150 THE MARQUETTE IRON-BEARING DISTRICT. 



part of tlie district it has been prosecuted only carefully enoug-li to insure 

 accurate mapping of the main areas. 



In consequence of the widespread occurrence of the granites and 

 schists beneath the sedimentary formations, and the complexity of their 

 structural relations, the term "Basement C'omplex," first proposed by 

 Ir^'iug, has been used to designate them, as indicating that they constitute 

 the basement upon which the Marquette sediments were laid down. 



This Basement Complex in the Marquette district, as elsewhere in the 

 Lake Superior region, consists of a series of acid and basic schists, cut by 

 veins and dikes of granite and other acid rocks, dikes of basic erujjtives, 

 and bosses of acid, intermediate, and basic materials. The whole comprises 

 a confusing mass of crystalline rocks, the relations between which may 

 sometimes be discovered, but which more frequently are not decipherable. 

 So different are these rocks from the members of the Marquette series in 

 appearance, structiu-e, and composition that even where there is no apparent 

 structural break between the two series, on lithological g-rounds alone they 

 would naturally be regarded as of different ages, or at least as having been 

 produced under very different conditions. It is partly for the ]nirpose of 

 contrasting- their characteristics with those of the Algonkian rocks that the 

 members of the Basement Complex are here described. 



The Marquette rocks are bounded both to the north and to the south 

 by areas of the Basement Complex. The northern area differs somewhat 

 from the southern one in the nature of its rocks, and therefore the two are 

 discussed separately. In addition to these tw<i large areas, there are smaller 

 ones tliat are entirely surrounded Ijy the Algonkian beds, like islands in a 

 sea of rocks. In these areas the rocks are not materially different from 

 those of the larger areas, but for the sake of clearness in picturing the 

 structure of the Marquette range they will be referred to separately and 

 described briefly. 



SECTIOX I. THE XORTHERN COMPLEX. 



Throughout nearly its whole extent, from Lake Superior as far west as 

 beyond Lake Michigamme, the Marquette series is limited on the north by 

 a belt of crvstalline and pyroclastic rocks, cut by basic, intermediate, and 



