136 ANNALS NEW YORK ACADEMY OF SCIENCES 



face. Yet the writer's own coiiceplion of the ori^iin of tliese ores is 

 much strengthened by these same conditions, which are indeed neces- 

 sary, according to his ideas, for the existence of the iron-bearing seams. 

 Because of these conclusions, it seems unnecessary to the writer to spend 

 further time on a theory tliat in the light of such conditions as have 

 been pointed out seems not only inadecjuate but impossible. 



« 



Secondary Enrichment Theories 



Under the head of secondary enrichment may be classed both ciiricli- 

 iiient due to replacements and that due to residual enrichment. Of the 

 theories of secondary origin, some depend upon leaching of soluble mat- 

 ter and a consequent enrichment of the iron-bearing deposits because of 

 relative insolubility of the iron, and others upon a combination of re- 

 placement with enrichment by a process of leaching of iron from over- 

 lying ferruginous shales and slates and replacement of the lime in ilio 

 underlying limestone by iron thus obtained. The most that can be said 

 regarding these possibilities is that undoubtedly these alterations luive 

 been made, but to a very limited extent, and such methods are wholly 

 inadequate to explain the distribution of the ore, as sliowii by icccut 

 borings and extensive underground mining, which have provetl tlint hard 

 ore does not change materially with depth, and that the above theories 

 only account for the very superficial fades called soft ores. Absence of 

 extensive exploration gave these theories plausibility and caused much 

 favorable comment upon them for a time, but more recent underground 

 mining has caused them to be more or less generally discredited. 



Sim.ple Replacement Theory 



c- 



We still have one well recognized theory to discuss before advancin 

 the theory of the writer, and that is simple replacement. The advocates 

 of this theory have seen extensive evidence of replacement of the calcite 

 by iron in the fossiliferous beds and have noted the replacement of the 

 lime cement by iron. Tn respect to the evidence advanced by these au- 

 thors, the writer is inclined to believe that to a large extent it is correct, 

 but in a few particuhirs he finds himself obliged to disagree with their 

 deductions. 



The first is the attempt to account for the iron as a leached product 

 from overlying shales. It seems incredible that such immense quanti- 

 ties of iron can have been derived from so limited a source. It also 

 appears, as Professor Smyth has well stated, that the intervening layers 

 of limestone which are comparatively free from iron would have offered 



