128 C. It. Van Hise — Iron Ores of Michigan. 



degree this process had already gone before Upper Marquette 

 time. Whatever the time at which the work was done, the 

 process seems to have been as follows : The upper part of the 

 ore formation was traversed by solutions more extensively than 

 the deeper lying portions. It naturally follows that the fer- 

 ruginous material was in part deposited about and through the 

 minute particles of silica, reddening them and changing the 

 material from white chert to red jasper. In some places this 

 jasperization has extended deeper than in others and, as already 

 said, it sometimes abruptly stops at an impervious mass of 

 soap-rock. Prof. Pumpelly suggests that before the secondary 

 concentration which formed the ore-bodies, the bands of silica 

 of the ore-formation were white and perhaps in a partly 

 amorphous condition ; that at the time of this concentration a 

 partial recrystallization of the silica occurred, affording an 

 opportunity for the ferruginous impregnation so characteristic 

 of the jaspers. A microscopical examination shows that while 

 most of the silica of the entire formation is now individualized, 

 much of the iron oxide of the jaspers is concentrated about 

 the particles of quartz, and that numerous minute flecks also 

 occur within their interiors, thus giving support to Prof. 

 Pumpelly's suggestion. 



One or two questions remain to be considered : First, why 

 the ore is so frequently hard and specular along the contact 

 horizon or in the jasper and is usually soft within the ferrugi- 

 nous chert. Second, why the magnetites when present occur 

 at the contact horizon. 



An examination of the jasper associated with the hard ores 

 shows that crystallized hematite and magnetite often occur in 

 cavities formed by the removal of the silica. In such geodal 

 cavities these materials have been deposited in a granular crys- 

 talline condition. In the continuation of the process the 

 silica was wholly removed and its j)lace taken by the crystal- 

 line hematite and magnetite. The adjacent jasper also shows 

 that numerous cracks and fissures have been filled with hema- 

 tite or magnetite. The manner in which these veins of coarser 

 ciwstallized material frequently cut across the finer grained 

 substance which represent the iron oxides present before the 

 concentration, shows conclusively that they are secondary in- 

 filtrations. The formation of the coarsely crystalline hematite 

 and magnetite thus appears to be connected with the abund- 

 ance of iron-bearing solutions along the contact plane. 



Oftentimes also the hard ores are of the brilliant micaceous 

 or specular variety. In a hand specimen composed of rapidly 

 alternating layers of ore and silica, where the folding has been 

 severe, micaceous ore is often found between the rigid bands of 

 quartz. Along these ferruginous zones is seeu all the evidence 

 of slickensides, and the micaceous character of the ore is seen 

 to be due to the shearing to which it has been subjected. 



