RATE OF WEATHERING. 535 



While the above is true, if one were to confine his attention entirely 

 to the decomposing effect of the weather and exclude disintegration, it is 

 probable that there is a limit beyond which increase in elevation decreases 

 the decomposing effect. As explained on pages 499-500, 502-503, great 

 elevation g-ives conditions unfavorable for the existence of life, and life is 

 one of the necessary agents in rapid decomposition. 



While the reactions of weathering are taking place throughout the 

 entire belt above the level of ground water, the process is at its maximum 

 efficiency near the surface and rapidly decreases in efficiency with depth. 

 This is largely due to the fact that life is confined to the upper few meters 

 and therefore the decomposing effects of life rapidly diminish with depth. 



It is a corollary from the relation of the thickness of the belt of 

 weathering and the rate of the process that geological periods in which 

 there are extensive land areas at a high elevation above the sea are periods 

 in which the processes of weathering are most rapid. These processes are 

 essential to the rapid formation of all classes of sediments', including 

 the limestones. The relations between weathering and sedimentation are 

 discussed on pag'es 555-560. 



STAGE OF WEATHERING. 



Where the rocks in the belt of weathering are fresh, and therefore have 

 not been adapted to their environment, the processes of weathering are 

 active. And, per contra, where the processes of weathering have trans- 

 formed the rocks to minerals adapted to the belt of weathering- the processes 

 are slow. For instance, the conditions are favorable for rapid weathering 

 in glaciated regions where all the weathered products have been recently 

 removed, and where, therefore, the rocks and minerals are not adapted to 

 the belt in which they are found. In such areas as the Piedmont Plateau, 

 on the contrary, where in some places the minerals have been very largely 

 transformed to those characteristic of the belt of weathering, and are there- 

 fore permanent under the conditions of that belt, the processes of weathering- 

 are very slow. 



Probably where weathering is in an early but not the earliest stage the 

 processes are at their maximum activity. Where by changes of physical 

 conditions rocks have been brought into the belts of weathering, at first the 

 conditions are very unfavorable for the existence of life, and the great role 



