338 PRACTICAL LESSONS IN SCIENCE. 



process till the cliff is reduced to sand and carried away by the 

 wind. 



The combined action of wide and sudden variation of tem- 

 perature and the wind, has removed a thickness of more than ten 

 thousand feet of strata, from an area of thousands of square 

 miles, of the plateau through which the Colorado river flows. 



The sand and dust eroded by the wind from one locality is de- 

 posited in another. The dust is widely disseminated, and settling 

 quietly is constantly adding to the growth of soil. In this way 

 dust from the high plateaus of Asia has built up a formation of 

 loess from 1,000 to 2,000 feet thick over regions between the 

 plateaus and the ocean. The loess is unstratified, fine yellow cal- 

 careous clay, containing abundance of land shells and remains of 

 vegetation. Other loess seems to have been formed with the aid 

 of water, and is called lake loess or river loess. It is said in some 

 places in Asia dust enough falls to serve as a fertilizer for the soil. 

 This dust, which pervades the air everywhere, is of interest in the 

 matter of rain-fall, for it seems that water-vapor condenses only 

 on free surfaces, and the cloud and mist is formed by the conden- 

 sation of moisture on the particles of cosmic dust. 



The sand may be carried into rivers or lakes or into the sea, or 

 may form sand hills or dunes on the margin of the dessicated 

 region, as in northern Africa, Arabia and western United States. 

 But the most interesting sand dunes are those formed along the 

 margin of large bodies of water. When the prevailing winds are 

 from the water toward the land they carry sand from the beach 

 over the land till, meeting some obstruction, a dune is formed. It 

 may be ridge-like or somewhat conical in form; the seaward 

 slope is long and gentle, the other as steep as loose sand will 

 stand. 



The wind not only carries or rolls sand from the beach, up the 

 gentle slope to the crest, but is continually carrying sand from 

 the slope itself to the crest and letting it fall on the landward 

 side, so that the dune travels inland, destroy ing forests and crops, 

 covering buildings, filling ponds, damming up streams, etc. In 

 time another dune forms between the first and the beach, which 



