Page:The Journal of geology (IA journalofgeology11893univers).pdf/59
current to scour and take on more load; but broadening channel or deepening water tends to cause it to deposit. The Gulf stream scours the straits of Florida and the Blake plateau, but deposits a silt bank on the lee side of the latter.[1] Only in the broad expanse of deep water does it widely distribute sediment.
(d) Size of particles.—Fine or light sediment is most widely distributed. The "blue muds" which form the terrigenous deposits beyond the littoral zone consist of particles of an average diameter of .05 mm.
Deposition occurs whenever a body of water becomes overloaded with substances in suspension or in solution. According to the condition which determines the result the deposits may be classified as mechanical, chemical or organic.
MECHANICAL DEPOSITION.
Favorable Conditions:
- (a) Arrest and retreat of waves; beaches and sand deposits from undertow.
- (b) Current entering still water and slowing; lake-deposits.
- (c) Alternating currents in fresh and salt water; estuarine deposits.
- (d) Rise of salt water surface at a river's mouth in consequence of winds, long continued from one direction; delta of the Mississippi.
- (e) Flotation of fresh water on salt; bars of the Mississippi.
- (f) Floculation of sediments in salt water.
- (g) Expansion and diffusion of a current in rapidly deepening water; silt deposits on the edge of continental plateaus.
- (h) Final subsidence from oceanic circulation.
Arrest of Waves.—(a) Beaches are formed where waves break. The rotary oscillation which constitutes waves in deep water becomes a motion of translation when the water shallows sufficiently and the mass of the broken wave, rushing forward,
- ↑ Agassiz.Three Cruises of the Blake.Bull. Mus. of Comp. Zoölogy.Harvard College.Vol. XIV.