PLATE LXXXII. 



GUNFLINT LAKE PLATE OF THE MESABI IRON RANGE, 1899. U. S. GRANT. 



The iron-bearing Animikie, within the area of this plate, presents interesting 

 variations. These are discussed fully in connection with the question of the origin 

 of the iron ores of the Mesabi range in Part III of volume v. The first suggestion 

 that the iron-bearing member of the Animikie was primarily an igneous rock, vary- 

 ing from rhyolyte or obsidian to volcanic obsidian sand, since silicified, was derived 

 from specimens collected on the north side of the peninsula between Gunflint and 

 North lakes. This suggestion led to the grouping in one general class of several 

 rocks that had before been supposed to have little or no genetic affinity. 



The earliest condition of the iron-bearing rock is seen at the westward from 

 Gunflint lake, where it (apparently) lies on the granite of the Giant's range- It is 

 in the isolated hill at the centre of sec. 24, T. 65-4. While the rock is considerably 

 charged with magnetite and siderite it is also slaty and but little silicified. It is 

 seen to embrace, near its upper surface, a singular breccia of flinty fragments, and 

 here the iron-bearing rock which forms a cement or matrix for this breccia presents 

 characters that show it was a surface lava. The flint fragments are apparently glassy 

 parts (now silicified) of the lava first cooled but still embraced in the general flow 

 (vol. v, p. 951). 



This condition was followed and accompanied by a glassy rock (obsidian) in 

 considerable amount. It forms breccias and conglomerates at the north of Gunflint 

 lake, but in the true taconyte (which was originally a clastic obsidian sand) it causes 

 the globular structure characteristic of that rock. 



When this obsidian is not silicified it is greenish and isotropic, and was at first 

 attributed to foraminiferal origin, and was called glauconite. It evidently formed 

 sand beds of greater or less thickness distributed the whole length of the Mesabi 

 range. The "soft" iron ore, which is the same as this obsidian sand, though con- 

 verted to hematite, is sometimes 100 or 200 feet in thickness. 



Associated with the obsidian sand was also rhyolyte, which possessed the 

 streamed, contorted, banding characteristic of rhyolyte. This rhyolyte, as well as 

 its fragmental debris, being usually amorphous, uncrystallized mineral matter, was 

 subject to easy silicification. It has been hence converted to the banded jaspers 

 (jaspilyte) which are occasionally seen associated with the taconyte of the Mesabi 

 range but which are more abundant in the Vermilion range. 



The silica which permeated these igneous rocks was derived in part from the 

 ocean by chemical precipitation, the ocean probably being alkaline and perhaps, in 

 the vicinity of igneous activity, also heated. 



