Van Hise — Iron Ores of the Penokee- Gogebic Series. 45 



probability, is like that of the typical deposits of the range. 

 Their concentration is apparently, however, more nearly analo- 

 gous to the narrow seams of ore described in the early part of this 

 paper than to the typical deposits. The formation here appar- 

 ently being cut by no impervious dykes, the waters have not 

 been carried over to the quartzite, thus forming main channels 

 of percolation, but the comparatively small ore-bodies have 

 developed here and there as favorable conditions for concen- 

 tration occurred.* 



The above explanation of the origin of the ore-deposits ac- 

 cords well with the facts of their occurrence, and also with the 

 idea that the iron formation deposits were originally an impure 

 cherty carbonate of iron. It explains perfectly the peculiar posi- 

 tion of the ore-bodies with reference to the dykes and the foot- 

 wall quartzite ; it explains their presence in a similar position 

 in the few instances in which the deposits are north of the 

 fragmental quartzite ; it explains the flat wedge-shaped charac- 

 ter of the ore-deposit ; it explains the nature of the ore, a soft 

 somewhat hydrated hematite, bearing more or less of manga- 

 nese ; it explains the excess of manganese which the ore car- 

 ries beyond the amount found in the unaltered carbonates, and 

 its relatively greater abundance in the south deposits ; it ex- 

 plains the presence of large quantities of unaltered carbonates 

 in the upper horizons of the iron formation, the gradual lessen- 

 ing of this carbonate in passing to lower horizons, and its 

 absence at the base of the formation ; it explains the large per- 

 centage of silica contained in the greater part of the lower 

 horizons and the low percentage at the apices of the troughs. 



Probable extent in depth of ore-bodies. — This explanation of 

 the origin of the ores may throw some light upon the depth to 

 which the ore-bodies extend. The fact that all of them have been 

 traced to the erosion surface, is favorable, rather than otherwise, 

 to their extending to a considerable depth. The ore-bodies, at 

 the depths now penetrated, must have formed almost wholly 

 before the sweeping away of the rocks of the iron formation 

 above them. They could, then, have received but little of 

 the iron they contain since the end of the Glacial epoch, for 

 erosion was then terminated by the mantle of drift dropped 

 over the region. The deposits, with some degree of proba- 

 bility, may be sai'd to continue to a depth at which the agencies 

 of concentration could effectively work. Whether this distance 

 will be found to be measured in hundreds or thousands of feet, 

 the data at present are too scant to indicate. I am inclined 

 to believe, however, that they may be depended upon to con- 



* As bearing upon the truthfulness of the above theory as a whole, it is an 

 interesting fact that the practical miners, in prospecting, eagerly follow under- 

 ground water channels, hoping that they will lead to ore-deposits. 



