Sect. I. 
KEELING ATOLL. 
9 
it is possible only under the most favourable circum- 
stances, afforded by an unusually low tide and smooth 
water, to reach the outer margin, where the coral is 
alive. I succeeded only twice in gaining this part, and 
found it almost entirely composed of a living Porites, 
which forms great irregularly rounded masses (like 
those of an Astraea, but larger) from four to eight feet 
broad, and little less in thickness. These mounds are 
separated from each other hy narrow crooked channels, 
about sis feet deep, most of which intersect the line of 
reef at right angles. On the furthest mound, which I 
was able to reach by the aid of a leaping-pole, and over 
which the sea broke with some violence, although the 
day was quite calm and the tide low, the polypifers in 
the uppermost cells were all dead, but between three 
and four inches lower down on its side they were living, 
and formed a projecting border round the upper and 
dead surface. The coral being thus checked in its up- 
ward growth, extends laterally, and hence most of the 
masses, especially those a little further inwards, had 
broad flat dead summits. On the other hand I could 
see, during the recoil of the breakers, that a few yards 
further seaward, the whole convex surface of the 
Porites was alive : so that the point where we were 
standing was almost on the exact upward and sliore- 
ward’limit of existence of those corals which form the 
outer margin of the reef. We shall presently see 
that there are other organic productions, fitted to bear 
a somewhat longer exposure to the air and sun. 
Next, but much inferior in importance to the 
