750 



C. R. VAN HISE 



correctness. 1 Above the level of groundwater, in the belt of 

 weathering, the chemical reactions of oxidation, carbonation, 

 hydration, and solution are the rule. The mechanical results are 

 disintegration, softening, and decomposition of the rocks. Below 

 the level of groundwater, in the belt of cementation, the chemi- 

 cal reactions of oxidation and carbonation are less active ; but 

 hydration occurs very extensively. Instead of solution, deposi- 

 tion is continually taking place. The mechanical result is that the 

 rocks, instead of being disintegrated, softened, and decomposed, 

 are hardened, the openings being cemented. Where comes the 

 material for cementation ? Why, from this belt of weathering 



Zone of Flowage 



Fig. 7. 



above, where solution is taking place. If the waters in the belt 

 of weathering are continuously taking materials into solution, 

 and are continuously depositing material below, as denudation 

 goes on these belts steadily migrate downward. The present 

 belt of weathering not long ago geologically was in the belt of 

 cementation. While, therefore, the belt of weathering at any 

 given time in the past, as now, was relatively thin, it may have 

 moved downward for thousands of feet. In many mining dis- 

 tricts it is estimated that from 1000 to 3000 or more meters of 

 rock have been removed from above the present surface. All 

 of this thickness has been in the belt of weathering; although 

 at any one time the belt of weathering may have been but a 

 score or few score meters in thickness. Therefore, from the belt 

 of weathering a great and adequate amount of material may 



1 For a fuller discussion see Metamorphism, cit., pp. 277-286. 



