44 Van Rise— Iron Ores of the Penokee- Gogebic Series. 



which came from the oxidation of carbonate in place, must 

 have come from such a source, while it is probable that a large 

 part of the deposits located at considerable depths have been 

 stored from rock which has been broken down and scattered 

 far and wide. These are not so much concentrations of iron 

 oxide which were originally deposited as a carbonate above 

 them, as from the layers which stretched to the southward, 

 but which were subsequently by upturning placed over the 

 ore-bodies. Also the large proportion of silica now found 

 near the surface, and particularly in the southern half of the 

 belt, is probably much greater than was here originally present. 

 The silica of these highly cherty rocks associated with the ores 

 may represent a concentration from many hundreds, or even 

 thousands of feet of rock which have been swept away, just as 

 the ore-bodies are concentrations from the iron carbonates 

 which these same rocks contained. A portion of the silica 

 may have come from the alteration of the dyke-rocks contained 

 in this removed material, but doubtless most of it came from 

 the original cherty carbonate. 



The only exceptions of moment to the facts as assumed in 

 the above discussion, are the occurrence 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 dyke- rocks. It has been said 

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

 north of the fragmental 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 in 

 their downward passage before reaching the fragmental quartz- 

 ite. Here the relation of the ore to the dykes, and its other 

 characters, are the same as when the ore is found upon the 

 fragmental quartzite. It is of interest to note that in one of 

 the largest mines of the region, the Colby, as shown by Fig. 4, 

 the north and south deposits have as their basement the same 

 great dyke, the two ore-bodies being separated merely by a 

 gigantic horse of rock which served as the impervious layer to 

 form the foot-wall of the north deposit. The explanation given 

 of the origin of the ore found upon the fragmental quartz- 

 ite, applies perfectly to these north deposits with the modifica- 

 tions above indicated. That there are layers of the iron-bearing 

 formation which are not readily pervious, and therefore become 

 basements along which the down-flowing waters passed, is not 

 at all strange. It would be stranger if, in a thickness of water- 

 deposited sediments of 800 feet, there were no layers which, at 

 least for a short distance, were effectual barriers to the passage 

 of percolating waters. The chemistry of the process of con- 

 centration of the ore-deposits east of Sunday Lake, in all 



