DEGRADATION. 195 



tion in terms of declivity, but Hopkins arid Babbage have shown that the 

 power to transport load of a given size of particles increases with the sixth 

 power of the velocity of water, and it is probable that the rate of degrada- 

 tion increases with the velocity of water in very nearly the same ratio, but 

 modified slightly by a multiplicity of climatic and petrologic conditions. 



It will be seen that in the above discussion I have neglected one term 

 of climate, that is temperature. My field of study has been limited, to the 

 frigid and temperate climates, and for the effect of a torrid climate I have 

 no facts to guide me to valid conclusions, but within my field of study the 

 temperature term, though modifying the methods and topographic results of 

 degradation, does not invalidate the result I have reached as to the over- 

 shadowing importance of declivity. Increased cold diminishes the protec- 

 tion derived from vegetation and permits greater rainfall to produce greater 

 degradation. But in a given latitude increased cold is due to increased ele- 

 vation. (This increased elevation is also increased declivity.) But this i& 

 again modified by the fact that moisture descends as snow, and to that extent 

 the beating power of rains is lost. Where these snows accumulate as ice, 

 forming glaciers, another modification of degradation is introduced, dimin- 

 ishing the effects of the water in degradation and changing the topographic 

 features; that is, any given amount of precipitation of water will produce 

 more degradation acting as rains and rivers than as snows and glaciers. By 

 the former condition atmospheric disintegration is more rapid, and corrasion 

 is confined to narrower channels, and thus sapping is promoted. Transporta- 

 tion in ice and water are governed by the same conditions of flotation, and 

 the greater heterogeneity of river channel promotes greater flotation, so that 

 degradation in each of its elements of disintegration and transportation is 

 faster by rains and rivers than by snows and glaciers. Hence, as cold 

 increases, degradation is promoted by the decrease of protection derived 

 from vegetation until that degree of cold is reached where the moisture is 

 precipitated as snow, when low temperature serves to decrease degradation. 



Having considered all the modifying conditions of climate and petrol- 

 ogy, it yet remains . that degradation increases with declivity through the 

 combined forces of water power and rock power in a rapidly-multiplying 

 rate from that low degree of declivity which permits the transportation of 



