Van Hise — Iron Ores of the Penokee- Gogebic Series. 37 



It is evident they incline to the east. The ore-bodies lie in 

 the apices of these roughly shaped troughs, figs. 3, 4, 7. Each 

 deposit of ore in following a trough will evidently be at differ- 

 ent depths at different places east and west, depending upon the 

 nearness of the dykes to the surface. All ore deposits in the 

 position described would reach the surface if the underlying 

 dykes dip to the east or to the west. As a matter of fact, many 

 of them were found at the rock surface, but others were found 

 after cutting an overlying rock. However, at present (October, 

 1888), mining developments have traced all deposits which are 

 large enough to warrant working, with two exceptions, to the 

 surface, and these exceptions are newly discovered deposits, 

 which in all probability will be trace'd to the surface in the 

 future. As would be expected, it is also true that the devel- 

 opment of the deposits which were originally found at the 

 surface have carried them, in every case in which they are of 

 any magnitude, below the surface of the country rock. Both 

 of these facts, the tracing of the ore-deposits discovered at 

 depth to the surface, and those discovered at surface beneath 

 rock, are inevitable deductions from what has preceded. 



Rock above the ore. — The rocks which are found above the 

 ore-deposits are the ferruginous cherts — the rocks which have 

 been spoken of as the characteristic ones near the base of the 

 iron-bearing member throughout the area in which the ores 

 occur. The upper boundary of the deposits differs from the 

 cmartzite and dyke-boundaries, in that the change from ore to 

 the cherty rock is a transition instead of an abrupt one. In 

 passing upward through an ore-deposit, as its border is reached, 

 the ore becomes mixed with chert until so poor in iron as to 

 become unsalable. In passing still farther upward, the amount 

 of chert becomes greater, until a fractured chert and iron ore, 

 known to the miners as " mixed ore," is found. In passing up 

 still farther this mixed ore grades into the ordinary ferrugi- 

 nous chert of the lower horizons. 



To summarize then, the boundaries of the ore-deposits are 

 to the south, either fragmental cmartzite or ferruginous quartz- 

 rock in the ore-formation — generally the former ; under the 

 ore, the dyke rocks; and above the ore, the typical ferrugi- 

 nous cherts of the region.* 



The horizon above the ferruginous cherts, is in most cases a 

 regularly banded red ferruginous slate. This slate is composed 

 of chert and iron peroxide and is as regularly bedded as the 

 nnaltered carbonates. Above this slate, constituting the upper 

 horizon of the ore formation, are often found cherty iron 

 carbonates. While this section is known to occur at several of 



* Evident practical deductions for carrying on prospecting and mining follow 

 from the foregoing, but in this paper my space is too limited to give them. 



