GYPSUM DEPOSITS OF NEW YORK 65 



minerals, apecially the carbonates, to produce an interchange of 

 bases. Wherever a source of sulfuric acid exists in nature, the 

 formation of gypsum may be expected under ordinary circum- 

 stances, as the other essentials are nearly always at hand. 



The derivation of sulfuric acid can be traced most commonly to 

 the oxidation of the sulfur occurring in metallic sulfids. The iron 

 sulfids — pyrite, marcasite and pyrrhotite — are particularly wide- 

 spread both as rock-forming minerals and in ores. When exposed 

 to atmospheric influences they are subject to rapid decomposition, 

 yielding such compounds as hydrogen sulfid, sulfurous and sulfuric 

 acids, ferrous sulfate and iron oxids. The presence of hydrogen 

 sulfid in the spring waters that issue from the shales and sandstones 

 of western New York is an illustration of the decomposition of 

 iron sulfids which are disseminated in the shales. In the Oak Or- 

 chard spring at Byron, Genesee co. sulfuric acid of similar deriva- 

 tion exists both free and combined with lime, magnesia and the 

 alkalis. Another source of the acid is from the decay of organic 

 matter, which yields hydrogen sulfid in the first instance. This gas, 

 as well as sulfur dioxid, is also given off by volcanos, fumaroles and 

 hot springs, and gypsum is frequently deposited near their vents by 

 the action of the acid vapors and waters upon lime minerals. 



With the supply of sulfuric acid that is made available in these 

 ways the formation of gypsum takes place very generally through- 

 out the zone of weathering and ground-Avater circulations. Under 

 some conditions the gypsum may accumulate directly in sufficient 

 amount perhaps to have economic importance, as when acid solu- 

 tions from the deco^mposition of pyrite come in contact with bed-s 

 of limestone. But more generally it is carried in solution until the 

 waters reach the surface and are concentrated by evaporation. 

 Though gypsum dissolves rather slowly in pure water, its solubility 

 is greatly increased in the presence of salts of the alkalis, specially 

 sodium chlorid, so that sea water for example is a much better 

 solvent than fresih water. It is by concentration of the surface 

 waters held in some inland basin, lake, or arm of the ocean that 

 the valuable deposits of gypsum are usually formed. 



Deposition of gypsum from sea water. The deposits that result 

 from the evaporation of sea water have been investigated by 

 J. Usiglio, Van't Hoff and others. Usiglio in 1849 carried out a 

 series of laboratory experiments whdch outline very well the gen- 

 eral conditions of their formation, thougli his results liave been 

 amended in some respects by the later works of \'an't IIofT and his 

 associates. The experiments were based on samples of water taken 



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