502 A TREATISE ON METAMORPHISM. 



of clays, of which kaolin is the chief constituent, proves the extensive 

 decomposition of the silicates, especially the feldspars. 



It is rather probable that decomposition is most rapid in regions of 

 moderate humidity, where there is moisture sufficient to carry on decom- 

 position, solution, and transportation of the disintegrated material with 

 considerable rapidity, and for animals and plants to be reasonably abundant, 

 but not sufficient to produce extraordinarily luxuriant vegetation, which 

 may practicallv stop mechanical transportation on all but the steepest 

 slopes, and thus produce a protective covering of decomposed materials. 

 (See p. 503.) 



While humidity is very favorable to decomposition, it must be remem- 

 bered that running water is the greatest of the eroding agents, and there- 

 fore that humidity is also very favorable to disintegration. 



Regions of low latitude. — Regions of low latitude are those of high tempera- 

 ture. Warmth is favorable to chemical decomposition. The great increase 

 in the activity of chemical agents in consequence of increase in temperature 

 has already been explained on pages 61-62, 78-81, and hence in warm 

 regions the chemical agents which are at work in the belt of weathering 

 have a potency which they do not approach in cold regions. Therefore 

 the usual deep belt of decomposition in the tropical and semitropical 

 regions and the general absence of such a belt in the colder regions- is 

 partly explained by high temperature. 



Consequent upon high temperature as one essential factor is abundant 

 life, which still further favors decomposition, as explained below. In 

 regions of high temperature the range of temperature may be great, 

 and therefore disintegration due to this cause be a concurrent process. 

 But in such regions the temperature does not often fall below zero, and 

 hence the disintegrating effect of freezing and thawing is nil, or nearly so. 



Regions of moderate topographic relief. — Regions of small topographic relief gener- 

 ally have gentle slopes, and gentle slopes are favorable to retention of 

 moisture and unfavorable to rapid transportation. Moreover, regions of 

 small relief usually have low elevations and are favorable to high tempera- 

 ture, and consequently to abundant life. Hence, regions of small elevation 

 are those of marked decomposition. 



If the elevation be slight the level of ground water may be at a very 

 small depth below the surface, or even reach the surface, and thus decom- 



