528 THE MARQUETTE IRON-BEARIXG DISTRICT, 



than the Upper Marquette sediments, and are genetically connected with 

 the great intrusions of diabases which are found abundantly in this series. 

 In one locality south of the Magnetic mine a dike of iiue-grained red 

 granite was found cutting the ordinary coarse gray granite. No other 

 clear proof of the existence of younger intrusive granites was found in the 

 Republic area proper. 



SECTION II.— THE LOWEB 3IjV.BQUETTE SERIES. 



The bedded rocks of the Republic area belong to two unconformable 

 series of Algonkian age. The lower of these, to which the name Lower 

 Marquette series has been applied, consists of two distinct members, a 

 lower fragmental member of small thickness, probably not exceeding 100 

 feet, and an upj^er iron-bearing member, which in its maximum develop- 

 ment, including intrusive greenstones, can not be less than 1,500 feet in 

 thickness. The lower member, from its usual lithological character, is 

 known as the Ajibik quartzite, while the upper member, from its constant 

 feiTugiuous character, may be distinguished as the iron-bearing member 

 or Negaunee formation. 



THE AJIBIK QUARTZITE. 



The lower member of the Lower Marquette series in this part of the 

 district is relatively a weak rock, and as its thickness is small it rarel}" 

 outcrops above the glacial mantle in the Republic area. At the jjresent 

 time but seven or eight localities are known. These are, however, so 

 widely distributed over the area that it is very probable that the lower 

 member is present wherever the Lower Marquette series is represented at 

 all. In these exposures the rock usually appears as a white quartzite, 

 sometimes vitreous, but often of an opaque white color from the large 

 amounts of contained muscovite or sericite. The mica is frequently pres- 

 ent in such abundance that the rock becomes properly a mica-schist. In 

 only one known locality, in which it is found to rest in direct contact 

 upon the Archean, does it appear as a coarse conglomerate made up of 

 recognizable fragments derived fi-om the underlying granites and crystalline 

 schists. 



