THE BUILDING OF THE LAND 21 



occasional interruptions around the Gulf of 

 Mexico to Yucatan, is a series of long, narrow, 

 sandy islands and peninsulas lying parallel with 

 the mainland shore and at no great distance from it. 

 They are usually low; rarely rising higher than 

 the limits of a storm tide, though in places they 

 assume the character of sand dunes, with a little 

 greater elevation. Between these islands and the 

 main shore there are usually shallow lagoons some- 

 times called rivers, though their water is salt or 

 brackish. In some places these lagoons fill up with 

 sediment with little or no water remaining and 

 thus form brackish swamps. It has often been 

 asserted that these sandy coastal islands result 

 from ocean currents running parallel with the shore 

 which carry and deposit sand in long, narrow bars, 

 constantly adding to these bars at the end where 

 the retreating water leaves them. While this may 

 be true in some cases I do not believe that the 

 action of such currents alone has formed most of 

 these islands. In some instances these narrow land 

 bodies run parallel with the shores of bays where it 

 seems unlikely that any ocean currents woxild sweep 

 along the deeply incurved beaches. Besides this, 

 such long-shore currents could only build up the sand 



