C. R. Van Hise — Iron Ores of Michigan. 131 



ondary concentrates which usually rest upon impervious for- 

 mations.* 



As instances among the mines figured by these authors may 

 be mentioned the following : The ore of the Ely mine is found 

 between impervious schists, the layers of which are inclined. 

 At the Stone mine the ore in its upper workings is in two 

 bodies, each of which rests upon green schists. In its deeper 

 workings these are found to come together and make a solid 

 mass of ore. The ore grades above into the jasper, which is 

 like a great horse separating the upper parts of the deposit. 

 In short, the relations so far as principles are concerned, are 

 much the same as shown in figure 8, taken from a deposit in 

 the Marquette district. At the Chandler mine the ore has its 

 greatest length along and rests upon as a foot- wall, a great mass 

 of greenstone. The upper boundary of the ore is irregular and 

 grades into mixed ore and jasper. It is said of the Ely that 

 the " ore is quite open to the action of percolating water," and 

 that the material shows the effects of crushing and folding. 

 The schists associated with the ores in the Vermilion district 

 are regarded by the Professors Winehell as well as by us as of 

 igneous origin, f 



The facts given by these writers then seem to show that the 

 ore is a secondary concentration, instead of that the " ore and 

 jasper" are "of similar and contemporaneous origin." Indeed 

 in the report upon the Vermilion ores, published soon after 

 returning from the field, Prof. 1ST. H. Winehell held, although 

 this position was later abandoned, that the iron ores are later 

 than the siliceous jasper and probably produced by metasom- 

 atic change. 



It is fully recognized that the explanation above given for 

 the ores of the Lake Superior districts in which mining is now 

 being done, is not wholly if at all applicable to the titaniferous 

 magnetites of northeastern Minnesota associated with the great 

 gabbro flows near the base of the Keweenaw series. In the 

 interstices of these magnetic ores are olivine, augite, feldspar, 

 and often secondary quartz, the whole having a completely 

 crystalline interlocking structure ; in other words, they are no 

 more than a very magnetic gabbro. Since these ores occur as 

 basal horizons of the great gabbro flows, it has always seemed 

 to me that this class of deposit is of direct igneous origin. In 

 the crystallization of basic rocks, magnetite is one of the early 

 minerals to separate, and in the immense masses of gabbro, 

 before the magma has solidified the crystals of magnetite have 

 slowly settled by virtue of their superior specific gravity to the 



* Fifteenth Ann. Kept. Geol. and Nat. Hist. Survey of Minn, for the year 1886, 

 pp. 24, 25, 221, 235, 255. Bulletin No. 6, Geol. and Nat. Hist. Survey of Minn., 

 pp. 63-67. 



•{■Fifteenth Ann. Eep. Minn., pp. 231, 245, 246; Minn. Survey Bull, vi, pp. 64. 

 394, 398, 399. 



