106 GEOLOGY, 



the sea in solution, the average rate of degradation for the Mississippi 

 basin is estimated at one foot in 3000 to 4000 years. 



It is not to be inferred that this rate is uniform, or even that erosion 

 at any rate whatsoever is taking place in all parts of the basin. Such 

 is not the fact. On the whole the rate of erosion is doubtless greatest 

 toward the margins of the basins where the land is in its topographic 

 youth or early maturity. It is notably less in the middle courses 

 of the valleys, and erosion is locally exceeded by deposition along 

 the lower courses of the Mississippi and some of its main tributaries. 



The average elevation of North America is not accurately known, 

 but it is probably not far from 2000 feet. If the present rate of degra- 

 dation, say one foot in 3500 years, were to continue, it would take 

 something like 7,000,000 years to bring the continent to sea-level. 

 But this rate of degradation could not continue to the end, for as the 

 continent became lower streams would become sluggish and erosion 

 less rapid. Long before the continent reached base-level the rate of 

 degradation, so far as dependent on mechanical erosion, would become 

 so slow that the time necessary to bring the continent to sea-level 

 would be almost inconceivably prolonged. Furthermore, it is quite 

 possible that the land is suffering, or is Hable to suffer, uplift, relative 

 or absolute. If the rate of rise were equal to the rate of degradation 

 the average height of the continent would of course not be affected. 



The amount of sediment carried by streams in suspension varies notably 

 according to the stage of the water. During a year when the stream was under 

 careful study the Mississippi at Carrollton (Miss.) was found to carry -^i^- of 

 its weight of sediment during the high-water stage of June, and -g^^^ during 

 the low-water of October, the average for the year being yg^g. The average 

 of a greater number of records gives about j-^\-^ as the average ratio between 

 the weight of the sediment and the weight of the Avater. This corresponds to 

 about -g-gVo by volume, the average specific gravity being about 1.9. The 

 amount of material carried in the upper part of the water was notably less than 

 that carried at greater depths, but that carried midway between top and bottom 

 was about the same as that carried at the bottom.^ 



The discharge of the Mississippi River is about 19,500,000,000,000 cubic feet 

 of water per year, and the sediment it carries in suspension is estimated to weigh 

 about 812,500,000,000 pounds. This is equivalent to about 6,714,694,400 cubic 

 feet. It is estimated that about 750,000,000 cubic feet of sediment is rolled 

 along the bottom, giving a total of 7,468,694,400 cubic feet as the aggregate 

 annual load carried to the Gulf by the river. This would be adequate to cover 

 an area one square mile in extent to the depth of 268 feet per year. 



^ Humphreys and Abbot. Physics and Hydraulics of the Mississippi River. 



