PLATE LXXVI. 



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



The Keewatin is nonconformably overlain by the Animikie along the southern 

 slope of the Giant's range. The only known place in Minnesota where, under such 

 relations, both formations are iron bearing is in sec. 11, T. 59-14. This repeats the 

 structural relations so common in the Marquette district of Michigan. The iron ore 

 of the Keewatin is jaspilitic rather than taconitic. Other rock of the Keewatin in 

 the immediate vicinity is not ore, but greenish gray, tine grained, and embraces 

 crystals of striated feldspar, evidently one of the fragmentals seen largely in the 

 Keewatin in other places. The jaspilitic ore varies to a quartzyte of fine grain. 

 Indeed, the most of the ridge exposed near the centre of section 11, as mentioned, is 

 composed of nearly pure " chalcedonic " quartz, the structure being nearly vertical. 

 The excavation formerly known as the Mallmann mine, a little northeast of this 

 jaspilyte ridge, is in Animikie strata, dipping southeastwardly. 



The quartzyte and taconyte at the bottom of the Animikie seem not to have 

 uniform and independent stratigraphic positions, but in some cases they are found 

 to alternate or to be wanting, one or the other. Besides, it is apparent that a large 

 part of the quartzyte is not of detrital origin, but more probably of the nature of a 

 chemical precipitate, or of chemical and detrital nature combined. When subjected 

 to metamorphism such a rock becomes finely crystalline gneiss. 



That exploration for ore in N. W. J sec. 18, T. 59-14, supposed to be in the Ani- 

 mikie, is probably wholly within the Keewatin, and the granular fine structure of 

 the ore is not due to a taconitic original structure, but to a fragmental jaspilyte such 

 as that seen in the conglomerate of the "south ridge" at Tower, the horizon being 

 at or near the bottom of the Upper Keewatin. This is the most likely explanation, 

 based on a re-examination of the hand samples collected by Mr t Meeds. 



The black slates are rendered crystalline, and converted to a cordierite mica 

 schist. They appear thus in N. E. ^ S. W. ^ sec. 9, T. 58-14, along the shores of 

 Partridge river. 



The gabbro is exposed in the vicinity of Allen and Allen Junction. It rises in 

 low, massive, gray domes, but in some places it appears to be undergoing a bouldery 

 disintegration, which is due to some original variation in the grain and texture of 

 the rock itself. This original difference in the gi'ain and texture of the gabbro is to 

 be ascribed to a coarsely fragmental or pebbly or agglomeratic composition of the 

 greenstone of the Keewatin, from which the gabbro has been generated by intense 

 metamorphism, but little short of fusion. N. H. w. 



