134 TUE -NIAKQUETTE IRON-BEAEING DISTRICT. 



The l)asement series — 



consists of a most intrit'ate mixture of nearly massive rocks, among whicli grranite 

 and granite gneiss are predominant; of gneissic aud schistose rocks, all of which are 

 completely crystalline, and so folded and contorted that nowhere has any certain 

 structure over been made out over considerable areas. * * » The minerals in the 

 rocks generally show evidence of dynamic action; they do not have the clear-cut, 

 definite relations characteristic of the later jdutonic rocks. » * * Further, the 

 basal complex is not only recognized by its positive but by its negative (-haracters. 

 Nowhere in It is a persistent thick formation of (juartz-schist (although vein-quartz 

 is abundant), of liuu^stone or marble, of a graphitic schist, or of a conglomerate. 

 (P. 470.) 



This complex, iu its positive as well as in its negative charactevistics, is 

 unique among the geological formations; hence, observes the author, it 

 deserves a descriptive name (Archean) to distinguish it from the clastic 

 formations above it. 



Concerning the origin and stratigraphy of this complex little is known. 



The only division generally applicable to the Archeau warranted by present 

 knowledge is its separation into (I) fine grained mica-schists, feldspathic mica-schists 

 (technically gneisses), hornblende-schists, hornblende-gneisses, etc., and (2) the granites 

 and granitoid gneisses, with their associates. (P. 488). 



The term Laurentian is propost-d for the lighter-colored, gneiss-granite 

 portion of the complex, and Mareniscan for tlie darker, schist j)ortion. 



The classification of the Marquette rocks, then, in its general featiu-es, 

 is as follows: 



Paleozoic Cambrian — . Potsdam. 



Affnotozoic or Proterozoic . .Alsonkian. 



eweenawau. 



K 



lluronian. . .| PP " 

 [ Ijower. 



Archean Archean Mareniscan and Laiu-entian. 



In the Lake Superior region, between the Archean and the Potsdam sandstone, 

 the great Algonkian system is subdivided into three series, which are separated by 

 very considerable unconformities. The lowest sei-ies is closelj^ folded, semicrystalline, 

 and consists of limestones, quartzites, mica-slates, mica-schists, schist-conglomerates, 

 and ferruginous aud jaspery beds, intersected by basic dykes, aud in certain areas 

 also by acid eruptives. It includes volcanic elastics, often agglomeratic, and a green 



