16 ON CORAL REEFS AND ISLANDS. 



wide plains are spread out with breadfruit and other tropical pro- 

 ductions. Ports, safe for scores of vessels, are also opened by the 

 same means, and some islands number a dozen, when the unpro- 

 tected shores would have hardly offered a single good anchorage. 

 Coral reefs are sometimes viewed as only traps to surprise and 

 wreck the unwary mariner. But one who has visited the dreary 

 prison-house, St. Helena, can have some appreciation of the ben- 

 efits derived from the growth of the zoophyte. 



The area of level shores, alluded to as added to many of the 

 high islands by this means, is one of the most striking of these 

 benefits. These plains are sometimes of large extent. The reefs 

 stop the detritus from the hills, and are thus the means of its be- 

 ing added again to the land: they prevent, therefore, the waste 

 which is constantly going on about islands without such barriers ; 

 for the ocean not only encroaches upon the unguarded shores of 

 the smaller islands, but carries off whatever the streams may 

 empty into it. The delta of Rewa, on Viti Lebu, resulting from 

 the detritus accumulations of a large river, covers nearly sixty 

 square miles. This is an extreme case in the Pacific, as few isl- 

 ands are so large, and consequently rivers of such magnitude are 

 not common. But there is rarely an island which has not at least 

 some narrow plains from this source ; and upon them the villages 

 of the natives are usually situated. Around Tahiti these plains 

 are from half a mile to two or three miles in width, and the co- 

 coanut and breadfruit groves are mostly confined to them. 



Beach sandrock. — Besides the accumulations from a shore 

 source, there are also beach formations derived from the reefs. 

 The tides and the attending currents carry to the shores more or 

 less coral sand with shells and other reef-relics, and these some- 

 times form large deposits. The material is mostly like common 

 sand in fineness, but often somewhat coarser, or even like a bank 

 of pebbles. When the barrier is distant, only the sand and smaller 

 pebbles are met with ; but if the reef is quite narrow, there may 

 be larger fragments and masses of coral rock. 



These deposits become cemented by being alternately moisten- 

 ed and dried, through the action of the recurring tides, and the 

 wash of the sea on the shores. The waters take up some carbo- 

 nate of lime which is deposited and hardens among the particles 

 on the evaporation of the moisture at the retreat of the tides. In 

 some places the grains are loosely coherent, and seem to be united 

 only by the few points in contact. ; and with a little care, the cal- 

 careous coating which caused the union may be distinctly traced 

 out. In other cases, the sand has been changed to an oolite, or 

 to a solid rock, the interstices having been filled till a compact 

 mass was formed. Generally, even the most solid varieties show 

 evidence of a sand origin, and in this they differ from the reef- 

 rock. The pebbly beds produce a pudding-stone of coral. 



