CHAPTEB XLV. 



GEOLOGICAL ACTION OF AIR AND WATER. 



AIR and water; winds, clouds and rain; frost, snow and ice; 

 springs, creeks and rivers ; waves, tides and currents, are the most 

 common objects and phenomena of the material world. They are 

 so common and familiar that most people fail to realize that they 

 are important, efficient, essential agents in nearly all geological 

 phenomena. 



Quiet, dry air of uniform temperature is not an efficient agent, 

 but changes of temperature, which may be mentioned here, are 

 of great importance. The daily variation of temperature in 

 some dry regions is very great. On the southwestern plateaus of 

 North America the temperature varies from 90 F. at mid-day to 

 20 F. during the night, and the same is true of wide regions in 

 Africa and Asia, a variation of 90 F. between day and night be- 

 ing by no means uncommon. The expansion and contraction 

 due to such great and rapid change of temperature soon causes 

 rocks to crack and flake off and rapidly disintegrate into sand. 

 Dr. Livingstone, while in Africa, found that rocks heated up to 

 130 F. during the day, cooled so rapidly during the night that 

 they cracked and split, throwing off angular fragments of all 

 sizes up to those of a hundred pounds weight. In the dry regions 

 the rocks are broken up in this way almost as rapidly as in 

 regions where frost is the active agent. 



The winds carry away the dust and sand formed, and the sur- 

 face of the region is gradually lowered by this dry erosion. The 

 sand carried in this way by the wind acts as a piece of sand- 

 paper, polishing and cutting down the rocks. These natural sand 

 blasts sometimes cut into the base of a cliff, and when under- 

 mined blocks fall, attack them in the same way, continuing the 



L.S.-22 (337) 



