450 THE JOURNAL OF GEOLOGY. 



have a direct transition into the green schists of the Keewatin. The green 

 schist has a nearly vertical cleavage. The schists do not always follow 

 the course of the granite range. They are unconformably covered in many 

 places by the quartzite. The quartzite never has a high dip. Near 

 the base it contains pebbles of quartz and granite, as well as jasper and 

 greenstone. This quartzite is correlated with the Pewabic quartzite of the 

 Gunflint lake, the Pokegama quartzite of the Mississippi river, that of Sioux 

 Falls, South Dakota, and that of Baraboo, Wisconsin. Conformable with the 

 quartzite is the iron ore and taconyte horizon. The strata are siliceous and 

 calcareous, and are banded with oxide of iron in beds of variable length and 

 thickness. The ore is sometimes magnetite and sometimes hematite. To the 

 banded jasper)^ quartzite associated with the ore the term taconyte is applied. 

 The greenish siliceous slates or cherts constitute a transition stage between 

 the rocks of the iron horizon and the black slates. There is a considerable 

 mixture of greenish material, apparently of eruptive origin. The greater part 

 of the rock is a red, yellow, black, white, or green chert, sometimes having a 

 thickness of 200 or 300 feet. It often has a peculiar brecciated appearance, 

 having been shattered into angular fragments, and recemented by the same 

 amorphous silica. The same fracturing is also visible in the iron ore. The 

 siliceous slates and cherts pass upward into a carbonaceous argillite of great 

 thickness, having a dip varying from the horizontal to 20° to the south or 

 southwest. Locally the dip is as high as 45°, in which case the ore deposits 

 lie close to the green schists. The gabbro flow is over all of the previous 

 strata. The effect of the heat on the molten gabbro was to make the iron ore 

 which already existed in the rocks hard and magnetic. There is good reason 

 to believe that the iron ore deposits in their present condition have been prin- 

 cipally formed since the gabbro overflow. The ore deposits occur as regular 

 beds, which lie in almost their original positions, usually having a dip of less 

 than 30° and passing into the jaspery quartzite or taconyte in three directions, 

 and occasionally on all sides. The theory of Irving as to the origin of the 

 Gogebic ores is partially adopted. The quartzite is impervious to surface 

 infiltration. The ore is regarded as produced by chemical replacement of 

 some mineral, chiefly silica, by oxide of iron. As evidence of this, all stages 

 of the process may be seen. Iron carbonate is found in the Mesabi rocks, but 

 it does not appear in sufificient quantity to permit the assumption that the 

 source of the ore was originally a carbonate. The solvent for the silica was 

 probably carbon dioxide, and its source may have been the atmosphere, the 

 black slates, recently decaying vegetation, or the ore deposits higher up the 

 hill. The silica removed from the location of the iron ores has been added to 

 the grains of quartz in the quartzite, has been deposited as chalcedonic and 

 flinty silica, and has been deposited in cracks and fissures in the slate, which 

 lie at a lower elevation, but stratigraphically above the ore. The source of 



