PLATE LXXIV. 



THE MOUNTAIN IRON PLATE OF THE MESABI IRON RANGE, 1899. N. H. WINCHELL. 



The granitic area, in this plate, has not been thoroughly examined. The topog- 

 raphy here is that of an Archean granite and gneiss somewhat modified by a thin 

 spreading of drift. But along the rough tract known distinctively as the Giant's 

 range the drift is more abundant and frequent boulders are on the surface. South 

 of the granite, while the elevation is less, the boulders are perhaps more common, 

 one of the most notable kinds being a fine-grained, white quartzyte which, at a 

 distance, appears like marble. These are from a quartzyte that accompanies, and 

 usually underlies, the taconyte, but quite different from the quartzyte that causes 

 the falls of Pokegama which, as already stated, is probably later than the age of the 

 Mesabi ore. South from the ore-bearing rocks the surface slopes gently to the flat 

 land and finally to the swampy tract which is included in the area of glacial lake 

 Upham. 



The first discovery of valuable iron ore on the Mesabi range was made at the 

 Mountain Iron mine. It was by Capt. J. A. Nichols, in November, 1890. A shaft 

 was sunk through fifteen feet of drift materials and entered a rich, soft hematite. 

 This was but a short distance south from an outcrop or nearly vertical bluff of jas- 

 pery ore which had invited the attention of several explorers, but which had not 

 rewarded them with satisfactory results. This bluff is near the granite and probably 

 lies on the granite, or, at least, on the Archean. In the light of later interpretations 

 of the features of the Mesabi ore, this bluff is supposed to represent a rhyolitic lava, 

 silicified, lying near the old beach of the Animikie ocean, and from it was derived a 

 large amount of basic obsidian sand, which was carried into the ocean and distributed 

 in shallow water near by. The old, glassy rhyolyte, and the sand which it produced 

 under the action of the beach line, have both been converted to siliceous and ferru- 

 ginous rock, one making a jaspilyte and the other granular taconyte, the latter at 

 this place varying to a rich pisolitic hematite. 



The ore-bearing rock is several hundred feet in thickness. The ore is not uni- 

 formly disseminated through it, but occurs in large, irregular lenses, dependent on 

 not only the existence of the original beds of greensand, i. e., obsidian sand, but also 

 on the ease with which water could enter them and carry on the progressive chemical 

 alteration which has resulted in iron oxide and silica. Such alteration has produced 

 a shrinkage in the bulk of the strata, at least when the change is wholly to oxide of 

 iron, and the strata, at the present time, show a prevailing dip toward the centre of 

 such ore lenses. 



