REVIEIVS 167 



concretions with concentric structure, which Van Hise has shown to develop during 

 the alteration of iron carbonate; and associated with these are rare granules of iron oxide 

 and chert in varying proportions, which may represent altered ferrous silicate granules 

 similar to those of the Mesabi district. Evidences of the existence of original ferrous 

 silicate granules in the Gogebic district are not sufficiently numerous to warrant modi- 

 fication of Van Hise's conclusion that the ores have developed from the alteration of 

 iron carbonate. 



C. K. Leith. "The Mesabi Iron-Bearing District of Minnesota." Mono- 

 graph XLIII, U. S. Geological Survey, 1903. 



Leith describes and maps the geology of the Mesabi district of Minnesota. The 

 district is two to ten miles in width, extending from near Grand Rapids on the Mis- 

 sissippi River to Birch Lake, a distance of approximately one hundred miles. The 

 main topographic feature is a ridge known as the Giant's (or Mesabi) Range, which' 

 extends the length of the district. The geologic formations represented in the district 

 belong, in ascending succession to the Archaean, Lower Huronian, Upper Huronian 

 Keweenavv^an, Cretaceous, and Pleistocene. They are all separated by unconformities. 

 The core of the Giant's Range is formed by Archaean and Lower Huronian rockS) 

 except for the portion in ranges 12 and 13, where Keweenawan granite forms the core. 

 On the south flank rest the Upper Huronian rocks, containing the iron-bearing forma- 

 tion, with gentle southerly dips. The ICeweenawan gabbro lies diagonally across the 

 east end of the district. 



The Archaean rocks consist principally of green rocks of great variety, including 

 dolerites, metadolerites, basalts, metabasalts, diorites, and hornblendic, micaceous, and 

 chloritic schists. The more massive rocks frequently have an ellipsoidal structure, 

 which is characteristic of the green igneous rocks of other parts of the Lake Superior 

 region. In addition to the green basic rocks, there are present small areas of granite 

 and porphyritic rhyolite. 



The Lower Huronian series consists of sediments and granite. The sediments 

 are graywackes, slates, and conglomerates, all metamorphosed, with bedding and 

 schistosity practically vertical. They may be as thick as 10,000 feet, but it is thought 

 more probable that the thickness does not exceed 5,000 feet. The Lower Huronian 

 sediments rest unconformably upon the Archaean rocks, as shown by basal conglomerates 

 containing fragments of all the varieties of rocks found in the Archaean. The Lower 

 Huronian granite forms the main mass of the Giant's Range westward from a point 

 near the east line of Range 14 W. It is intrusive into both the Archaean rocks and 

 the Lower Huronian sediments, and has produced strong exomorphic effects in both. 



The Upper Huronian or Animikie consists of three formations — -the Pokegama 

 quartzite at the base, above this the Biwabik formation (iron-bearing), and above this 

 the Virginia slate. 



The Pokegama quartzite comprises vitreous quartzite, micaceous quartz-slate, and 

 conglomerate. The thickness ranges from to 500 feet, averaging about 200 feet- 

 The conglomerate at the base indicates unconformable relations of the Pokegama 

 formation to the Archaean and Lower Huronian rocks. 



The Biwabik formation, the iron-bearing formation, comprises ferruginous, 

 amphibolitic, sideritic, and calcareous cherts, siliceous, ferruginous, and amphibolitic 

 slates, paint rocks, "greenalite" rocks, sideritic and calcareous rocks, conglomerates 

 and quartzites, and iron ores. Cherts make up the bulk of the formation. The 



