﻿132 GEOLOGICAL ACTION OF 



in our great estuaries, the Delaware and Chesapeake Bays, must be attributed to the con- 

 fluent action of the tides. 



The normal currents of the ocean may, however, take the place of the tidal cur- . 

 rents, and exhibit the same effects. The constant current flowing into the Mediter- 

 ranean Sea may be cited as a striking example. At the Rock of Gibraltar this current 

 divides into two branches, one of which enters the Bay of Gibraltar, while the other 

 passes to the eastward of the Rock ; and the conflicting action of these two streams 

 has built up the long and narrow ridge of sand, known by the name of the Neutral 

 Ground, that unites the fortress to the Spanish peninsula. The large bank to the east- 

 ward of this ridge is an instance of bay deposit. In the Nehrungen of the Baltic, the 

 external current is probably one that is caused by the prevailing winds. I have no doubt 

 that these formations are to be accounted for in the same manner as all similar phe- 

 nomena, but am not sufficiently well informed concerning the currents of the Baltic to 

 speak of them in detail. Bars or belts of the same kind are found at the mouths of 

 the rivers emptying into Lake Superior, and on the south shore they are so long that the 

 river stream runs parallel to the land for some distance. These belts follow a constant 

 direction, different on different sides of the lake, determined, no doubt, by the currents 

 caused by the prevailing winds and by changes in the atmospheric pressure. 



It follows from what has been said, that there are three distinct conditions under 

 which the formation of belts and lagoons may take place. They may be the product 

 of tides only, of which the currents act internally and externally, as in the ponds and 

 small harbours, or partially inclosed basins, of Nantucket and Martha's Vineyard. They 

 will occur on a much larger scale where the tidal action is assisted by rivers, as in the 

 sounds and inlets of the Southern coast. And finally, the absence of a regular tide- 

 stream may be supplied by a normal current, as at Gibraltar and in the Baltic. In all 

 cases the direction of the belts is closely allied to that of the coast, being in general a 

 little convex towards that quarter from which the external current approaches. When 

 the tide is small, and the river rapid and loaded with matter, there is a strong tendency 

 to fill up the lagoons and transform them into swamps. Large and muddy rivers pene- 

 trate beyond the sea-border and build up that form of deposit called deltas. The deltas, 

 accordingly, are never surrounded by belts, like lagoons ; and as these belts are produced 

 by external currents, it follows that there can be no strong tidal or uniform ocean current 

 where there is a delta. At the mouths of all those rivers most distinguished for their 

 deltas, as the Mississippi, the Nile, the Po, the Rhone, and the Orinoco, there is little or 

 no tide. 



While, on the other hand, there are tides of a marked and decided character at»the 



