238 CORALS AND CORAL ISLANDS. 



the ocean swell, like the wind, comes from the southeast, to 

 the force of which the south side of the island is exposed, 

 while the western side is protected. In consequence, the sands 

 of the beach that have been accumulating during the summer 

 on the south side, are all washed around the southwest point 

 and are heaped up on the western side, forming a plateau 

 along the beach two or three hundred feet wide, nearly cover- 

 ing the shore platform, and eight or ten feet deep. With 

 October and November comes the winter swell from the north- 

 northeast, which sweeps along the western shore, and from 

 the force of which the south side is in its turn protected. 

 Then the sand begins to travel from the western to the south- 

 ern side ; and, after a month or two, nothing remains of the 

 great sand plateau but a narrow strip ; while on the south 

 side, the beach has been extended two hundred or three hun- 

 dred feet. This lasts until February or March, when the 

 operation is repeated." 



II. CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 



Coral reefs, although (1) dependent on the configuration of 

 the submarine latids for many of their features, undergo vari- 

 ous modifications of form, or condition, through the influence of 

 extraneous causes, such as (2) unequal exposure to the waves ; 

 (3) oceanic or local currents ; (4) presence of fresh or impure 

 waters. In briefly treating of these topics, we may consider 

 first, reefs around high islands, and afterward, atoll reefs. 

 The effect of the waves on different sides of reefs has already 

 been considered, and we pass on, therefore, at once to the 

 influence of oceanic or local currents, and fresh or impure 

 waters. 



