68 SEA AND LAND 



the sands are constantU- and rather rai)idly moving to the 

 southward. At the last-named cape the coral reefs carry the 

 shore out to the edge of the northward-flowing Gulf Stream, 

 which speedily conveys a great part of the mobile waste into 

 the deep valley between the Bahama Islands and the margin 

 of the continent. Where these coast-moving sands abound the 

 beaches usually lie upon the seaward face of long, low, sandy 

 islands separated from the shore by lagoons. These sand-bar 

 islands are a very conspicuous feature along all the shore 

 from Portsmouth, N. H., to Cape Florida, and again along 

 the Gulf of Mexico from the western coast of Florida to the 

 mouth of the Rio Grande. This peculiar type of beaches 

 appears to be originated under the following diverse condi- 

 tions : Whenever by an elevation of the shore-line a new 

 beach-line is to be formed farther out to sea than that which 

 previously existed, the construction is begun, not at the very 

 water's ^A^o., but at a distance out from the shore at the 

 depth where the storm-waves break, in perhaps twenty feet 

 of water. When the w^ave topples over in the surf, all the 

 sand which it has swept forward from the seaward falls down, 

 and each successive wave adds to the supply, until the mass 

 reaches to the top of the water and forms a new bar. Upon 

 this elevation the storms Ijiiild yet more sand, grasses take 

 root, and low dunes are formed. As the waves bring in not 

 only sand but much shelly matter and the bones of fishes, the 

 deposit may make tolerably fertile land, such as is found on 

 the long beach islands which border the great lagoon known 

 as Indian River in eastern Florida. In other cases, where 

 the sea-shore slowly sinks, similar islands may be formed in 

 a rather different way. If the continental land is low, as it is 

 next the sea in all the Southern States in this country, the 



