181 



the decomposing sulphuret of iron, which is often copiously 

 present. 



To the reaction of the alumina and sulphuret of iron, assisted 

 by the oxygen of the atmosphere, we are to attribute the incrus- 

 tation of sulphate of iron and sulphate of alumina sometimes seen 

 upon its surface, imparting to the mass an astringent and acid 

 taste, and endowing it with properties very noxious to vegetation. 

 Some layers, containing a larger quantity than usual of the sul- 

 phuret of iron, acquire even a brown colour at the surface, caused 

 by the amount of the hydrated oxide of iron produced by decom- 

 position. The impervious texture of this material, causes the 

 water which reaches it through the overlying porous sandy beds, 

 to appear along its upper margin in the ravines and other de- 

 pressions of the surface wherever its outcrop occurs. Much of 

 this water passing through the adjacent beds of greensand, which 

 are ofien charged like the blue clay with the decomposing sul- 

 phuret of iron, becomes impregnated to some extent with the 

 sulphate of iron, producing a precipitation under favourable cir- 

 cumstances, of the hydrated peroxide of iron, in the form of bog 

 iron ore, and yellow ochre. 



To exhibit more definitely the composition of this blue earth, I 

 here submit a chemical analysis of a specimen procured from the 

 base of the Nevesink hills, on the Raritan bay, which will show 

 the average composition of a very abundant variety of the 

 stratum. 



Jlnalysis of the Blue Astringent Micaceous Clay. 



97-60 

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