20!) 



very richest soil niaterial. tlic ni am oT the soil. Tlic \aluo i>r ilic inaiei'ial 

 is not easily fixed, luit at a luodi'ratc atipraisal tlie annual loss woul'l 

 exceed all tlie laud taxes of the eouuti y. I'.esides impoverishing the soii. 

 tile si'dinient iioUntes the water, redncin.t; their value for domestic and inan- 

 nfa'-turing purposes and eudan.nerinii- the li^■(•s of those roni]>i'llf(l to use 

 them, and causin;,' streams In scour their channels and lauld bars: .'nid 

 through scouring and hnildin;;' it compels the lower rivers to slnt't and 

 overflow, tliereby reducinj^ the value of fertile Itottom lands, llowevci- 

 estimated the loss is enormous, and the chain of evils resultint; from tli.' 

 annual erosion of this billion tons of soil is loni;- and complex and leads 

 directly back to the fann."' 



How easily and rapidly watei- may Iians|iort objects with the increas- 

 iuij: swiftness of the (airrcnt is seen from the folhiwinu- experiment ,ij;ivcn 

 by Page'-': "It has been found Vi.\ experiment that a current mo\ing at the 

 rate of three inches per .second, will take up and carry Jiin- rlaii; moxing 

 six inches per second, will '-jii-ry /iiic siuid: t^ght inches pei- second. ■f«//-.s" 

 sand, the size of lin.seed ; twelve inches, gravel; twenty-four inches, peb- 

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



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

 c^rn/ui()-i)()nrr incfeascs uiinh iiii.ic niitidi// than the vclocitii. 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 

 niug three I'eet per secojid, or about two miles per hour, will move frasr- 

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

 Then i'nan the law established we say" "a cairrent of ten miles an hour will 

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

 hour will carry fragments of 300 tons' ^veight. We can thus easily under- 

 stand the destructive efferts oi mouidain-torrents wh.en swollen by Hoods." 



Hall and MaxwelT"' 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 rcjnove thi^ soil and uully the surface until all fertility has 



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

 tion, pp. IS-f). 



"he Conte, 1. c, p. 19. 



" Le Conte, 1. c, p. 20. 



's Hall, V\m. L. and Maxwell, Ha. Surface Condilions and Stream Flow.— Forest Service Circu- 

 ar 176, p. 10. 



1 4—101 i) 



