Rivers and Valleys 



about five hundred cubic miles of matter. Now, 

 this great river carries out to sea about one- 

 twentieth of a cubic mile of sediment each year. 

 This sediment which goes into the sea is in small 

 part directly derived from the action of the 

 mountain-torrents; in larger part, it is composed 

 of waste taken from the alluvial plains by the 

 wanderings of the various streams which con- 

 stitute the Mississippi system of waters. It 

 therefore follows that the average time required 

 for the sediment discharged from the mouth of 

 the Mississippi to make its way from the head- 

 waters to the sea is not less than ten thousand 

 years. As soon as a pebble or other bit of rock 

 is laid away in the alluvial terrace, it begins to 

 decay; the vegetable acids which penetrate the 

 mass in which it finds lodgment favour its dis- 

 integration. When it is turned over by the 

 stream at the time of encroachment on its resting- 

 place, it probably falls to pieces, the finer bits 

 are hurried onward by the stream, those too 

 coarse for the current to control are again stored 

 away in the bank to await further decay. In 

 this manner the alluvial material lying on either 

 side of rivers is a great storehouse, or rather we 

 should say laboratory, in which sediments are 

 divided and brought into a chemical condition 

 which permits them to be taken into the control 

 of the waters and borne away to the ocean, in 

 order to become rebuilt into strata, which are in 

 time, with the growth of the continents, to be- 

 come dry land and be again subjected to this 

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