STRUCTURE OF CORAL /SLA .YDS. 



147 



l)Iocks and those cemented below had the same general cliarac- 

 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° 44' on a continuous mass of Porites whicJi 

 was at least twenty feet across, and it seemed to ])ass down 

 wards into the mass of the reef below water without any 

 disconnection. 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 attracted our attention for a distance of three miles." 



The shore of the lagoon is generally low and gently inclined, 

 yet in the larger islands, in which the waters of the lagoon are 

 much disturbed by the winds, there is usually a beach resem- 

 bling 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 out- 

 ward in an abrupt descent, of a few yards or fathoms, to a 

 lower area of growing corals, or a bottom 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, charac- 

 terizing 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 tliere is another descent to the depth 

 which prevails over the lagoon. On some small lagoons the 

 shore is a thick plastic mud, either white or brownish, and 

 forms a low flat which is very gently sloping. On Henuake, 

 these mud deposits are quite extensive, and of a white colour. 

 At Enderbury's Island, another having a shallow lagoon, the mud 

 was so deep and thick that there was some difficulty in reaching 

 tlie waters of the lagoon; the foot sank in eight or ten inches, 



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