MUSEUM OF COMPARATIVE ZOOLOGY. 



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The McComber mine, Negaunee, is a mine of "soft hematite"; this, 

 in connection with others lying along the same line, is near the "dio- 

 rite." Part of these mines, however, lie between two hills of the "dio- 

 ritc." Two deposits occnr on the McComber property, separated by a 

 dike of the "diorite." The ore here, like that in the Salisbnry mine, 

 seems to occnr only where the rocks have been shattered and acted npou 

 by water. The jasper has decomposed (244+), yielding a hydrous 

 silicate of alumina (Kaolin, Brush) (2454-, 24G+), quartz, hydrous and 

 anhydrous oxide of iron, etc. The minerals associated with the ore are 

 evidently water deposits, —barite (253), rhodochrosite (249H-, 250+, 

 25 1+, 252+), manganite (247+, 248+), etc. Figure 25 shows the fornia- 

 tion of the ore under a jasper cap, the water working in by the means 



of fissures on both sides. 



The Pcndill mine, worked for a similar ore, contains much botryoidal 

 limonite (254). In one part the ore was worked out upon the top and sides 

 of an oven-shaped mass, the ore following the curvature of the oven. 

 This is similar to a form observed in the Lake Superior mine, associated 

 likewise with limonite. In the Jackson mine "soft hematite" occurs in 

 places, and it is seen to be associated with and to pass directly into the 

 jasper and " hard ore," showing that they were originally the same. Water- 

 deposited quartz (vein-stone) w^as found in places in this "soft ore," and 

 a specimen was taken for microscopic examination (27G), for the pur- 

 pose of seeing whether it contained fluid cavities or not. It is found to 

 be full of inclusions, most of which contain bubbles. These bubbles 

 were, in the majority of cases, seen to exhibit motion of greater or less 

 rapidity. Some were seen containing double inclusions, and vapor cavi- 

 ties were observed. All would indicate that the water by which the 

 quartz was deposited was at a higlier temperature than the rock has 

 at present. This then would seem to confirm the views of all the 

 writers quoted, as well as of ourselves, that these ores are derived- from 

 the decomposition of the ferruginous, siliceous rocks. That this is the 

 correct view of their origin it would seem Dr. ITunt denies.* 



The various soft ores are partly true hematites and partly limonites, 

 mixed with more or less impurities, but of course, in goncral, have none 

 of the characters of an ordinary bog-iron ore. 



Our theory, of course, depends upon the relation of the " dioritcs " 

 to the jasper and ore, which near Ishpeming and Negaunee is some- 

 what uncertain ; yet there is but little doubt that the "diorite " is the 

 later. The dip of the jiisper increases as it approaches the "diorite," 



* Geol. of Wiac, III. 660. 



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