]30 ANNALS NEW YORK ACADEMY OF SCIENCES 



Eckel (37, J). 33) . claims that I\us.sell was niistakcii and tliat ihc Al- 

 tai la ores do not vary witli depth. 



Replacement Theory 



Tlie replacement theory holds that the iidii content of the Clinton 

 rocks in the foi-m found at present has resulted from a replacement of 

 lime carbonate by iron, long after the roc;!^s had been deposited. The 

 iron was introduced by descending waters charged will) imn wliich they 

 had dissolved out of overlying ferruginous rocks. 



J. J. IJutledge (293, pp. 254-2r)5) : 



"The conclusion that tlie iron content of the Clinton iron ore beds of Stone 

 Valley, Penn., is due mostly to rei)lacement by removal and enrichment, seems 

 unavoidable, when it is considered that but a portion of the fossiliferous lime- 

 stone or of the hard ore is found to contain iron oxide when examined in thin 

 sections under a microscope. Calcite cement makes ui> liy fai- the greater 

 portion of the section. 



"An analysis of the limestone shows that it contains but 2.12 per cent of 

 FeO and 2.35 per cent of FcjOs. These seem much too small an iron content 

 to yield as rich an ore as the soft ore, simply by the removal of the calcium 

 carbonate. Field conditions such as the occurrence of weathered shales, dull 

 colored clays and iron-stained sandstones, prove that the action of replacement 

 is still going on. 



"The iron came originally from the overlying shales and was transferred 

 later to the beds of fossiliferous limestone." 



The following reasons are advanced by Eutledge for the statemeni 

 that the addition of iron was not due merely to the i-emnval i>f the lime- 

 stone, as would be the case under the enrichment theory : 



"(rt) The character of iron ore concretions where associated with silica. 



"(&) The invariable association of the soft ores (rich) with the leached, 

 decolorized shales and the hard (lean) ores with unweathered. bright red 

 shales. 



"(c) The relations of the ores to the shattered sandstones and to the topo- 

 graphic situation of the ores. 



"{d} The fact that analogous replacements are now going on in the Medina 

 formation. 



"(e) The observed in'ogressive steps in the transformation of the limestone 

 to an ore, which may be followed in the field, in the sections under the micro- 

 scope and in chemical analyses. 



"(/) The absence of conditions such as local crumpling, including a .shrink- 

 ing of the strata, pointing to a relative rather than an .ibsolntc enrichment 

 of the ores." 



I. C. White (198, pp. 135-137) : 



"The iron has evidently been filtered into the bed as the lime has beea 



