290 THE PENOKEE IRON BEAKINU 8E1UES. 



nally present. As erosion steadily carried away the upturned edge of the 

 ore formation, percohxtiug waters took in solution a jjart of the silica which 

 it contained, penetrated the deeper parts of the ore formation, and from it 

 silica was precipitated, as explained above, by the ore-forming process. The 

 highly cherty rocks associated with the (^res may then represent a concen- 

 tration from many hundreds or even thousands of feet of rock which has 

 been swept away, just as the ore bodies are concentrations from the iron 

 carbonates which these same rocks contained. A portion of this silica niay 

 have come from the alteration of dikes contained in this removed material, 

 but doubtless much of it came from the original cherty carbonate. 



Exceptional localities. — At three ])laces iron carbonate is found near the 

 base of the formation: Sec. 6, T. 45 N., R. 2 W., Wisconsin, Sec. 13, T. 47 

 R. N., 46 W., Michigan, and Sec. 18, T. 47 N., R. 45 W., Michigan. At the 

 iirst of these localities the rock contains a considerable quantity of clayey 

 material. At the second and third, the rocks are exceedingly dense, finely 

 laminated, and bear contortions, as shown in the exposures, without breaking 

 the stratiiication. In all of these places, then, the original cherty iron car- 

 bonate had exceptional characteristics, which prevented ready penetration 

 by water. The presence of an altered carbonate at these low horizons is, 

 however, interesting on two accounts. In the first place, it greatly 

 increases the probability that the whole base of the formation, which now 

 contains so little carbonate, was once a cherty iron carbonate ; secondly, , 

 one of these exposures shows how completely the presence of a little clay in 

 a rock may prevent the action of percolating waters, even when the remain- 

 der of the belt is completely altered by such agencies. 



The only other exception to the facts as assumed in the above discus- 

 sion are the occurrences of iron ore at a higher horizon than the foot-wall 

 quartzite and the ore bodies east of Sunday lake, which so far as present 

 developments go are not known to be associated with dike rocks. It has 

 been said that those ore bodies in the main part of the range which are 

 north of tlie fragniental quartzite have a well defined cherty quartz foot- 

 wall of a regular character. In these cases this quartz rock has served as 

 the plane which checked the waters on their downward passage before they 

 reached the fragraental foot wall which constitutes the basement of the 



