EARLE, INTERBEDDED IRON ORE DEPOSITS 135 



do not thri\e in the still waters of inclosed basins but require agitated 

 waters in the open sea. lu places, also, the sediments contain a large 

 amount of water-worn material, fossils badly broken and coarse-textured 

 conglomerates. So, although we do see in places such testimony as Pro- 

 fessor Smyth (317) has suggested, yet we find also much evidence of 

 quite different conditions; and, therefore, it seems to the writer that 

 little importance can be attached to the supposed basins as an aid to the 

 •determination of the origin of these ores. 



THEORIES OF REPLACEMENT AND SECONDARY ENRICHMENT 



Persistence of Ore Seams 



The question of depth and distance from the outcrop to which the ore 

 is known to extend is an interesting one, and the facts are inconsistent 

 with the theories of secondary enrichmeftt and replacement, where such 

 theories depend upon leaching of slates and shales or vertical descent of 

 ground waters. One boring has shown good ores, 1,903 feet deep and 

 two and one-half miles from the outcrop ; another over 800 feet deep and 

 more than ten miles from outcrop and with a very low dip. The writer 

 is inclined to agree with Professor Smyth (loc. cit.) in part in regard to 

 such data; but as to using this great depth as an argument in favor of 

 original deposition, he cannot convince himself that it applies. Some of 

 the deposits in that event must have extended into waters of very con- 

 siderable depth and distance from shore; and as depth increased, the 

 amount of iron necessary for keeping up the same degree of richness as 

 nearer to the shore must have been great indeed. It would seem highly 

 improbable that these iron-bearing marine waters could circulate over 

 and through the sediments without becoming diluted in the great expanse 

 of water, as currents carried them far out to sea. The present writer 

 would, on the other hand, lay claim to the argument of great depth in 

 support of his ideas of origin, which differ widely from those of the advo- 

 cates of original deposition. 



Unreplaced Limestone 



Finally, as to the argument that some overlying beds of limestone 

 would be excellent for replacement of lime by iron and yet remain prac- 

 tically untouched with clean-cut contacts although in close proximity to 

 iron-bearing seams. Here again the writer agrees with Professor Smyth 

 that the facts are against the replacement theories as ordinarily ad- 

 vanced, especially since many layers of impervious rock lying in a more 

 or less horizontal position intervene between the ore seams and the sur- 



