STRUCTURE OF CORAL REEFS. 153 



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 pro- 

 duce 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 con- 

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

 stances observed, the layers dip at an angle of six to eight de- 

 grees down the beach. This dip is nothing but the slope of the 

 beach itself, and arises from the circumstance that the sands 

 are deposited by the incoming waves, or tides, on such a slop- 

 ing surface. Tutuila and XJpoH, in the Navigator Group, and 

 Oahu in the Hawaiian, afford many examples of these beach 

 formations. At certain localities the beach sand-rock has been 

 washed away after it was formed ; and occasionally large mass- 

 es or slabs have been uplifted by the sea and thrown high up 

 on the beach. 



Deposits of the same kind sometimes include detritus from 

 the hills. Black basaltic pebbles are thus cemented by the 

 white calcareous material, producing a rock of very singular 

 appearance. Near Diamond Hill, on Oahu, is a good locality 



