DISCUSSION OF PLANT INDUSTEJES 441 



sunshine, and freedom from sudden changes in climate, it 

 is evident that there is a great future for irrigated lands. 

 Already over 13,000,000 acres are under irrigation, and other 

 projects that are now under way will add largely to that 

 amount. Even with this large acreage added to our tillable 

 soil, it must be kept in mind that only a very small portion 

 of the arid lands has been or apparently can be supplied with 

 water from the sources that are now available. 



408. Removal of soil by winds and water. Currents of air 

 constantly carry particles of dust. During periods when the 

 earth's surface is dry the amount of dust thus carried is large. 

 If a pane of glass that has been moistened with oil is exposed 

 for a time to the wind on a dry day, and then examined with 

 a strong magnifying glass, it will furnish a good demonstra- 

 tion of the dust-carrying power of moving air. Further illus- 

 trations are found in the dust that strikes our faces on windy 

 days, and in that which is deposited on window sills. Win- 

 dow panes in the houses near the end of Cape Cod finally 

 become translucent, like ground glass, from the action of sand 

 driven by the wind. When cultivated fields become dry the 

 wind may carry away large quantities of soil. This is some- 

 times well shown in winter when snow covers the ground in 

 such protected places as the leeward slopes of hills. Soil which 

 has been frozen dry is often carried from other regions by the 

 wind and dropped upon these leeward snow banks in such 

 quantities as to bury the snow completely. Good windbreaks 

 about cultivated fields help to prevent loss of soils by wind. 



Rapidly running surface water often carries away part or 

 all of the fertile soil. In grasslands, meadows, and forested 

 areas, surface water is retarded in its rate of flow, and conse- 

 quently does not carry away much soil. In regions that were 

 once forested and from which the timber has now been largely 

 removed, the surface water soon erodes ditches (Fig. 343), 

 which, with rapidly deepening channels and developing tribu- 

 taries, will in a few years carry away much of the fertile soil 

 of the forest floor. After forest fires, which themselves destroy 



