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 Portias 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 hi 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 cov- 

 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 



