292 THE PENOKEE IRON-BEARING SERIES. 



more or les.s of manganese; it explains tlie excess of manganese which the 

 ore carries beyond tlie amount found in the unaltered carbonates and its 

 relatively greater abundance in the south deposits ; it explains the presence 

 of large quantities of unaltered carbonates in the upper horizons of the iron 

 formation, the gradual lessening of this carbonate in passing to lower hori- 

 zons, and its absence at the base of the formation ; it explains the large 

 percentage of silica contained in the greater part of the lower horizons and 

 the low percentage at the apices of the troughs. 



The exceptions of carbonate near the base of the formation, the 

 occm-rence of ore deposits at horizons above the foot-wall quartzites, and 

 the unusual deposits east of Sunday lake, are all due to exceptional char- 

 acters at these places. These exceptions, with the ready explanations, are 

 thus rather in favor of than against the general idea of the concentration 

 of the ores. 



Probable extent in clcjifh <if ore bodies. — This explanation of the origin 

 of the ores may throw some light upon the depth to which the ore bodies 

 extend. The ftict that all of them have been traced to the erosion surfoce 

 is favoraljle, rather than otherwise, to their extending to a considerable 

 depth. The ore bodies at the depth now penetrated nmst 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 district. The deposits may be said, 

 with some degree of probability, to continue to a depth at which the 

 agencies of concentration could effectively work. In estimating the dei)th 

 it must be remembered, in the part of the range in which the ore deposits 

 are close together nnd parallel dikes consecjuently not far apart, that the 

 deeper dikes will l)e screened in part from percolating water by overlying 

 dikes. When the dikes are very deep an overlying dike may pene- 

 trate the u})per slate to the north before reaching the surface in this 

 direction, and the lower dike can receive iron ore, if it holds any itt 

 all, only by the creeping down of the surface waters from the area 

 between the outcrops of the two dikes. Whether tlie depth of the ore 

 bodies will be found to be measured in hundreds or thousands of 



