iconom- 

 iijues envoy fes a I'exposiUon universelle de Paris, 1855, par W. E. LOGAN et T. STERRY-HUNT. 



