670 DYNAMICAL GEOLOGY. 



begins to ascend upon the salt water of the Gulf; and here this material "is left upon 

 the bottom, in the dead angle of salt water. A deposit is thus formed, whose surface 

 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 of these sand-bars or barriers, off the mouth of a 

 river, 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. 



The preceding map of Pamlico Sound and the region about Cape 

 Hatteras (Fig. 1099) 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 

 toward the ocean. This gives greater force to the river-current ; 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 outflow- 

 ing, stream and tidal current carry a greater quantity 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 main- 

 land 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 bordering 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. 1093, p. 652). Incipient sandbars at times 



