EROSION AND SEDIMENTATION 307 



The great importance of erosion of soils is suggested by 

 the fact that the Mississippi River is estimated to carry over 

 1,000,000 tons of sediment into the Gulf of Mexico every 

 day, or over 400,000,000 tons per year. Professor Salisbury 

 says : " It would take nearly 900 trains of 50 cars each, each 

 car carrying 25 tons, to carry an equal amount of sand and 



FIG. 148. A late stage in erosion of deforested area 



The gullies have broadened until the whole of the original surface has been 



removed excepting where vegetation has protected it. Photograph by the United 



States Forest Service 



mud to the Gulf. All the rivers of the earth are perhaps 

 carrying to the sea forty times as much as the Mississippi." 



310. Vegetation and erosion. When visiting the banks of 

 a stream which has recently overflowed we usually find silt 

 or mud deposited upon the plants along the banks. The more 

 dense the vegetation, the larger the quantity of silt deposited. 

 In many overflowed regions deep layers of rich soil have been 

 built up through periodic deposits made in this way (fig. 149). 

 Where the water of the stream is flowing rapidly, it can carry 



