﻿THE CURRENTS OF THE OCEAN. 131 



rent performs the circuit of a bay, and in which it operates in building up the belt 

 or outer border of the pond or harbour, after the style of a hook. That part of the 

 tidal current which enters the lagoon, and revolves around its inner limit, not only 

 assists this operation, but probably determines the place of the belt, and in some decree 

 its form. Its place must be that of the confluence or meeting of the internal and exter- 

 nal currents. When these lagoons are in the process of building up, a ripple upon the 

 extremity of the belt, and beyond it in the line of its direction, indicates the seat of 

 conflict between these two opposing forces. Now, in the case of the great lagoons of 

 the Southern coast, this conflict derives a more distinct and permanent character from 

 the currents of the rivers. There is a more powerful and complete action on the inside, 

 arising from the cooperation of the tidal and river currents, the latter acquiring additional 

 strength on the ebb from their being checked by the flood. The opening to the sea is 

 preserved by the rivers that demand an outlet for their discharge, assisted by the flux 

 and reflux of the tide. Across these openings lie bars of sand, formed, as has been said, 

 by the deposits of the ocean tides, and not by the river deposits, which are wholly dis- 

 similar in mineralogical composition.* The depth of water upon the bar will depend 

 upon the size of the rivers, the capacity of the lagoons, and the rapidity of the tidal or 

 ocean currents. Therefore, under favorable circumstances, the bar might, in the case of 

 the Southern lagoons, as of the Northern ponds, rise to the surface, and, by completing 

 the belt, break off the communication with the sea. The streams would discharge them- 

 selves into a lagoon thus inclosed as into a lake, and change them from salt to fresh 

 water. Finally, the lagoon might be filled up by the river deposits, and a soil be accumu- 

 lated suitable for tillage, even below the surface of the sea. Mr. Lyell's similar cases 

 may be again referred to. (Principles of Geology.) 



By these successive changes a home is created for animals of salt, brackish, and 

 fresh water, in the same spot, where their remains may be found buried together; 

 as they are now in the tertiary basins of Paris and London. The principle of conflu- 

 ence is illustrated wherever two moving streams of water meet, whether they belong to 

 the ocean or to an inland sea, to tides or to rivers, or to both combined. 



An instance of an imperfect lagoon formation, of classical celebrity, is that of Venice. 

 The lidi are belts resulting from the confluence of the gulf and river waters. At the 

 meeting of the Rhone and Arve a similar deposit in ridges is formed. In the beds of 

 the small rivers that empty into the harbour of Boston the same results are produced, 

 though here, of course, the influence of the tidal currents predominates. The shoals 



* Professor H. D. Rogers has shown that these formations are caused by the meeting of the rivers with an 

 external current. But he considers the Gulf Stream as the external current. 



