ACTION OF MARINE CURRENTS 65 



the breakers would commit it to the strand. The result is 

 that the waves, which are only powerful when they run toward 

 the shore, oppose the work of the tides, and tend to return 

 to the margin of the continent the fine detritus which the tides 

 labor to carry away. 



While these two irregular classes of marine movements of 

 tides and waves are contending with the grains of sand and 

 mud, the ocean currents in their continuous, though slow, 

 pace share in determining the fate of the wandering bits of 

 stony matter. Almost all parts of the shallow water near the 

 shore are the seat of these oceanic rivers, which flow with tol- 

 erable steadiness in one direction. In their currents, which 

 generally flow at right angles with, the run of the tides and 

 waves, the particles of sand and mud are given a decided set 

 in one direction. Thus on the eastern coast of North America 

 there is a prevailing, though variable, southerly current which 

 skirts near the shore over the surface of the continental 

 shelf, and to a certain extent operates to convey all the sandy 

 matter from the northern part of the continent to the southern 

 part of its shallow waters next the coast. It is now contribut- 

 ing to the growth of the vast submerged sand-plains which 

 extend from Cape Cod to Cape Florida, and in past geologi- 

 cal ages has doubtless helped to bring to their place the exten- 

 sive deposits of sand and clay once a part of the continental 

 shelf but now uplifted into the dry land of the Southern 

 States. We may recognize the fact that these lowlands were 

 originally portions of the continental shelf, by their gently 

 undulating surface, which is exactly like that of the neighbor- 

 ing sea-floors, by the nature of the fossils in their beds, as 

 well as by other indications of a more recondite nature. In 

 fact, the greater part of the surface of the continents was 



