EEACTIONS IN BELT OF WEATHERING. 163 



delimited by the level of ground water. The separation of the belt of 

 weathering from the belt of cementation is therefore based upon the posi- 

 tion of an agent of metamorphism. It has been seen that the zone of kata- 

 morphism is separated from the zone of anamorphism by a reversal of the 

 physical-chemical factors. As one would suppose, the latter distinction is 

 of much more fundamental importance than the former. 



BELT OF WEATHERING. 



By some it has been proposed to call the belt of weathering that of 

 demorphism; and to call the alterations of all rocks below this belt 

 metamorphism. The fact that the alterations in the belt of weathering are 

 very different from the belts below has been well known for many years. 

 But it has not been generally recognized that the belts of weathering and 

 cementation are delimited by the level of ground water. This is doubtless 

 due to the fluctuations of that level and to a considerable transition band 

 between the two belts (see pp. 423-429, 560-561); but in many places the 

 change in the character of the alterations in passing from the belt of 

 weathering to the belt of cementation is very sudden, and at such places 

 is very clearly connected with the level of ground water. 



The belt of weathering is therefore defined to extend from the surface 

 to the level of ground water. In this belt all of the very important reac- 

 tions characteristic of the zone of katamorphism — viz, oxidation, carbona- 

 tion, hydration, and solution — are at their maximum activity; but on the 

 whole, of these three reactions the most characteristic, but not the dom- 

 inant one, is that of the carbonation of the silicates.' This reaction takes 

 place on a vast scale, producing carbonates from the silicates, and at the 

 same time setting free silica or colloidal silicic acid. Hydration is the most 

 extensive simple reaction in the belt of weathering. Oxidation is also very 

 important. As will be seen, this reaction is very general in this belt, 

 because not being- saturated with water the oxygen of the atmosphere very 

 rapidly makes its way through the porous rocks and continually supplies 

 oxygen to replace that element used in the process of oxidation. The total 

 effect of these chemical reactions is decomposition. While hydration and 

 oxidation are usual for this belt, under special conditions these reactions may 

 be reversed. In places of luxuriant vegetation and very high humidity 

 deoxidation may take place. In regions of great heat and temporary or 



