620 DYNAMICAL GEOLOGY. 



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 been elevated, 

 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 remarkable and instructive 

 fact. 



2. A compact oolyte, consisting of rounded concretionary grains, and generally with- 

 out 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 exceedingly 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 a coral pudding-stone. It forms layers, or a laminated bed, along 

 the beach of the lagoon, and also on the sea-shore side, sloping gener- 

 ally at an angle of five to eight degrees toward the water, but some- 

 times at a larger angle, this depending on the slope of the beach at 

 the place. 



Formation of the Coral Reef. — 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 in the distribu- 

 tion of vegetation over the land. Some of the kinds branch like trees 

 of small size, or shrubs (Madreporce) ; others form closely-branched 

 tufts (Pocilliporce, many Porites) ; others resemble clustered leaves 

 (Merulince, Montiporce), or tufts of pinks (Tubiporce), or lichens and 

 fungi (Agaricice, etc.) ; others grow in hemispherical or subglobular 

 forms (Astrcece, Meandrince and some Porites) ; and others are groups 

 of slender, brilliantly-colored twigs {Gorgonice). 



When alive in the water, all these corals are covered throughout 

 with expanded polyps, emulating in beauty of form and colors the 

 flowers of the land. 



Each of the polyp-cells in these corals corresponds to a separate animal or polyp (p. 

 130). In the Madreporce, the polyps when expanded have twelve rays, or tentacles, 

 and a diameter of an eighth to a quarter of an inch. Those of the Pocilliporce and 

 Porites are also twelve-rayed, but smaller. The Astrcece have an indefinite number of 

 rays, or tentacles: in some species of the family, the expanded flower-like polyp is an 

 inch or more in diameter. In the Meandrince and related kinds, the polyps coalesce in 



