156 CORALS AND CORAL ISLANDS. aan 
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often forty feet or more; and in a region subject to Vio- 4 | 
lent tempests or cyclones, like those of the Bahamas and/ | 
the Bermudas, they raise hills to a height of one or nce 
hundred feet, that are sometimes one or two miles broad, | | 
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and may make banks that are many miles in width. J 
About cavities over the surface, the rock is usually very 
compact to a depth of half an inch or more, almost horny 
in texture, owing to the infiltration of lime from the 
waters often occupying them; but this is an exceptional 
variety of the rock. 
Odlitic beds appear to be confined to the superficial forma- 
tions of a reef, that is, to the beach and wind-drift accumulations. \" 
No example has come under the notice of the author of odlite \ | 
constituting the foundation rock of a reef orisland. Itis possi- . 
ble that such beds might in some cases be the basement rocks to a 
considerable depth below ; for a reef-island might subside so | 
much more slowly than coral formations grow and accumulate, ( 
that a succession of beach-made beds would be produced even 
to a great thickness. Yet the probability is that the subsi- 
dence would sink the surface beneath the water, and put an 
end to beach and wind-drift work. The beach slope of 6° to 
8°. is an almost constant mark of beach-made beds. 
Vill. THICKNESS OF REEFS. 
We have considered in the preceding pages the peculiari- 
ties of form and structure characterizing the reef formations | 
bordering islands and continents, and their influence upon the 
enclosed land. Could we raise one of these coral-bound islands 
from the waves, we should find that the reefs stand upon the 
submarine slopes, like massy structures of artificial masonry ; 
some forming a broad flat platform or shelf ranging around 
