STRUCTURE OF CORAL ISLANDS. 



27 



The reef-rock, wherever broken, shows a detritus origin. 

 Parts are of compact homogeneous texture, a solid white lime- 

 stone, without a piece of coral distinguishable, and rarely an im- 

 bedded shell. -But generally the rock is a breccia or conglomer- 

 ate, made up of corals cemented into a compact mass, and the frag- 

 ments of which it consists are sometimes many cubic feet in size.. 



It is apparent that we are describing a second 

 time an outer reef. Without dwelling farther 



upon its characters, we may pass to the features 

 of the reef when raised above the waters and 

 covered with vegetation. 



Sections of coral islands and their lagoons have 

 been given by Captain Beechey and Mr. Darwin. 

 We add another, by way of illustration, although 

 little may be presented that is novel after the ex- 

 cellent descriptions of these authors. Sketches of 

 several of these islands, showing the general rela- 

 tion of the rim of land to the reef and the lagoon 

 within, are given in the Plate of the Kingsmill 

 Group. The following sketch represents a sec- 

 tion of the rim of land from the sea on one side, 

 (the left,) to the lagoon on the other. In the 

 view, the part ma, represents the shallow sea 

 bordering an island, and abruptly deepening one 

 to six hundred feet from the line of breakers. 

 In these shallow waters are the growing corals ; 

 yet, as before stated, a large part is barren sand 

 or coral rock. 



From a to b is the shore platform of reef-rock, 

 nearly at low tide level, with the margin (a) 

 slightly elevated, and much incrusted at the top 

 with Nullipores. From the platform there is a rise 

 by a steep beach (be,) of six or eight feet, to the 

 wooded part of the coral belt represented between 

 c and d. From d to e there is a gently sloping 

 beach bordering the lagoon. Beyond e, the wa- 

 ters of the lagoon at first deepen gradually, and 

 then fall off more or less abruptly. 



In the Paumotus, the shore platform, the steep |lfc > 



beach, and the more gently sloping shore of the f/ ! 



lagoon are almost constant characteristics. s «i vt 



The width of the whole rim of land, when the island gives 

 no evidence of late elevation, varies from three hundred yards to 

 one-third of a mile, excepting certain prominent points, more ex- 

 posed to the united action of winds and waves and from oppo- 

 site directions, which occasionally exceed half a mile.* 



* Beechey states that the rim is generally three to four hundred yards in width, 

 and never exceeds half a mile. — Voyage, Amer. ed., p. 160. 



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