toa solid Yock, the interstices having been fille 
368 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 sometithes viewed as only traps to surprise and 
wreck the unwary mariuer. But one who has visited the dreary 
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 OF 
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 . 
k 
These deposits become cemented by being alternately moisten- 
ed and dried, through the action of the recurring tides, and the 
mass was formed. Generally, even the most solid varieties 
e of a sand origin, and in this the 
| be -pebbly beds produce a puddin 
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