■tOO Decomposition of Iron Pyrites'. 



direct derivation in many cases, by oxidation, from ancient beds 

 of pyrites, as claimed by Hunt and Prime 78 , and even of such 

 beds in situ, as held by Shepard ;md Percival, e. (/.', when the 

 two materials lie in contact, as in Carroll County, Virginia, 74 etc. 



More recently, another theory has been advanced by J. D. 

 Dana 75 to account for these iron-ores, which may be briefly pre- 

 sented in the following quotations : 



" But the nearly total absence from the ore of sulphur (seldom 

 over one-tenth of a per-cent.) appears to be evidence that pyrite 

 played a very subordinate part in the production of limonite. 

 It is a question of interest, whether the iron was in the state 

 of carbonate when the deposits were originally made by sedi- 

 mentary action, or, whether in some different state, from which 

 it was converted into carbonate as one of the results of meta- 

 morphism." 



He apparently inclines to believe that it has been caused by 

 drainage deposits of iron carried into the marshes as carbonate, 

 a "result no doubt favored by the excess of carbonic acid in the 

 Lower Silurian atmosphere and waters," this action having taken 

 place during some epoch of long-continued marshes during 

 which the schists were laid down. 



In a more recent "note on the making of limonite ore beds," 76 

 Prof. Dana states the view he is disposed to favor as to the 

 origin of the irregular ferriferous areas in the limestone forma- 

 tion, as follows : 



"The stratigraphical change, in the region, from limestone 

 to slate, indicates that a change took place in the era of their 

 formation from limestone-making seas to mud-distributing 

 seas. During the transition from one to the other, iron was 

 washed down from not distant land, in the state of bicarbon- 

 ate or a salt of an organic acid, over limited areas of the 

 calcareous deposits. These areas, so invaded by the iron solu- 

 tion, during the transition-epoch, were within interior seas or 

 basins, or marshes, half shut off from the ocean. The calcare- 



73 Trans. Am. Inst. Min. Eng. (1874-75), III, 410. 



14 Idem (1876-77), V, 82. 



™ Am. Jour. Sci. (1877), 3, XIV, 139. 



' 6 Idem (1884), 3, XXVIII, 398. 



