CONDITIONS OF SEDIMENTARY DEPOSITION. 485 



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 de- 

 posits beyond the littoral zone consist of particles of an average 

 diameter of .05 mm. 



Deposition occurs whenever a body of water becomes over- 

 loaded with substances in suspension or in solution. According 

 to the condition which determines the result the deposits may 

 be classified as mechanical, chemical and organic. 



MECHANICAL DEPOSITION. 



Favorable Co?iditio?is : 



[a) Arrest and retreat of waves ; beaches and sand deposits 



from undertow. 



(b) Current entering still water and slowing ; lake-deposits. 

 (V) Alternating currents in fresh and salt water ; estuarine 



deposits. 



(d) Rise of salt water surface at a river's mouth in conse- 



quence of winds, long continued from one direction ; 

 delta of the Mississippi. 



(e) Flotation of fresh water on salt; bars of the Mississippi. 

 (/) Floculation of sediments in salt water. 



(g) Expansion and diffusion of a current in rapidly deepen- 

 ing water ; silt deposits on the edge of continental 

 plateaus. 

 (^) 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, 



1 Agassiz. Three Cruises of the Blake. Bull. Mus. of Comp. Zoology. Har- 

 vard College. Vol. XIV. 



