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STRUCTURE OF CORAL ISLANDS. 181 
described were found isolated, and only at considerable inter- 
vals. In no instance were they observed clustered. The loose 
blocks and those cemented below had the same general charac- 
ter, and must have been placed where they were by the same 
cause, though it may have been at different periods. 
Such blocks are of course not confined to coral island reefs, 
but belong to barrier reefs generally. 
Jukes says, ‘I once landed close to the edge of the Aus- 
tralian barrier on the south side of the Blackwood channel, in 
south latitude 11° 45’, on a continuous mass of Porites which 
was at least twenty feet across, and it seemed to pass down- 
wards into the mass of the reef below water without any dis- 
connection. It was worn into pinnacles above, so that two or 
three of us could stand in the different hollows without seeing 
each other; and it was one of a line of such masses that at- 
tracted our attention for a distance of three miles.” 
The shore of the lagoon is generally low and gently in- 
clined, yet in the larger islands, in which the waters of the 
lagoon are much disturbed by the winds, there is usually a 
beach resembling that on the seaward side, though of less 
extent. A platform of reef-rock at the same elevation as 
the shore platform sometimes extends out into the lagoon; | 
but it is more common to find it a little submerged and coy- | 
ered for the most part with growing corals; and in either case, | 
the bank terminates outward in an abrupt descent, of a few. 
yards or fathoms, to a lower area of growing corals, or a bot- | 
tom of sand. Still more commonly, we meet with a sandy 
bottom gradually deepening from the shores without growing 
coral. These three varieties of condition are generally found 
in the same lagoon, characterizing its different parts. The 
lower area of growing corals slopes outward, and ceases where 
the depth is 10 to 12 fathoms or -sooner; from this there 
