OF PYRITES AND GYPSUM. 321 



In the Coal Measures, the pyrites has no doubt been derived 

 from circulating ferruginous waters by the reducing action of 

 the carbonaceous matter. A similar origin may account for the 

 pyrites found elsewhere in connexion with organic structures, as 

 in the fossil wood occurring in the clifts of Walton-on-the-Naze. 

 Solutions of iron salts, like the carbonate in carbonated water, 

 brought into relation with sulphates in the presence of decom- 

 posing organic matter might readily yield pyrites. 



Bischof, the great chemical geologist, obtained pyrites by. 

 adding sugar to a solution of sulphate of soda containing a little 

 iron, and allowing a pitcher of the liquid to remain in a cellar 

 for four years. A well-known case of the formation of pyrites 

 was that cited a century ago by Pepys, who having prepared . 

 hydrogen by the action of dilute sulphuric acid on iron filings, 

 left a jar of the by-product in his laboratory for a year, and then 

 tound that some unfortunate mice which had fallen into the 

 liquid had effected its reduction to the condition of sulphide, in 

 the form of pyrites. In nature, ferrous sulphate may occur in 

 percolating waters and readily suffer reduction oy any decaying 

 organic matter with which it may come in contact. But in such 

 cases the sulphate itself may have been derived from the 

 oxidation of pyrites elsewhere ; so that the changes are cyclic. 



Pyrites sometimes occurs crystallized in alluvial deposits of 

 recent age, and is found in the muds forming around the coasts 

 at the present day. According to the report of the Challengev the 

 colour of the *' blue muds," which are frequently found either in 

 the deeper waters around continental land or in enclosed seas, is 

 due to organic matter and to sulphide of iron in. a fine state of 

 division. When first dredged, it emits the odour of sul- 

 phuretted hydrogen. 



Annelids pass mud through their alimentary tract, and during 

 its passage any sulphates may be reduced to sulphides. 



The bluish colour of certain clays like the gault, and some of 

 the Jurassic limestones like the Forest marble, is usually connected 

 with the presence of finely-divided iron-pyrites. 



The decomposition of pyrites often contributes to the mineral 

 character of waters percolating through rocks in which it occurs. 

 It is curious to find that Harrison, the good old rector of Rad- 

 winter, in Essex, in writing in his quaint history, more than three 

 hundred years ago, about the waters of Bath, refers their 



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