LAND WATERS STREAMS 151 



High temperature favors chemical action, and the weathering 

 of rock by decomposition is at its best in the presence of abundant 

 'moisture, in regions where the temperature is uniformly high. 

 Furthermore, a warm, moist climate favors the growth of vegeta- 

 tion, the decay of which supplies the water with organic acids 

 iwhich greatly increase its solvent power. The climatic conditions 

 favoring mechanical weathering are therefore different from those 

 'favoring chemical weathering. High temperature and abundant 

 moisture and vegetation are found in many tropical regions, and 

 here the rock is decomposed to greater depths, on the average, 

 than in high latitudes. How far this is the result of rapid weather- 

 ing, and how far of slow removal, due in part to the protective 

 influence of the plants, cannot be affirmed. 



So long as the water of the surface and that in the soil remains 

 unfrozen, temperature affects neither corrasion nor transportation. 

 But in middle and high latitudes the surface is frozen for some part 

 of each year. During this time corrasion is at a minimum, for 

 although the streams continue to flow, there is relatively little 

 water running over the surface outside the drainage channels, 

 and that little is relatively ineffective. Under some conditions, 

 therefore, temperature affects both corrasion and transportation. 



The humidity of the atmosphere has an influence even more 

 important than that of temperature upon the rate of erosion, and 

 its influence is exerted on each of the elements of that complex 

 process. A moist atmosphere favors oxidation, carbonation, 

 hydration, and the growth of vegetation, all of which promote 

 certain phases of rock weathering. On the other hand, humidity 

 tends to prevent sudden and considerable variations in temperature, 

 thus checking the weathering effected by this means. Precipita- 

 tion, the most important single factor in determining the rate of 

 erosion, is dependent on atmospheric humidity. Its amount, its 

 kind (rain or snow), and its distribution in time, are the elements 

 which determine its effectiveness in any given place. 



Other things being equal, the greater the amount of precipita- 

 tion the more rapid the corrasion and transportation. Much, how- 

 ever, depends on its distribution in time. A given amount of 

 rainfall may be distributed equally through the year, or it may fall 



