THE IRON SERIES (IN PART). 85 



York and Adains counties, Pennsylvania, found the hydromica 

 slates filled with the casts of pyrite crystals, and held these to 

 have been the sources of the iron, by affording ferrous sulphate 

 and sulphuric acid. The latter reacted on the alkali of the slates, 

 producing sodium sulphate. This, meeting calcium carbonate, af- 

 forded calcium sulphate and sodium carbonate, which later precipi- 

 tated the iron. Calcium carbonate alone is, however, abundantly 

 able to precipitate iron carbonate and oxide from both ferrous and 

 ferric sulphate solutions (even when natural) without the intro- 

 duction of the alkali, although this might account for the alteration 

 of the slates. 1 



2.01.24. J. D. Dana has written at length on the New England 

 and New York deposits, and finds them always at or near the junc- 

 tion of a stratum of limestone, proved in many cases to be ferrif- 

 erous, and sometimes entirely siderite, and one of hydromica 

 slate or mica schist. In several mines bodies of unchanged spathic 

 ore are embedded in the limonite. Hence Professor Dana explains 

 the limonite as derived by the weathering of a highly ferruginous 

 limestone, from which the limonite has been left behind by the 

 removal of the more soluble elements, so as practically to replace 

 the limestone in connection with other less soluble matter. The 

 limonite has also at times replaced the schists, probably deriving 

 its substance in part from iron-bearing minerals in them, and 

 changing these rocks to the ochers and clays now found with the 

 ores. These views are undoubtedly very near the truth for the 

 region studied. (Cf. also Example 4.) Weathering limestones do 

 furnish residual clay ocher, etc., as is shown by the deposits of 

 western Kentucky and Tennessee under Example 2. 



2.0 .25. Another hypothesis early formulated and advocated 

 by many is that the limonites have been derived by the surface 

 drainage of the old Appalachian highlands and then precipitated in 

 still water where they are now found. A precipitation around the 

 shores of a ferruginous sea has also been urged on the analogy of 

 certain explanations of the Clinton ore. (Example 6.) Their sup- 

 posed Tertiary age has already been remarked. All these views are 

 essentially hypothetical. 2 



1 See F. P. Dunnington, " On the Formation of Deposits of Manga- 

 nese," Amer. Jour. Sci., iii., XXXVI., p. 175. (Experiments 10 and 11.) 



3 See H. D. Rogers, Trans. Asso. Amer. Qeol. and Nat., 1842, p. 345 ; 

 E. Hitchcock, Geol. Vt., Vol. I., p. 233 ; J. P. Lesley, Iron Man., p. 501 ; 

 Rep. A, Second Penn. Survey, p. 83 ; J. S. Newberry, International Re- 

 view, November and December, 1874. 



