RHYTHMS lis DENUDATION 759 



From these considerations it appears that the corrasive power of a 

 stream varies perhaps near the square of the slope. 



On the waste slopes leading down to the stream the component of 

 i^ravity varies with the sine of the angle of slope. For slopes low enongh 

 to hold soil it may be said to vary with the angle of slope, as the sine and 

 its arc for small angles are approximately equal. The greater time taken 

 to remove the material from a flat slope is shown by its reduction by 

 weathering to a finer state. In humid regions this soil becomes bound by 

 vegetation which prevents a ready washing away, even though reduced in 

 size of particle. AVhere the lateral slopes of valleys in humid climates are 

 mantled with clastic material the rate of removal probably varies betweeji 

 the first and second power of the angle of slope. It cannot, however, obey 

 any regular law, but ^■aries with many factors. 



The action of ground wdtev must be considered as an important factoj'. 

 In humid climates it is most important as the preparatory agent in me- 

 chanical erosiou as wel] as in solvent denudatiou, but to be effective it 

 must circulate, not lie stagnant, and for circulation there must be "head."^ 

 Where the head is high \A'ith respect to the length of course, the velocity 

 of luulerground lio\\' increases. The maximum rate oF cbemical decay is 

 therefore pi'oduced in a well dissected country, but one witli slopes flat 

 enough to hold soil and present a high ratio of surface run-off to total 

 rainfall. Deeply incised valleys are a facto l- fa\oring rock decay, but very 

 thick soil mantles act, on the contrary, as a check to further decay and 

 testify to the efficiency of the iiatural vegetation to prevent soil waste 

 rather than to a high rate of present chemical denudation. Denudatioji 

 in such regions is potential rather than actual. In a topography of con- 

 siderable relief the stimulus of more rapid circulation of ground water 

 causes the zone of oxidation as well as that of hydration to affect a gi'eater 

 volume of rock. Much of this water returns to the surface to swell the 

 volume of the mam streams and give them increased carrying power. With 

 very steep slopes, however, there is a lesser proportion of ground water tu 

 run-off, but if the intervening hilltops are fiat from a previous cycle of 

 erosion, then there is not only abundant ground water, but a more vig- 

 orous circulation. 



It would appear on reviewing the complex factors that the rate of 

 chemical denudation does not vary with slope to such a high degre6 as 

 does mechanical denudation. In fact, the steepest slopes are unfavorable. 

 On the other hand, a considerable head promotes chemical denudation. 



The conditions for limestone regions are exceptional and quite different 

 from those of compact igneous rocks. In limestones a vigorous circula- 

 tion and solittion by underground waters may go forward with moderatf' 

 LVIT — Bull. <tKOL. Soc. Am., Vol. 28, lOlfi 



