THE WORK OF THE ATMOSPHERE 25 



fantastic forms (Fig. 16). Abrasion by wind-driven sand is of little 

 consequence in a plain country where the climate is moist, and 

 where the rock is covered with soil; but it is of much consequence 

 in arid and semi-arid regions where the topography is rough, and 

 where hills and points of bare rocks are numerous. Wind-driven 

 dust wears the surface of rock much less than sand does. 



2. THE CHEMICAL WORK OF THE Am 



The chemical changes produced by the air on soil and rocks can- 

 not be understood without some knowledge of chemistry; but 

 certain familiar facts will help us to see the general nature of these 

 changes. 



When a piece of iron or steel, such as a knife-blade, is left in the 

 moist air, it rusts. In the process of rusting, both oxygen from the 

 air and water enter into combination with the iron, and the rust 

 contains all three substances united into one. The iron rust scales 

 off, and a knife-blade will soon be " eaten away" if the rusting is 

 allowed to go on. Similar changes take place in some rocks. Iron 

 is present in many of them, and this iron rusts much as the knife- 

 blade does. Other parts of the atmosphere also help to change 

 some of the minerals of the common rocks, and in most cases the 

 rocks crumble in consequence. 



Weathering. All changes of the surface rocks which make 

 them crumble are parts of the general process of weathering, which 

 includes most of the processes by which rock at or near the surface 

 is made to change its form or color, or to lose its solidity. The 

 processes of weathering are very important. Much of the soil and 

 subsoil (mantle rock) of the earth have been made by them, and 

 the weathering of rock prepares it for transportation by wind and 

 water. 



3. CHANGES BROUGHT ABOUT UNDER THE INFLUENCE OF THE AIR 

 The surface of the land is subject to great changes of tempera- 

 ture, which are of importance in various ways. 



Freezing and thawing. In many regions where the surface is 

 well covered with soil, it freezes in winter; that is, the water in it 



