NATURE AND FORMATION OF SOILS. 7 



ring up the ocean into waves and l)illows, which 

 beat upon the rocks, carr^ino- with them sand 

 and pebbles, which grind each other into powder. 



On the sandy beach of the ocean and of the 

 great lakes, and in the great sandy plains, or 

 wherever the sand is loose and unprotected by 

 vegetation, the wind becomes a potent factor 

 (Figs. 1-2). Along the shore of Lake Michigan 

 sand-dunes are destroying forests^ and often 

 when the forests have been cut off, fertile farms 

 are covered by these great accumulations of 

 wind-blown sand. In conjunction with sand, 

 the wind builds or destroys islands. The loess 

 in China is a deposit of wind-blown soil. 



In the desert of Sahara and in our gfreat 

 western plains great blinding storms of dust 

 and sand occur. The sand, too heavy to be lifted 

 more than a few feet hiofh, is rolled alon^ and 

 drifted in wave-like mounds, which change their 

 shape and position with the changes in the direc- 

 tion of the wind — just as the snow-drifts are 

 formed in waves — and the particles are sucked 

 up into the whirling air, and redeposited in a 

 new place by the force of gravity as the motion 

 subsides. One of our "blizzards" is a o-ood il- 

 lustration of a sand-storm, only the substance 

 transported by the wind is snow instead of dust 

 and sand. 



3. Che77iical Action. — Another phase of atmos- 

 pheric work is that of chemical action. Dry air 



