396 Decomposition of Iron Pyrites. 



durable form of iron pyrites." Me also states that the " pyrites 

 of the nodules oxidizes with great rapidity; specimens kept for 

 only a few months rapidly fall to powder, and become incrusted 

 with crystals of iron-sulphate." 



Distribution of the Iron Pyrites. 



We may sum up the general distribution of the three minerals 

 as follows : 



Pyrrhotite is comparatively of uncommon occurrence and 

 limited abundance, and generally more or less mixed with the 

 other pyrites. It is found in eruptive rocks, e. g., most basalts; 

 also in many granites, diorites, and serpentines; but mainly in 

 clay slates, mica slates, and rocks of incipient metamorphism, 

 e. g., in beds of limestone which have plainly escaped the altera- 

 tion into marble prevailing in adjacent districts. 



Marcasite constitutes largely or altogether the pyrites occupy- 

 ing thin seams and coatings in the Coal-beds, lignitic shales, the 

 dolomytes of Saxony and Cornwall, the Chalk, many limestones 

 of America, and, in general, the unaltered sedimentary rocks, 

 such as sandstones, graywacke schists, peat, clay, bituminous 

 coals, casts of fossils, and also veins of galenite. 



Pyrite has a far wider distribution, from its stability, but 

 nevertheless tends to predominate in the crystalline rocks, largely 

 constituted of magnesia- and iron-micas, hornblende, chlorite, 

 and serpentine, such as dioryte, chlorite- and hornblende-schists, 

 etc. It is prominent as well in weathered eruptive rocks and 

 most granites, marbles, argillytes, the scattered particles in an- 

 thracite, and nearly all metalliferous veins, e. g., the sulphuret 

 lodes of Colorado. 



In clay-beds, the pyritous nodules generally consist of succes- 

 sive crusts or transition mixtures of this and the preceding min- 

 eral, as in the clays of Schoharie, N. Y., and Amboy, N. J. 



The distribution of the three pyrites along the Appalachian 

 chain presents points of interest, in view of the great series of 

 extensive pyritous deposits which are scattered along, from Ala- 

 bama through the Carolinas, Virginia, Pennsylvania, New York, 

 and New England, into Newfoundland. 



The association of the pyrites at the principal localities is about 

 as follows : 



