RESUME OF LITERATURE. 115 



contain angular and subaugular fragments of rock almost like themselves, 

 and some may be regarded as composed of fragmental volcanic rocks. 

 Associated with the greenstones, especially in sees. 22, 23, and 24, T. 65 

 N., E. 5 W., are small masses of more acid rocks, quartz-porpliyries, and 

 quartzless porphyries, which are probably younger than the greenstones. 



The pre-Animikie granite has its typical development on the shores of 

 Saganaga Lake. In a number of places it may be seen in intrusive 

 relations with the greenstone. A quarter of a mile south of the northeast 

 corner of sec. 23, T. 65 N., R. 5 W., many granite dikes cutting the green- 

 stone are seen, and on the south shore of West Sea Gull Lake granite dikes 

 of the same nature as the immediately adjacent main mass of granite of 

 Saganaga Lake are seen cutting the greenstone. Both granite and green- 

 stone are cut by another series of finer-grained, more acid granite dikes. 



The Animikie rocks rest uncomformably upon the pre-Animikie rocks, 

 and are usually exposed on the south slope of the Giants range, which is 

 composed essentially of granite. The strike is apjDroximately east-northeast, 

 and the dip in general about 10° SE. The thickness varies from nothing 

 to 4,000 feet. The Animikie is separable into four conformable divisions — 

 (1) the lower or quartzite member, called the Pewabic quartzite; (2) the 

 iron-bearing or taconite member; (3) the black-slate member; (4) the 

 graywacke-slate member. 



(1) The quartzite member is well developed in Itasca County, but 

 disappears before reaching the eastern side of St. Louis County. 



(2) The rocks of the iron-bearing member are similar to those in St. 

 Louis County on the western end of the range, described by Spurr." They 

 differ, however, in two features. They are more completely crystalline, 

 and the iron is magnetite instead of hematite. The rocks consist chiefly 

 of jaspers, amphibole- (griinerite) schists, greenish siliceous slates, cherts, 

 chertv carbonates, and magnetite slates. It is believed that these rocks 

 were originally glauconitic greensands; that the ore has been derived from 

 the iron in the glauconite, and that the ore bodies result from concentration 

 and replacement. In this part of the Mesabi range no ore bodies liave 

 yet been found which are at the same time both rich enough and large 

 enough for profitable mining, although vast quantities of magnetite ore 



«Geol. and Nat. Hist. Survey Minnesota, Bull. No. 10, 1894. 



