CHAPTER VI. 



THE BELT OF WEATHERING. 



In Chapter IV general statements were made as to the nature of the 

 alterations of the belt of weathering and the relations of these to alterations 

 in the belt of cementation and the zone of anamorphism. The statements 

 were mainly from the physical-chemical point of view, and no attempt was 

 made to give in detail the facts upon which they were based. It is the 

 purpose of the present chapter, first, to consider fully the phenomena of the 

 belt of weathering from a geological point of view, and, second, to interpret 

 these phenomena in terms of the physical-chemical principles which have 

 been developed in the previous chapters. However, the two are not 

 separately considered, but are interwoven. 



BELT OF WEATHERING DEFINED. 



The belt of weathering has been shown to be a part of the zone of 

 katamorphism. (See Chapter IV.) From a physical-chemical point of view 

 this belt is one in which the reactions take place with liberation of heat, 

 and with expansion of volume provided all the compounds formed remain 

 in situ. 



At a variable depth below the surface of the earth ground water is a 

 connected body which fills all of the openings. The position of the surface 

 of this body of water may be called the level of ground water. Above the 

 level of ground water the openings are ordinarily not filled with water. 

 From a geological point of view the belt of weathering is the surficial belt 

 extending from the surface of the earth to the level of ground water. The 

 thickness of this belt varies greatly. At or near streams, lake, or ocean, 

 and in areas where the surface is not much higher than the adjacent bodies 

 of water, the level of ground water may reach near or to the surface, and 

 thus there be for these areas either a very thin upper belt or even none. In 

 regions of average precipitation, moderate elevation, and moderate irreg- 

 ularities of topography the level of ground water is usually from 3 to 30 



