CHAPTER IV 

 DESTRUCTIVE PROCESSES — THE ATMOSPHERE 



The atmospheric agencies are by far the most important of the 

 destructive or denuding agents, because no part of the land surface 

 is altogether exempt from their activity. Their work is described 

 by the general term weathering, and is shown at once by the 

 different appearance of freshly quarried stone from that which has 

 been long exposed in the face of a cliff, or even in ancient build- 

 ings. While such agents as rivers and the sea do work that is 

 much more apparent and striking than that of the atmosphere, 

 yet they are much more locally confined, and even in their opera- 

 tions the atmosphere renders important aid. Though no part 

 of the land surface is entirely free from the destructive activity of 

 the atmosphere, the rapidity and intensity of this activity vary 

 much in different places. There are, in the first place, the great 

 differences of climate to be considered, differences in the amount 

 and distribution of the rainfall, of temperature, and of the winds. 

 In the second place, the resistance offered by the various kinds 

 of rocks to the disintegrating processes differs very greatly, in 

 accordance with the differences of hardness and chemical compo- 

 sition. Again, the presence or absence of a covering of protective 

 vegetation has an important influence upon the amount and char- 

 acter of the destruction effected. 



The outcome of all these varying factors is to produce very 

 irregular land surfaces. While the tendency of the atmospheric 

 agencies is gradually to wear down the land to the level of the 

 sea, yet in that process some parts are cut away much more rapidly 

 than others ; and hence the first effect of denudation is an increas- 

 ingly irregular surface. The overlying screen of soil conceals many 

 of these irregularities, especially the minor ones ; and were that 



7 2 



