WATER. 663 



The accompanying map of Pamlico Sound and the region about 

 Cape Hatteras (fig. 947) illustrates this feature of the continent. 



The numerous rivers of this well-watered coast carry great quan- 

 tities of detritus to the ocean, — part of which is borne out to sea to 

 raise the great submarine plateau of the coast, and another part is 

 added to the barrier and to the banks and flats of the Sound. The 

 contraction of the Sound, which is going on by the additions to the 

 flats and over its bottom, gradually prolongs the channel of the 

 river towards the ocean. This gives greater force to the river-cur- 

 rent, and it acts in conjunction with the strong ebb tide against the 

 inner side of the barrier, in slowly wearing it away. At the same 

 time, the outflowing stream and tidal current carry a greater quan- 

 tity of detritus into the ocean, contributing sand to the beach and 

 finer detritus to the plateau, the nature of wave-action on a beach 

 being such as to leave only the sand or coarser material. Thus, by 

 a slow process, the mainland gains in breadth, and the river in 

 length, and the barrier moves gradually seaward. In other cases, 

 the lagoons inside of the barrier become filled, and a continuous 

 marsh, and ultimately dry land, is made out to the barrier. All the 

 low lands along the eastern coast of the continent, and that border- 

 ing on the Gulf of Mexico, in most parts many scores of miles in 

 breadth, have been made in the manner here pointed out. 



When the tides are very small, or fail altogether, the rivers may 

 reach the sea by many mouths without the formation of barriers, 

 or, in other words, may form true deltas. The height of the tide 

 of the Mexican Gulf along the north shore is but twelve to fifteen 

 inches; and, consequently, while most of the streams, before even 

 this small tide, have their bars and barriers, the great Mississippi 

 sends its many arms far out into the Gulf, prolonging its channels in 

 the face of winds, waves, and tide (fig. 942, p. 64G). Incipient sand- 

 bars at times form ; but these serve only to divide one of the great 

 channels, and make a new branch. 



The course of the out-flowing currents during ebb tide, in con- 

 junction and alternation with the in-flowing tidal current and waves, 

 determines the position of the channels and sand-bars, and causes 

 the prolongation of hooks off prominent capes. ' In some cases, 

 wind-currents are concerned in the action. The general process is 

 the same as when only the currents of the ebb and flow are con- 

 cerned; but the ebb tide has far greater effect from the added 

 volume, velocity, and detrital material of the river. The out-flowing 

 currents are deep and strong, sweeping out the channels of bays and 

 lagoons, and moulding the sand bars and spits ; while the in-flowing 

 move in a more diffused manner, and with much less rapidity and 



