ACTION OF WIND 85 



3. Changes of Temperature 



These changes, by alternately expanding and contracting the 

 rocks, widen the crevices and fissures which traverse them. In 

 humid climates this agency is a very subordinate one, and acts 

 chiefly in preparing an easier path for percolating waters, but 

 in dry regions it becomes much more important. In the latter 

 case, the naked rocks are, during the day, heated to a high 

 temperature by the full blaze of the sun, and at night the rapid 

 radiation which occurs in dry air, cools them very quickly. When 

 radiation begins, the outer layers of the rock chill rapidly and 

 attempt to contract upon the still heated and therefore expanded 

 interior ; thus strains are set up which the rock cannot resist, and, 

 therefore, great pieces are split off. In this fashion talus slopes of 

 angular blocks form at the foot of the cliffs, just as in the case of 

 frost-made talus, and this work goes on in all arid regions which 

 have hot days and cool nights. The agency is purely mechanical 

 and effects no chemical change ; it is also entirely superficial and 

 is prevented by even a thin covering of soil. 



The work of destruction due to changes of temperature goes 

 farther than merely splitting blocks off the faces of cliffs, and may 

 result in breaking up a rock into minute fragments. Compara- 

 tively few rocks are made up of a single mineral, and in many 

 rocks several varieties of minerals occur. Each of these minerals 

 will expand and contract, when heated or chilled, at a slightly 

 different rate from the others, and thus the particles are subjected 

 to stresses which will gradually loosen them, causing the rock to 

 disintegrate. 



4. Wind 



Of itself the wind is unable to accomplish in any important 

 degree the disintegration of firm rocks, but when it can drift along 

 sand and fine gravel, it may effect much. Except on sandy coasts, 

 this agency is of small importance in regions of ordinary rainfall, 

 because in these the soil is protected and held together by its 

 covering of vegetation. In arid regions and deserts, on the con- 



