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CONDITIONS OF SEDIMENTARY DEPOSITION.
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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 narrow strip of deposition during rising tide.

Flocculation in salt water.—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 suspension 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. Practically 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 force of attraction or

  1. ↑ Humphreys and Abbott; op. cit. p. 445.
  2. ↑ Memoirs of the National Academy of Sciences, Vol. II, 1883, p. 168.
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