CORALS AND CORAL LSLANDS. 



VI. BEACH SAND-ROCK. 



Besides the ordinary coral rock, there are also beach for- 

 mations made of coral sands, worn shells, &c., thrown up by 

 the tides and waves. Their mode of formation is like that of 

 any sea-beach. The material is mostly like common sand in 

 tineness, but often much coarser. When the beach is fronted 

 by a distant barrier to shield it from the force of the waves, 

 the material is usually sand and small pebbles j but if the reef 

 is narrow, so that the sea breaks over it with full force, it may 

 consist even of cobble stones, as on any other shore, and 

 include also huge masses of coral rock. 



These deposits become cemented by being alternately mois- 

 tened and dried, through the action of the recurring tides and 

 the wash of the sea on the shores. The waters take up some 

 carbonate of lime, and this is deposited and hardens among the 

 particles on the evaporation of the moisture at the retreat of 

 the tides. In some places the grains are loosely coherent, and 

 seem to be united only by the few points in contact ; and with 

 a little care the calcareous coating which caused the union 

 may be distinctly traced out. In other cases, the sand has 

 been consolidated into a solid limestone rock, the interstices 

 having been filled till a compact mass was formed. Generally 

 even the most solid varieties show evidence of a sand origin, 

 and in this they differ from the reef rock. The pebbly beds 

 produce a pudding stone of coral. 



In most localities the rock is an oolite or oolitic limestone. 

 The grains become coated by the agglutinating carbonate of 

 lime, and each enlarges thus into a minute sphere — a spherical 

 concretion ; and the aggregation of these concretions makes 

 the oolite. The grains are usually much smaller than the roe 

 of most fishes, a resemblance which is alluded to in the name, 

 from the Greek wov, egg. 



These beach deposits consist of regular layers, commonly 

 from a few inches to a foot in thickness, and are generally 

 consolidated up to a line a little above high-tide mark. In all 



