494 THE JOURNAL OF GEOLOGY. 



(4) Since tidal deposits are imperfectly sorted, they form 

 under shelter from waves or in the presence of waves of force 

 insufficient to handle the volume of sediment. The shelter may 

 be a point of land before a bay or a barrier of beach sand 

 before a lagoon ; in either case clean sands and mud deposits 

 may be contemporaneous. Or the feeble waves may be 

 unequal to the task of sorting, because of short fetch in a nar- 

 row sea. 



(d) Long continued or powerful winds. — The fall of a river 

 determines its current, other things being constant, and therefore 

 its transporting power. The fall near the mouth is lessened in 

 any given stream if the level of discharge is raised, and vice 

 versa, and the influence of tides in this respect has just been 

 discussed. Winds may exercise a no less important influence. 

 Revy (p. 27) describes an instance in which the effect upon the 

 tides of a storm approaching from the east, combined with its 

 subsequent direct effect in heaping up waters, was to raise the 

 level of the La Plata fifty inches at ebb tide, and to reverse the 

 current of the Parana for a hundred miles. An extraordinary 

 result like this is probably balanced in its effect upon deposition 

 by the scouring which takes place when the wind changes direc- 

 tion, or calms, and the mass of water returns to its normal level. 

 But the influence of long continued winds blowing periodically 

 during certain seasons of the year must be effective in causing 

 deposition from silt-laden rivers. Humphreys and Abbott briefly 

 discuss the nature of winds affecting the level of the gulf at the 

 mouth of the Mississippi, and assign an important share of the 

 results from deposition to the influence of the southeast winds. 1 



(e) Flotation of fresh water on .m/A— Fresh water is lighter 

 than salt water, hence a river discharging into the ocean rises 

 and spreads over the surface. The volume of the river, advanc- 

 ing, holds back the salt water, and the fresh water flows up an 

 incline which is the surface of contact between the media of 

 unlike densities. This checks the river's current and forms a 



1 Physics and Hydraulics of the Mississippi. Page 450. 



