498 T. C. EtOPItlNS — CAMBRO-StLtfRiAN LIMONITE ORES 



below. There is probably no appreciable addition of ore at the top of 



the mass or any marked enrichment except on the steep slopes, where 

 there is little or no vegetation, and the rivulets wash away part of the 

 imbedding clay into the surface streams. 



There is probably to some extent a segregation of the iron oxide into 

 lump ore in the residual material, but I suspect that any such segrega- 

 tion is largely confined -to the margin and bottom of the mass, as the 

 body of the clay-ore mass is not readily permeable to water. 



The conclusion is that the diffused iron of the limestones and slates 

 is segregated into nodular masses, pipes, and sheets of ore in large 

 measure in the limestone or in the seams and cavities in the same and 

 in or on the top of the slates previous to the final dissolution of these 

 rocks. The leaching away of the limestone leaves the ores scattered 

 through the residual material. The segregating of the oxide may be 

 continued in the residual clays, especially around the margins, but less 

 actively than in the original beds. 



Summary 



Extensive deposits of limonite ores occur in the residual clays on the 

 Ordovician and Cambrian limestones and slates in the Great valle}', Nit- 

 tany, Kishacoquillas, and Chester valleys, in central and eastern Penn- 

 sylvania. The ores are hydrous ferric oxides, consisting largely of 

 limonite, associated with which are local occurrences of turgite and 

 goethite and very limited quantities of hematite, magnetite, pyrite, and 

 siderite. The ores are associated with manganese ores, wavellite, quartz, 

 chert, and fluorite. They occur.as pipes, shells, nodular and brecciated 

 masses, and irregular fragments mingled with more or less residual clay 

 and sand lying in irregular pocket like deposits of varying sizes in cav- 

 ities on the limestones or on beds of white clay or sandstone. 



The original source of the iron is primarily the Cambro-Ordovician 

 limestones and slates, with smaller quantities from the overlying Ordo- 

 vician and possibly Silurian strata and the underlying slates and quartz- 

 ites. The iron occurs in these strata as carbonate, sulphide, and silicate, 

 the first being probably the most common. 



The segregation of the diffused iron into the ore lumps is brought 

 about by the meteoric waters. The higher oxide is reduced by the 

 organic acids. The ferrous oxide is taken into solution by the organic 

 and carbonic acids, possibly sulphuric acid in some measure. The iron 

 is precipitated from the solution in part as the hydroxide, in part as the 

 carbonate, which is later oxidized. Some of the ores have been concre- 

 tionary segregations, probably as the carbonate, in the original rock, and 



