128 GEOLOGY, 



The effects of variations in temperature on rock weathering have 

 already been discussed (p. 43). They are chiefly mechanical, and 

 are seen at their best where the daily range is great. 



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 vegetation, the decay of 

 which supplies the water with organic acids which 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 often decomposed to greater depths, 

 on the whole, than in high latitudes. How far this is the result of 

 rapid weathering, and how far of slow removal, due in part to the 

 protective influence of the plants, cannot be affirmed. If the weathered 

 material is not removed, it will presently become a mantle thick enough 

 to retard the processes which brought it into existence. 



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 rela- 

 tively ineffective. Under some conditions, therefore, temperature 

 affects both corrasion and transportation. 



The humidity of the atmosphere has an influence even more im- 

 portant than that of temperature on the rate of erosion, and its in- 

 fluence 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 weather- 

 ing effected by this means. Precipitation, 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 precipitation 

 the more rapid the corrasion and transportation. Much, however, 



