STRUCTURE OF CORAL ISLANDS. 143 



In the Paiimotus, the shore platform, the steep beach, and 

 the more gently sloping shore of the lagoon are almost con- 

 stant characteristics. 



The width of the whole rim of land, when the island gives 

 no evidence of late elevation, varies from three hundred yards 

 to one-third of a mile, excepting certain prominent points, 

 more exposed to the united action of winds and waves and 

 often from opposite directions, which occasionally exceed half 

 a mile. 



The shore platfonii is from one to three hundred feet in 

 width, and has the general features of a half-submerged outer 

 reef. Its peculiarities arise solely from the accumulations 

 which have changed the reef into an island. Much of it is 

 commonly bare at low tide, although there are places where it 

 is always covered with a few inches or a foot of water ; and 

 the elevated edge, the only part exposed, often seems like an 

 embankment preventing the water from running ofif. The 

 tides, as they rise, cover it with water throughout, and bear 

 over it coral fragments and sand, comminuted shells and other 

 animal remains, to add them to the beach. The heavier seas 

 transport larger fragments ; and at the foot of the beach there 

 is often a deposit of blocks of coral, or coral rock, a cubic foot 

 or so in size, which low tide commonly leaves standing in a 

 few inches of water. On moving these masses, which generally 

 rest on their projecting angles and have an open space beneath, 

 the waters at once become alive with fish, shrimps, and crabs, 

 escaping from their disturbed shelter; and beneath, appear 

 various Actiniae or living flowers, the spiny Echini and sluggish 

 biche de mar, while swarms of shells, having a soldier crab for 

 their tenant, walk off with unusual life and stateliness. More- 

 over, delicate corallines, Ascidiae and sponges tint, with lively 

 shades of red, green, and pink, the under surface of the block 

 of coral which had formed the roof of the little grotto. 



Besides the deep channels cutting into the margin of the 

 reef and giving it a broken outline, there are in some instances 

 long fissures intersecting its surface. On Aratica (Carlshoff) 

 and Ahii (Peacock Island) they extended along from a fourth 



