Decomj^osition of Iron Pyrites. 125 



VIII. — On the Variation of Decom^josition in the Iron Pyrites; 

 its cau.se, and its relation to density. 



BY ALEXIS A. JULIEX. 



Part II. 

 Continued from \u\. Ill, l>Mge 404. 



Read April 18tli, 1887. 



We btive thus far considered ordinary forms of decomi)osition 

 of the natni'al iron-sulphides, and also tlie principal facts known 

 as to their close association, intermixture, and mutual replace- 

 ments. There are local conditions, however, of extreme expo- 

 sure to the atmosphere, or of protection from its action, Avhich 

 in many instances ai)pear to affect the stable character of varie. 

 ties of these three minerals. The following example of the lat- 

 ter has been communicated to me by Prof. D. S. Martin. 



"' As to the two forms of pyritous decomposition, I have ob- 

 served them esi;)ecially in the nodules from the clays at Cliff- 

 wood, N. J., opposite Keyport. Those on the beach, washed 

 out of the bluff and exposed to the salt water, are limonite- 

 coated and permanent ; while those uncoated, taken out of the 

 bank and brought home, cannot be preserved, splitting up and 

 going into vitriol, with a development of free acid that eats 

 through any cotton, paper, or pasteboard. Sometimes those on 

 the beach are coated on their upper, exposed surfaces, and not 

 on their lower sides, or only partially, leaving spots or portions 

 that in time become starting-points for the vitriolic decomposi. 

 tion." 



In this instance we have simply evidence of decomp )sition of 



