662 



DYNAMICAL GEOLOGY. 



in direct opposition. The in-flowing tide sets back the rivers, quiets 

 the waters, and floods the adjoining tidal flats ; and consequently a 

 deposition of detritus takes place over the flats, and the bed of the 

 stream. The turn of the tide sets the river again in full movement, 

 and it takes up the detritus deposited over its bed (but only little 

 of what fell over the flats) and bears it to the ocean. Here the cur- 

 rent loses much of its velocity in the face of the waves and with the 

 spreading of the waters, and hence a deposition of detritus goes on : 

 this continues until the next tidal flow dams up the fresh-water 

 stream anew. Between the ocean and the river there is a region 

 of comparative equilibrium in 

 the two movements, and there 

 the accumulations of sand or 



detritus take 

 sand-bars. 



place, forming 



Humphreys and Abbot observe, in 

 speaking of the Mississippi delta, 

 that as the river-water rises above 

 the salt water, from its low density, 

 there is a dead angle between the 

 two. The current out the Passes 

 pushes sand and earth before it, un- 

 til, reaching, it begins to ascend upon, 

 the salt water of the Gulf, and here 

 this material " is left upon the bot- 

 tom in the dead angle of salt water. 

 A deposit is thus formed, whose sur- 

 face is along or near the line upon 

 which the fresh water rises on the 

 salt water as it enters the Gulf; and 

 this action produces the bar." 



The distance off the mouth 

 of a river of these sand bars or 

 barriers will depend on the 

 size and strength of the rivers 

 on one side, and the height and 

 force of the tides on. the other. 

 Small streams are often blocked 

 up entirely by a sand-bar across 

 their mouths, and the waters reach the ocean only by percolation 

 through the beach. Large streams make distant sand reefs and 

 barriers even in the face of the ocean. The North American coast 

 from Long Island to Florida is fronted by ranges of barrier reefs 

 shutting in extended sounds or narrow lagoons. 



Fluvio-marine formation along the coast of 

 North Carolina. 



