122 CRITIQUES AND ADDME8SE& [vi. 



and chalky, but, more often, the infiltration of water, 

 charged with carbonic acid, dissolves some of the cal 

 careous matter, and deposits it elsewhere in the inter 

 stices of the nascent rock, thus glueing and cementing 

 the particles together into a hard mass ; or it may even 

 dissolve the carbonate of lime more extensively, and 

 re-deposit it in a crystalline form. On the beach of the 

 lagoon, where the coral sand is washed into layers by 

 the action of the waves, its grains become thus fused to 

 gether into strata of a limestone, so hard that they ring 

 when struck with a hammer, and inclined at a gentle 

 angle } corresponding with that of the surface of the beach. 

 The hard parts of the many animals which live upon 

 the reef become imbedded in this coral limestone, so that 

 a block may be full of shells of bivalves and univalves, 

 or of sea-urchins ; and even sometimes encloses the 

 eggs of turtles in a state of petrifaction. The active and 

 vigorous growth of the reef goes on only at the seaward 

 margins, where the polypes are exposed to the wash of 

 the surf, and are thereby provided with an abundant 

 supply of air and of food. The interior portion of the 

 reef may be regarded as almost wholly an accumulation 

 of dead skeletons. Where a river comes down from the 

 land there is a break in the reef, for the reasons which 

 have been already mentioned. 



The origin and mode of formation of a fringing reef, such 

 as that just described, are plain enough. The embryos of 

 the coral polypes have fixed themselves upon the sub 

 merged shore of the island, as far out as they could live, 

 namely, to a depth of twenty or twenty-five fathoms. One 

 generation has succeeded another, building itself up upon 

 the dead skeletons of its predecessor. The mass has been 

 consolidated by the infiltration of coral mud, and har 

 dened by partial solution and rcdcposition, until a great 

 rampart of coral rock 100 or 150 feet high on its sea- 



