48 PROCEEDINGS OF WASHINGTON MEETING. 



influences which eventually caused the precipitation of the soluble salts of iron 

 contained in them, and this action may have been greatly facilitated by the con- 

 siderable evaporation that probably took place. 



The form in which the iron was precipitated depended on the local conditions 

 surrounding each area : Where iron in the form of sulphate came in contact with a 

 reducing agent, or wdiere other salts of iron were in the presence of sulphuretted 

 compounds derived from decaying organic matter or from other sources, then the 

 iron was often deposited as sulphide (iron pyrites) ; otherwise the iron might have 

 been laid down as either oxide or carbonate, or as glauconite. Subsequent segrega- 

 tion doubtless often assisted in the accumulation of the ore in certain areas. 



Though the large bodies of iron ore now found in the Tertiary area are in the 

 form of oxides, there is decided evidence that they were originally segregated as 

 carbonate and sulphide. It is very probably, however, that the original deposition 

 may often have been as oxide, and that the forms of sulphide and carbonate were 

 produced during a subsequent segregation into nodules and layers. 



The nodular ores already described have doubtless been largely derived from the 

 oxidation of an impure carbonate of iron in the form of the so-called clayironstone. 

 This material is of common occurrence throughout the Tertiary strata, though it is 

 usually seen only in protected places, such as in well-borings, in some creek bluffs, 

 and in other places in which it has not been exposed for a sufficiently long time to 

 undergo oxidation ; while, where it has been so exposed, it has been converted to a 

 more or less hydrous sesquioxide. The strongest evidence of this derivation of the 

 nodular ores is that in many places they can be seen in the actual process of tran- 

 sition, and it is not an uncommon occurrence to find masses of the as-yet unox- 

 idized clayironstone forming the kernels of the nodules. Moreover, the masses of 

 ore are often composed of aggregations of angular geodes, the angles of which are 

 so arranged that if they were brought together they would form one solid mass of 

 geodes. In most of the unaltered clayironstone masses there are numerous 

 shrinkage cracks, and it seems probable that the shape of the angular geodes has 

 been regulated by the directions of these cracks, which caused the mass to be more 

 or less divided into separate parts, each part afterward funning a separate geode. 



The clay already mentioned as often occurring in the geodes doubtless represents 

 the residual insoluble product left after the oxidation of the clay-ironstone. 



This mode of derivation is by no means confined to the Tertiary ores : It is de- 

 scribed by many writers in iron ores in various Paleozoic horizons. Dr. T. Sterry 

 Hunt* explains the formation of the geodes by the gradual shrinkage in the 

 transition from carbonate to oxide of iron, causing a diminution of volume equal to 

 19.5 per cent of the original mass. The transition progresses from without inward, 

 forming layer after layer of oxide, often separated by spaces as a result of contrac- 

 tion, while in other nodules the whole shrinkage is represented by the central 

 cavity alone. Hence sometimes the concentric nodules ; at other times the hollow 

 geodes. 



The laminated ores, which are especially well developed in Cherokee county, 

 Texas, appear to have been derived largely from iron pyrites, assisted probably in 

 some cases by carbonate of iron and glauconite. As already stated, the laminated 

 ore directly overlies a large glauconite bed in which iron pyrites is of common 

 occurrence. In some few places, when natural conditions have protected the beds 

 from atmospheric influences, it is found that the pyrite is especially abundant at 



* Mineral Physiology and Physiography, ls.so, p. 262. 



