THE OCEAN. 683 



The distance of sand-bars or barriers off the mouth of a river gen- 

 erally depends on the size and strength of the river 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 or barriers, and keep open channels 

 even in the face of the ocean. The attempt to block up the channels 

 of a large harbor or bay by obstructions, like that in the case of the 

 harbor of Charleston, where ships were sunk to fill the channels, is 

 always a failure, because the force that made the channel still remains. 

 Only by diverting the rivers that do the chief part of the work could 

 such an attempt be effectual. 



The North American coast, from New York to Florida, is fronted 

 by ranges of barrier reefs, shutting in extended sounds or narrow la- 

 goons. 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 quantities 

 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, and is drifted by 

 the tidal current along the coast, in the same manner as along the 

 south side of Long Island. 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 outflowing stream and tidal current carry 

 a greater quantity of detritus into the ocean, contributing sand to the 

 beach, and through the drifting tidal movements, the 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 and keeps its even outline. 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 those bordering on 

 the Gulf of Mexico, in most parts many scores of miles in breadth, 

 have been made by such means and methods. 



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 streams, before even so small a tide, 



