RESIDUAL, GRAVITY-LAID, WATER-LAID DEPOSITS 35 



a film .01 mm. thick has a specific gravity of 1.0002. This is so near 

 the specific gravity of water that the particle will remain in suspen- 

 sion indefinitely. Professor King estimates that a force of only 4.4 

 pounds is necessary to keep the 11 tons of sediment in suspension 

 that is delivered at the mouth of the Mississippi Kiver each second. 

 The increase of the effective diameter of the particle augments the 

 effective cross-section 441-fold, and so only a very small vertical 

 motion would he required to maintain suspension. The effective 

 volume would he increased 9,201-fold by the adhering film. When 

 a particle of dust is suspended in the atmosphere it attracts a film 

 of air which moves with it in the same way as the film of water and 

 lessens its specific gravity, thus enabling it to be held in suspension 

 with a much smaller force than would otherwise be necessary. The 

 specific gravity of a clay particle .001 mm. in diameter, with an 

 adherent film of air, is 1.233G. For computing the specific gravity 

 of a particle immersed in water the following formula may be used: 



Sp. Gr.= 



?rd 3 X sp.gr. 

 0~ 



d 3 X sp.gr. + 

 D 3 



*D 



6 



irD 3 



- = volume of a sphere. 

 o 



d = diameter of particle or solid nucleus 



sp. gr. = specific gravity of the nucleus 



D=- diameter of solid-fluid system 



Sp. Gr. = specific gravity cf the solid-fluid system 



The amount of material carried by the Mississippi River and 

 deposited in the (Julf of Mexico annually is equivalent to the re- 

 moval in (5,000 years of a layer one foot thick over the entire drain- 

 age area. 



Amount of Sediment Carried in Suspension Annually 5 



