CONDITIONS OF SEDIMENTARY DEPOSITION. 495 



vertical eddy or " dead angle," in which material rolled on the, 

 river's bottom is left and some sediment is dropped. Thus bars 

 are formed in advance of deltas. 1 With rising tide or on shore 

 winds the elevation of the salt water surface will- increase this 

 effect and force the zone of maximum deposition shoreward, 

 while the reflux with the ebb or change of wind will lower the 

 incline and assist wider distribution of sediment. Hence there 

 is most rapid accumulation in the comparatively harrow strip 

 of deposition during rising tide. 



Flocculation in salt ivater. — Acids and salts in solution cause 

 fine particles of sediment to draw together in flocculent form 

 and therefore to settle more rapidly than they would in fresh 

 water. W. H. Brewer states that clay which has been in sus- 

 pension thirty months in fresh water had not settled out as 

 clearly as the same clay from a solution of common salt in less 

 than thirty minutes, 2 and he describes a number of experiments 

 tending to show that " when a muddy river enters salt water 

 chemical laws interfere with the purely mechanical ones. Then 

 the rate of deposition is affected by the salt more than by the 

 current, and velocities which would be more than sufficient to 

 carry the finer suspended matter indefinitely, if the water were 

 fresh, entirely fail where the water is brackish or salt. Practi- 

 cally it is the degree of saltiness which controls deposition." 



Brewer applies this principle to a discussion of the formation 

 of the bars of the Mississippi and concludes that the zone of 

 maximum deposition retreats and advances as the greater or less 

 volume of the river changes the position of the opposing salt 

 water. It is obvious that this condition would be combined with 

 that of the "dead angle" produced by the rise of the fresh 

 water on salt. 



The phenomena of flocculation have been attributed by 

 Hilgard, Brewer and Barus to chemical reactions, but Milton 

 Whitney finds a readier explanation in the forces of attraction or 



1 Humphreys and Abbott ; op. cit. p. 445. 



2 Memoirs of the National Academy of Sciences, Vol. II, 1883, p. 168. 



