﻿THE CURRENTS OF THE OCEAN. 133 



mouths of other rivers equally strong and muddy in their currents, and magnificent in 

 their dimensions, but having no delta deposits; as the Canton, the Guayaquil, the Ama- 

 zon,* the Paraguay and others emptying into the Rio de la Plata, the rivers of Western 

 Europe, and all the rivers on the eastern coast of North America, north of Florida. 



Not only are deltas wanting where there are strong tides, but the mouths of the rivers 

 affected by the tidal currents branch out into wide bays or estuaries filled with shoals 

 composed of materials brought by the ocean (in our country quartzose sand), while the 

 greater part of the river matter is carried off through the channels leading into the sea, 

 owing to its lightness, and to the rapidity of the reflux caused by the retention of the 

 river streams during the flood tide. 



The conclusion, then, is drawn from these views of constructive action, that tides 

 and delta deposits are incompatible with each other ; that where there is a regular tidal 

 or normal current of any consequence there can be no delta formed, and that such a cur- 

 rent will always be a characteristic feature of those wide bays and river outlets where 

 deltas do not exist. This fact of the absence of deltas where there are regular tides is 

 distinctly stated by M. de la Beche in his " Manual," who accounts for the apparent ex- 

 ception in the case of the River Ganges by remarking that the formation of the delta 

 occurs in the rainy season, when the outward flow of the river is constant, overcoming 

 the current of the flood tide, and keeping the water fresh for many miles at sea. 



The most common and the best known of all the forms of ocean deposit remains 

 to be noticed ; that is, beaches. This term is applied indifferently to the borders of 

 seas and lakes composed of loose materials, whatever that material may be. Where 

 there is tide, it defines particularly the space included between low water and the 

 highest reach of the tides. Beaches of sand line the whole coast of the United 

 States, from Florida to the northern extremity of Cape Cod, sand being the material of 

 which this part of our coast is exclusively composed. And here every variety of form and 

 condition of beach is to be met with. If the shores are washed away by the destructive 

 action of the waves, and its materials taken off by the currents, the beach marks the 

 present limit of alteration. This is the case at the southeast part of Martha's Vine- 

 yard, where one mile in breadth of beach has disappeared in fifty years. 



* The effect of the equatorial current (passing Cape St. Roque) is to take off to the north and west 

 the solid matter brought down by the Amazon, and deposit it along the shore of French Guiana, or in the 

 adjacent sea, which it has made shallow to the distance of several leagues from this coast. I have not over- 

 looked this, and the remarkable character of the tides of the Amazon, nor the irregularities resulting from 

 the conflict of the tidal and equatorial currents. 



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