FORMATION OF CORAL REEFS AND ISLANDS. 203 



mulating during the summer on the south side, are all washed 

 around the south-west point and are heaped up on the western 

 side, forming a plateau along the beach two or three hundred 

 feet wide, nearly covering the shore platform, and eight or ten 

 feet deep. With October and November comes the winter 

 swell from the north- north-east, 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 southern 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 

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

 operation is repeated," 



IT. CAUSES MODIFYING THE FORMS AND GROWTH OF 



REEFS. 



Coral reefs, although (i) dcpciident oiithe coniiguration of the 

 snbuiai'ine lands for ma7iy of their features, undergo various 

 modifications of form, or condition, through the influence of 

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

 (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 afterwards, 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. 



I. BARRIER AND FRINGING REEFS. 



The existence of harbours about coral-bound lands, and of 

 entrances through reefs, is largely attributable to the action of 

 tidal or local marine currents. The presence of fresh-water 

 streams has some effect toward the same end, but much less 

 than has been supposed. These causes are recognized by 

 Mr. Darwin in nearly the same manner as here : yet the views 



