LIFE : ITS MECHANICAL WORK AND ROCK CONTRIBUTIONS. 147 



cemented into a porous calcareous 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 seashore side, sloping generally at an angle of five 

 to eight degrees toward the water, but sometimes at a larger angle, this 

 depending on the slope of the beach at the place. The rock is sometimes 

 an oolyte, owing to the coating of the grains with the calcareous cement as 

 solidification goes on. Oolyte is especially common where accumulations of 

 sand make large sand-flats partly emerged at low tide. 



4. Formation of the coral reef. — A reef-region is a plantation of living 

 corals, in which various species are growing together in crowded thickets, 

 or in scattered clumps, over fields of coral sand. Besides corals and shells, 

 there are also calcareous plants, called NulUpores, growing over the edge of 

 the reef, in the face of the breakers, as shown by Darwin, and attaining 

 considerable thickness. Even the delicate branching kinds sometimes make 

 thick beds, as observed by Agassiz in the Florida seas. Bryozoans add a 

 little to the material, occasionally making large massive corals. In Paleozoic 

 time, both branching and massive kinds contributed largely to limestone 

 formations. 



5. Action of the ivaves. — The waves, especially in their heavier move- 

 ments, sweeping over the coral plantations, may be as destructive as winds 

 over forests. They tear up the corals, and, by incessant trituration, reduce 

 the fragments to a great extent to sand ; and the debris thus made and ever 

 making is scattered over the bottom, or piled upon the coast by the tide, or 

 swept over the lower parts of the reef into the lagoon, or drifted off by the 

 currents for deposition elsewhere. The corals keep growing ; and this sand 

 and the fragments go on accumulating: the consolidation of the material 

 thus accumulated makes the ordinary reef-rock. Thus, by the help of the 

 waves, a solid reef-structure is formed from the sparsely growing corals. 



Where the corals are protected from the waves, they grow up bodily to 

 the surface, and make a weak, open structure, instead of the solid reef-rock ; 

 or, if it be a closely branching species, so as to be firm, it still wants the 

 compactness of the reef that has been formed amid the waves. 



6. History of the emerging atoll. — The growing corals and the accumulat- 

 ing debris reach, at last, low-tide level. The waves continue to pile up on 

 the reef the sand and pebbles and broken masses of coral, — some of the 

 masses even 200 or 300 cubic feet in size, — and a field of rough rocks 

 begins to appear above the waves ; and finally a beach is completed. The 

 sands, now mostly above the salt water, are planted by the waves with 

 seeds : trailing shrubs spring up ; and afterward, as the soil deepens, 

 palms and other trees rise into forests, and so the finished atoll receives 

 its foliage. 



The windward side of such islands is the highest, because here the winds 

 and waves act most powerfully. But where the leeward side of one part of 

 the year is the windward of another, the two may not differ much. The 

 water that is driven by the winds or tides over the reef, into the lagoon. 



