Van Hise — Iron Ores of the Penokee- Gogebic Series. 11 



tie rock. These joints do not affect the underlying slates, for 

 these thinly laminated clayey rocks are so. flexible that under 

 the slight bowing which they have received, they are scarcely 

 fractured at all. 



Process of Concentration.— Am attempt will now be made 

 to trace the passage of percolating waters through the inclined 

 layers of the iron formation. Fig. 9 is a section showing the 

 condition of this member at the present time at the surface, 

 and illustrating how this state of affairs was reached. The 

 strata of the formation are now exposed by their dipping at a 

 high angle, 65°, to the north. The whole Penokee-Gogebic 

 series, more than 13,000 feet thick in some places, of which 

 the iron formation is a part, is exposed in the same fashion. 

 Therefore thousands of feet of the iron member have certainly 

 been carried away by erosion. The figure assumes that about 

 2000 feet have been eroded from this member since it was 

 upturned. It would, however, make no difference with the 

 argument if this erosion occurred during the time of the upturn- 

 ing. The upper part of the figure represents the surface 

 of the iron formation, and a part of the underlying and overly- 

 ing rocks at some past time. Near the bottom of the figure 

 is the present land surface, Showing the succession of rocks 

 from north to south which are now actually found. A tran- 

 sition from unaltered cherty carbonate to completely decom- 

 posed carbonate is noted. At the time when the upper sup- 

 posed land surface was an actual one, the present surface would 

 be but little exposed to the action of percolating water. It 

 could not pass through the slates which overlie the iron forma- 

 tion ; neither could it get in through the underlying feld- 

 spathic quartz-slates. Therefore, most of the water which at 

 that time was able to reach the present land surface, must 

 have done so by passing down, along and through the layers 

 of the iron formation itself. The dotted broken line repre- 

 sents a perpendicular course which the water would follow 

 were its passage not deflected by the laminated character of 

 the rocks ; but there would be a tendency for this water to 

 follow the bedding, so that, entering the iron formation at its 

 uppermost horizon, it would follow an irregular course marked 

 by a broken line and would reach the foot-wall quartzite at the 

 line of the present surface of the country. It is immaterial 

 to the argument whether this line ought to vary farther from 

 the perpendicular than marked or not ; for, in any case, nearly 

 the whole of the present surface of the iron formation would 

 escape the percolating waters, or if not this surface, some 

 other yet lower down. It is, however, probable that the 

 lowest horizon would not thus escape until a great depth 

 was reached; for the waters entering the formation, would 



