ORIGINAL SOURCE OF THE ORES 



493 



occurs in pockets in eroded depressions in the limestone. These are in 

 many instances at the bottom of former sink-holes at the surface, and, as 

 shown by the present conditions, these sink-holes may collect the drainage 

 from areas many acres in extent. Both the surface and the subterranean 

 drainage would naturally transport the iron both while in solution and 

 later in the lump form toward the sink or cavity. Further evidence in 

 support of this view is the customary mode of occurrence of the ore in 

 the erosion cavities which are separated by more or less barren areas. 



While the percentage of iron in the limestone is small, it forms a com- 

 paratively high ratio of the insoluble residue, averaging 24 per cent for 

 the whole 16 analyses. Considering the proportional loss of iron to be 

 no greater than that of the other materials, this would apparently furnish 

 a sufficient supply of iron for 

 all the ore deposits. ?-.. 



Analyses of the limestones / ,,..,, \ 



(see page 494) indicate that / /..O* \ 



the iron occurs both as the / / .'\-, , \ \ 



carbonate and as the sul- / • / / \ \ \ \ 



phide, with the carbonate in •' / ,■ •' -., \\ \ \ 



excess in all cases. The an- / .' ,' : / \ '. \ \ \ 



alyses, while not many in . ; ■ / / / f\ \ \ \ \ \ 



number, are from several / / / /' / ; \ \ \ \ \ \ 



quite widely separated local- 

 ities and presumably repre- 

 sent fairly well the relative 

 amounts of the two com- 

 pounds, although there may 

 be local enrichments of iron 

 not shown by the analyses. 

 As mentioned under associated minerals, both the carbonate and the 

 pyrite occur in limited quantities in a few places, associated with the 

 limonites, but they are evidently secondary forms, and cannot be con- 

 sidered as indicating in any way the original form of the iron in the 

 limestone. No pyrite crystals have been observed in any of the many 

 limestone exposures examined by the writer. The intercalated hydro- 

 mica slates in many places contain considerable iron, apparently in the 

 form of silicate, which Bischof states is soluble in carbonate waters, and 

 part of which is presumably added to the supply from the limestone. 



The conclusion is that some of the iron in the limonite ore deposits is 

 derived from the overlying Ordovician shales and some from the under- 

 lying Cambrian slates, but the chief source is the diffused iron carbonate. 



Figure 7. 



-Cross-section of Anticline in JSittany Valley, 

 Pennsylvania. 



