62 SEA AND LAND 



generally in the regions neighboring to the tropics, the beds 

 assailed by the sea are of more friable nature than those of 

 high latitudes, and yield vast (juantities of sand, which are dis- 

 tributed far and wide over a rather shallow sea-Hoor. The 

 greater part of the vast accumulations of sand along our 

 southern coasts come to the shore from the bottom of the 

 neighboring shallow sea. The way in which this sand w^orks 

 in against the coast can easily be understood through a 

 knowledge of the processes which are brought into action by 

 the movements of the sea-waves over the shallow continental 

 shelf. First, as to this continental shelf or fringe of shallows 

 which skirts the ocean shores. 



If the reader will take the admirable general charts of the 

 Atlantic shore and neighboring sea which have been prepared 

 by the United States Coast Survey, he will find, on examin- 

 ing the soundings of the district from the St. Lawrence to 

 Florida, that the water very gradually deepens from the sea- 

 margin for a varying distance, amounting in places to as 

 much as one hundred miles, declining generally at the rate of 

 onh' four to six feet in a mile, for a distance from the shore, 

 and then plunging down steeply to the abysmal depths of the 

 sea. On the coast of Europe there is a similar shelf, and 

 researches in otlier parts of the ocean seem to indicate that 

 this broad platform is a common feature of the continental 

 margins, being present along all parts of the great lands 

 which ha\e long been elevated above the se;i and thus much 

 exposed to the action of the tides and waves. This shelf is 

 doubtless in the main composed of the waste from the neigh- 

 boring land which has been taken to sea by the rivers, or con- 

 tributed by the sea-waves to the ocean bottom. These con- 

 tributions of sediment have been borne to their places mainly 



