CORAL FORMATIONS. 617 



rods. At c is the beach on the lagoon side, and the commence- 

 ment of the lagoon. Corals grow over portions of the lagoon, — 

 although in general a large part of the bottom, both of the lagoon 

 and the sea outside, is of coral sand. 



Beyond a depth of 100 feet there are no growing corals, except 

 some kinds that enter but sparingly into the structure of reefs, 

 the largest of which are the Dcndrophyllim. 



Coral reef-rock. — The rock forming the coral platform and other 

 parts of the solid reef is a white limestone made out of corals and 

 shells. Its composition is like that of ordinary limestones. 



In some parts it contains the corals imbedded, but in others it is 

 perfectly compact, without a fossil of any kind, unless an occasional 

 shell. In no case is it chalk. The compact non-fossiliferous kinds 

 are formed in the lagoons or sheltered channels ; the kinds made 

 of broken corals, on the sea-shore side, in the face of the waves ; 

 those made of corals standing as they grew, in sheltered waters 

 where the sea has free access. 



The following are the principal kinds of coral rocks : — 



1. A fine-grained, compact, and clinking limestone, as solid and flint-like in 

 fracture as any Silurian limestone, and with rarely a shell or fragment of coral. 



This variety is very common ; and, where coral reefs or islands have heen ele- 

 vated, it often makes up the mass of the rock exposed to view. The absence of 

 fossils, while the rock was evidently made out of corals and shells, is a remark- 

 able and instructive fact. 



2. A compact oolite, consisting of rounded concretionary grains, and gene- 

 rally without any distinct fossils. 



3. A rock equally compact and hard with No. 1, but containing imbedded 

 fragments of corals, and some shells. 



4. A conglomerate of broken corals and shells, with little else, — very firm 

 and solid ; many of the corals several cubic feet in size. 



5. A rock consisting of corals standing as they grew, with the interstices 

 filled in with coral sand, shells, and fragments. In general the rock is exceed- 

 ingly solid ; but in some cases the interstices are but loosely filled. 



Coral beach-rock. — The beach-rock is made from the loose coral 

 sands of the shores which are thrown up by the waves and winds. 

 The sands become cemented into a porous sandstone, or, where 

 pebbly, into amoral pudding-stone. It forms layers, or a laminated 

 bed, along the beach of the lagoon, and also on the sea-shore side, 

 sloping sometimes at an angle of five or six degrees towards the 

 water. 



Formation of the coral structure. — A reef region is a plantation of 

 living corals, in which various species are growing together, — at 

 one place in crowded thickets, at another in scattered clumps over 

 fields of coral sand. There is the same kind of diversity that exists 



