CHEMICAL WORK. 125 



bination, as carbonic acid, it may take the FeO and make iron carbonate. Or 

 if the acid is a humus acid, this acid may combine with the FeO, and, as such 

 a compound is soluble, the waters may carry it to the marshes for deposition 

 and re-oxidation. 



Since the compounds so made are colorless or nearly so, fragments of a 

 plant in a rock may whiten the rock around them, thus making blotches in 

 red sandstones, or a zone may be bleached around stems and roots. Also, 

 the soaking down of soil waters may make a whitish streak along the top 

 of the less permeable layers. 



In like manner iron sulphate or copperas, FeO.SOg.Taq (which oxidation of reS2 

 often produces, as above explained), may be deoxidized and reduced to FeS2 ; that is, either 

 pyrite or marcasite. Fossil wood may be replaced by pyrite or marcasite as decomposition 

 goes on, and shells may be changed in like manner, as acid waters at hand dissolve and 

 remove the calcareous material. 



Calcium sulphate, or gypsum, is, by similar deoxidation, converted into calcium sul- 

 phide, CaS ; zinc sulphate, into zinc sulphide, ZnS, the mineral, sphalerite ; and lead 

 sulphate, into lead sulphide, PbS, which is the common lead ore, galena. After the deox- 

 idation of a sulphate, as gypsum (calcium sulphate), to calcium sulphide, the re-oxidation 

 of the sulphide may take place, and hydrogen sulphide (sulphuretted hydrogen) may result 

 through the agency of the water at hand, thus : Ca takes oxygen from the water, making 

 CaO, or lime (which may combine at once with CO2 to make CaO.C02, or calcium carbo- 

 nate), and the sulphur, S, takes the hydrogen thus set free from the water, making SH2, 

 or hydrogen sulphide (sulphuretted hydrogen) ; for CaS + H20 = CaO-fH2S. This is the 

 ordinary process by which the gas of sulphur springs is made, as for example those of 

 western New York and Virginia. 



By the oxidation of the hydrogen of the hydrogen sulphide makfaig H2O, or water, tne 

 sulphur, S, becomes deposited. This is a very prominent source of sulphur ; and il 

 accounts for its frequent association with gypsum and limestone. 



Further, hydrogen sulphide, SH2 (sulphuretted hydrogen), by action on zinc sulphate, 

 will deoxidize the sulphate and make zinc sulphide ; on iron sulphate, it will make an iron 

 sulphide ; on lead sulphate, lead sulphide. 



But under warm and moist conditions the sulphur may oxidize and make sulphuric acid, 

 SOg+water; and some sulphuric acid springs in New York have this source. Gypsum 

 may be formed by such waters if limestone is within their reach. Pfaif states that at 

 depths in water under a pressure of 40 atmospheres anhydrite will probably form, and not 

 gypsum. Anhydrite is gypsum minus the water. 



It may be added that sulphurous acid, SO2 , is formed by the combustion of sulphur 

 (as in volcanoes) ; and when this gas comes into contact with hydrogen sulphide (SH2), 

 the sulphur of both is deposited, the oxygen and hydrogen combining to form water ; and 

 this is one source of the sulphur about volcanoes. 



With heat, carbon deoxidizes iron oxide and oxides of other metals, producing the 

 pure metal. 



4. Destructive effects. — Since nine tenths of rocks not limestones contain 

 one or more of the common iron-bearing silicates, pyroxene, hornblende (or 

 other species of the hornblende family), or black mica, and almost all rocks 

 have a sprinkling of pyrite or marcasite, the oxidation process is all-pervading 

 in its destruction. The presence of water and air being necessary, the more 

 porous the rock, the deeper and more rapid the decay. The rocks where the 



