Decoritposition of Iron Pyrites. 401 



ous material, wherever receiving the iron-bearing waters, became 

 changed more or less completely to ferriferous limestone or 

 ferriferous dolomite, or received pure iron-carbonate," etc. 



This theory appears to me incomplete, in that it does not ac- 

 count for two important facts : first the destination of the vast 

 quantity of iron which was separated and transported somewhere, 

 during the metamorphism of the schists and the alteration of 

 the greater part of their contained pyrrhotite into the higher 

 sulphides. The whole mass of the Appalachians is so saturated 

 with pyrites, that the slight divergence of the line of heavier 

 deposits of limonite from that of the concentrated bodies of 

 pyrites in Western New England — to which Prof. Dana calls 

 attention — seems to have little bearing on the main question. 

 For such a comparison, in my view, the kind of pyrites should 

 be distinguished ; and it is worthy of attention that the more 

 abundant limonite-deposits are not in the vicinity of pyrrhotite 

 lodes (e. g., Strafford, Gap Mine, Ore Knob, and Ducktown), 

 but of masses of rock saturated by pyrite — as well as other iron 

 minerals, of course — and of the pyrite lodes, e. g., the limonite 

 beds at Brandon, [Salisbury, Amenia, Kittany Valley, etc., or 

 even underlaid by pyrites, e. g., in Louisa County, Va." 



Secondly, the improbability of the preservation down to this 

 day of any large bodies whatever of so unstable an ore as siderite, 

 intercalated in strata so long'exposed to sub-aerial atmospheric 

 action, seems inconsistent with the ancient origin involved in 

 Prof. Dana's theory of its contemporaneous deposition. In my 

 view, the ferruginous marsh-deposits of that distant period may 

 be represented by the present pyritous "lodes," pyritiferous 

 schists and limestones, and crystalline iron-oxides ; the concen- 

 tration of the siderites and the saturation of beds by iron- 

 carbonate attended the long subsequent metamorphism of those 

 sediments; and the limonites, pipe-ores, etc., have owed their 

 development to oxidation, still in progress, throughout the up- 

 lifted terrane of crystalline rocks, during the comparatively re- 

 cent though very long subsequent period. 



Intermixtures of the Iron Pyrites. 

 These intermixtures are now known to be very common, and 



« Trans. Am. Inst. Min. Eng. (1876-7), 529. 



