Page:The Journal of geology (IA journalofgeology11893univers).pdf/68

(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 narrow 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 Paraña 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 direction, 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 salt.—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, advancing, 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