Sect. III. REEF-BUILDING CORALS LIVE. 109 
into which the lead sinks many feet. Outside this 
irregular border of Madrepores, the water deepens 
gradually to 20 fathoms, which depth generally is 
found at the distance of from half to three-quarters 
of a mile from the reef. A little further out the depth 
is 30 fathoms, and thence the bank slopes rapidly 
in co the depths of the ocean. This inclination is very 
gentle compared with that outside Keeling and other 
atolls, but compared with most coasts it is steep. The 
water was so clear outside the reef, that I could 
distinguish every object forming the rugged bottom. 
In this part, and to a depth of 8 fathoms, I 
sounded repeatedly, and at each cast pounded the 
bottom with the broad lead ; nevertheless the arming 
invariably came up perfectly clean, but deeply in- 
dented. From 8 to 15 fathoms a little calca- 
reous sand was occasionally brought up, but more 
frequently the arming was simply indented. In all 
this space the two Madrepores above mentioned, and 
two species of Astrsea with railier large stars, seemed 
the commonest kinds ; and it must be noticed that 
twice at the depth of 15 fathoms, the arming was 
marked with a clean impression of an Astraea. 
Besides these lithophytes, some fragments of the 
Millcpora alcicornis which occurs in the same relative 
position at Keeling Island, were brought up ; and in 
the deeper parts there were large beds of a Seriato- 
pora, different from S. subulata, but closely allied to 
it. On the beach within the reef, the rolled fragments 
consisted chiefly of the corals just mentioned, and of 
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