WORK OF THE ATMOSPHERE 35 



as the atmosphere is concerned, in oxidation, hydration, and car- 

 bonation. Solution, which also goes on actively here, is to be re- 

 garded as the work of the ground water, rather than that of atmo- 

 spheric moisture. The two agents are, however, so intimately re- 

 lated that it is impossible to draw a sharp line between them and 

 their respective accomplishments. 



Carbonation of silicates is on the whole the most characteristic 

 reaction. This takes place on a vast scale, producing carbonates 

 from the silicates, and setting free, thereby, silica or colloidal silicic 

 acid. Hydration is the most extensive single reaction in this belt, 

 while oxidation is very pronounced, owing to the free access of 

 oxygen to the pores, from the absence of water. Solution removes 

 many of the products of decomposition from this belt of weathering, 

 and carries it away over ground or under ground. As a result of 

 this, the volume of rock in the belt of weathering becomes greatly 

 reduced, the resultant material in the end occupying only a small 

 fraction of the volume it once had. 



Oxidation. "The oxygen is chiefly utilized in the oxidation of 

 iron, surphur, and organic material. In the upper part of the belt 

 [of weathering], where oxygen is abundant, large amounts of hema- 

 tite and limonite may be produced by the oxidation of the ferrous 

 particles, but for the greater part of the belt of cementation, where 

 oxygen is somewhat deficient, the ferrous oxide is oxidized only to 

 the form of magnetite, since this requires, per unit of iron, only two- 

 thirds as much oxygen as to produce hematite and limonite." (Van 

 Hise-ioo:(5od.) Magnetite may be directly oxidized into hematite, 

 involving an increase in volume of 2.5 per cent. Hydration may 

 occur simultaneously, changing the mineral to limonite with a cor- 

 responding increase in volume of 64 per cent. Iron carbonates are 

 oxidized to ferric oxide, with a corresponding decrease in volume, 

 owing to loss of CO.. This is 49 per cent., if hematite is the result, 

 but only 18 per cent., if the carbonate is changed to limonite. Sul- 

 phides are oxidized to sulphites and sulphates. Sulphurous and sul- 

 phuric acids are formed, which may enter into various combinations. 

 Sulphuretted hydrogen is a common result of the oxidation of iron 

 pyrites, and, from the further oxidation of this, pure sulphur may 

 be precipitated. 



Some of the changes occurring may be expressed in the follow- 

 ing formula (for others see Van Hise-ioo:-^//) : FeS.-|-60=: 

 FeSO^ -|- SOo. If the water is present, we may have: 



Fe.S,,-f30+HX):=FeS0,-fH,S. 



The ferrous sulphate may be removed in solution or may fur- 



