122 T. C. CHAMBERLIN AND R. T. CHAMBERLIN 



and Kwang Si cases. In the Canton and Kowloon cases granular 

 disintegration prevailed in a marked degree, as is well known to be 

 common in low latitudes, but in these cases the rock was granitic 

 and peculiarly susceptible to this mode of disaggregation. In the 

 Mexican and Oahu localities the rocks were mainly close-textured 

 lavas; in Kwang Si fine-grained sediments, in the main. Under the 

 influence of a warm climate, aided by the tropical vegetation, the 

 rocks of these localities seem to pass into soil on the surface with 

 little detachment of blocks, bowlders, or other coarse material. The 

 rains of these regions are sufficient to keep the soil thin in spite of the 

 vegetal protection and thus to keep the decay working actively on 

 the whole outer surface. 



While this seems to explain the nature of the surface and the thin 

 surface mantle, we are not fully persuaded that it altogether explains 

 the abruptness of the change from the side slopes to the valley bottoms. 

 Without attempting to give very cogent reasons for the interpreta- 

 tion, we are disposed to refer this to a mode of hydraulic action 

 which is really normal but which seems abnormal to us because in 

 mid-latitudes it is commonly thwarted by an overburden of detritus. 

 During a rain the water on the slopes grows in amount as the slope 

 is descended and the wash-action is normally greatest at the foot of 

 the slope if the water has not become overloaded with detritus in its 

 descent. The velocity of flow also increases as the slope is descended 

 and this further adds to the erosive power toward the base of the slope. 

 A normal slope should therefore approach more and more to the 

 vertical as it gains in descent. This is almost universally true of the 

 brow and upper part of the slope in all latitudes. Why does it not 

 persist to the bottom ? In certain cases it does. Certain mesas, buttes, 

 and outliers, particularly in arid lands, possess essentially vertical 

 sides which reach down either to coarse talus piles or to the horizontal 

 beds of the surrounding region. In some instances there is sapping 

 at the base due to more perishable layers but in other cases there is 

 no sign of this and the cutting at the base appears to be due to the 

 superior volume and velocity of the water rushing down the sides. 

 In these cases the effect is probably correlated with the absence or 

 scantiness of vegetation which when present restrains the rush of the 

 waters. A similar effect is shown in waterfalls, though here differ- 



