Kimball — Siderite-basins of Hudson Hive?" Ej)oc?t. 159 



more or less calcareous as well as ferruginous. While, on the 

 one hand, in the southern portion of the second basin well 

 under cover, the iron-stone has been metamorphosed into 

 sub-crystalline spathic siderite, weathering decomposition, on 

 the other hand, has wrought the partial alteration of all 

 exposed parts of the beds into limonite. Metamorphism of 

 amorphous ferrous carbonate into spathic siderite appears to 

 be due to the unimpeded crystallization of such parts of the 

 deposits as were the freest from siliceous admixtures in the 

 form of mechanical sediments. That this has resulted from a 

 condition of exceptional purity is well shown by numerous 

 analyses as well as by the fact of the absence of crystallization 

 from parts of the deposits richer in siliceous matter. All the 

 familiar phenomena of alteration of iron-stone into limonite 

 are well exhibited in weathered parts of' all the basins,' espe- 

 cially exfoliation of hydrous ferric oxide with the elimination 

 of silex and clay. These insoluble residuums are sometimes 

 preserved in situ as contents of drusy cavities. 



This remarkable series of ore-basins seem to owe their origin 

 to depressions on an in-shore mud bottom fed by waters from 

 decomposing basic rocks. From such waters ferric oxide was 

 precipitated along with mechanical sediments from the land 

 and calcareous sediments from the sea. Currents and occa- 

 sional perturbations introduced detritus, while vegetable and 

 animal life found conditions favorable for existence in degree 

 inversely to the predominance of ferric precipitate. This is 

 indicated by the presence in the ore of phosphoric acid in 

 inverse ratio to the proportion of iron, the metamorphic or 

 spathic ore of the southern part of the second basin alone 

 being below the Bessemer limit in phosphorus, and up to the 

 shipping standard in units of iron. Submergence of the basins 

 by rapid accumulation of sediments, and probably also by sub- 

 sidence below the range of atmospheric action was followed 

 by decay of buried organic matter attended by reduction of 

 ferric to ferrous oxide, whence ferrous carbonate in the pres- 

 ence of carbonic acid and absence of atmospheric oxidation. 

 To some extent, also, carbonate of lime has probably been 

 replaced by carbonate of iron. 



Senft's theory of the formation of argillaceous iron-ores 

 through saturation of sedimentary beds with ferrous bi-car- 

 bonate, and the formation away from atmospheric oxidation of 

 insoluble ferrous carbonate, while perhaps adequate to explain 

 certain occurrences of thin deposits and alternations of clay 

 iron-stone, notably in the coal measures, in circumstances of 

 emergence, fails to satisfactorily account for its formation on 

 a large scale under conditions distinctly pointing to submer- 

 gence. 



