(J. B,. Van Hise — Iron Ores of Michigan. 121 



Of course the entire contact horizon of the quartzite and 

 ore formation is not occupied by ore-bodies as represented in 

 the generalized figure. The deposits occur at places along the 

 contact where sharp subordinate folding has occurred, or 

 where the jasper formation is broken by cross joints, or where 

 a soapstone dike cuts the contact plane forming a trough ; or, 

 finally, by a combination of two or more of these phenomena. 

 The last is well illustrated by the position of the great deposit 

 of the Republic, which is at the southeast corner of the horse- 

 shoe, where the curve, instead of being gentle, is abrupt, caus- 

 ing the jasper formation to become sharply plicated and often 

 fractured (fig. 3), and where there are numerous dikes of 

 soap-rock which usually form one of the boundaries of the 

 ore-deposits. The mining engineer remarked that the ore- 

 deposits cannot live long when they become separated from the 

 soapstone. Upon the east side of the Republic horse-shoe the 

 ore-bodies are in chimney-like forms which often continue for 

 a considerable depth and which are often usually rather sharply 

 separated from the banded ore and jasper adjacent. (Fig. 4). 

 However, here, as at the great deposits, the boundaries are 

 generally found at fractures or flexures. 



A detailed examination was made of the manner in which 

 the change occurs between the jasper and the ore. While, as 

 remarked, the ore often terminates somewhat abruptly, it also 

 frequently grades into the jasper. In following a jasper band 

 toward the ore it was found that instead of remaining solid it 

 becomes porous and frequently contains considerable cavities. 

 These spaces in the transition zone are lined with crystalline ore. 

 In passing on toward the ore-deposit more and more of the silica 

 is found to have been removed and the ore has replaced it to a 

 corresponding degree. An examination at many localities led 

 to the conclusion that the transition from the banded ore and 

 jasper to the ore takes place as a consequence of the removal 

 of silica and the substitution of iron oxide. Often in these 

 cases the fine-grained part of the ore is that of the original 

 rock, while the coarser material is the secondary infiltration. 



The above details as to the occurrence of the hard ore at 

 Republic are typical of this class in the remainder of the dis- 

 trict. In the Ishpeming-^Negaunee area, for instance, the ore 

 often terminates along a slip or joint crack. Also the ore- 

 bodies are likely to be found where the folding or crushing of 

 the jasper ha.s been severe, and especially where these phe- 

 nomena are accompanied by soap-rock. 



The ore-bodies between the ore formation and overlying 

 quartzite are not always wholly in the jasper, but often extend 

 upward to a greater or less degree into the quartzite-conglom- 

 erate, and some deposits wholly occupy the horizon of the 



