209 



very richest soil matei'iiil, the crciiiii of the soil. The x'iiliie of the material 

 is not easily fixecL but at a moderate aiipraisal tlie aiuinal loss wouM 

 exceed all the laud taxes of the country. Besides impoA'erishiug tlie soil, 

 the sediment pollutes the water, reducing their value for domestic and man- 

 ufacturing purposes and endangering the lives of those compelled to use 

 them, and causing streams tv scour their channels and build bars; and 

 through scouring and liuilding it compels the lower rivers to shift and 

 overflow, thereby reducing the value of fertile bottom lands. However 

 estimated tlie loss is enormous, and the chain of evils resulting fi-om tlu- 

 annual erosion of this billion tons of soil is long and complex and leads 

 directly back to the farm." 



How easily and rapidly water may traiisi)ort objects with the increas- 

 ing swiftness of the current is seen from the following experiment given 

 by Page'-: "It has been found by experiment that a current moving at the 

 rate of three inches per second, will take up and carry fine clay; moving 

 six inches per second, will carry fine sand; eight inches per second, coars'J 

 sand, the size of linseed ; twelve inches, gravel ; twenty-four inches, pel>- 

 bles; three feet, angular stones of the size of a hen's egg." 



"It will be readily seen from the above," says Le Conte'", "that the 

 cirnjing-poiver increases much more rapidly than the velocity. For instance, 

 a current of twelve inches per second carries gravel, while a current of 

 three feet per second, only three times greater in velocity, carries stones 

 many hundreds of times as large as grains of gravel." "A current" run 

 ning three feet per second, or about two miles per hour, will move frag- 

 ments of stone the size of a hen's egg. or of about three ounces' weight." 

 Then from the law established we say'' "a current of ten miles an hour will 

 carry fragments of one and a half ton, and a torrent of twenty miles an 

 hour will carry fragments of iOO tons' weight. We can thus easily under- 

 stand the destructive effects oi: mountain-torrents when swollen by floods." 



Hall and Maxwell"' state that "when the slope exceeds 10 per cent., 

 cultivation does not long go on before erosion sets in. and erosion If un- 

 checked will remove the soil awO. sully the surface until all fertility has 



'2 Pa e's Geology, p. 28. Quoted by Joseph Le Conte in his Elements of Geology, Fourth Edi 

 tion, pp. 18-19. 



13 Le Conte, 1. c, p. 19. 



"Le Conte, I. c, p. 20. 



" Hall, Wm. L. and Maxwell, Hu. Surface Conditions and Stream Flow.— Forest Service Circu- 

 ar 176, p. 10. 



] 4—101 9 



